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​Over the past three decades, molecular studies of the pathogenesis of microorganisms have yielded an explosion of information about the various microbial and host molecules that contribute to the processes of infection and disease. These processes can be classified into several stages: microbial encounter with and entry into the host; microbial growth after entry; avoidance of innate host defenses; tissue invasion and tropism; tissue damage; and transmission to new hosts. Virulence is the measure of an organism’s capacity to cause disease and is a function of the pathogenic factors elaborated by microbes. These factors promote colonization (the simple presence of potentially pathogenic microbes in or on a host), infection (attachment and growth of pathogens and avoidance of host defenses), and disease (often, but not always, the result of activities of secreted toxins or toxic metabolites). In addition, the host’s inflammatory response to infection greatly contributes to disease and its attendant clinical signs and symptoms.

MICROBIAL ENTRY AND ADHERENCE ENTRY SITES
A microbial pathogen can potentially enter any part of a host organism. In general, the type of disease produced by a particular microbe is often a direct consequence of its route of entry into the body.The most common sites of entry are mucosal surfaces (the respiratory, alimentary, and urogenital tracts) and the skin. Ingestion, inhalation, and sexual contact are typical routes of microbial entry. Other portals of entry include sites of skin injury (cuts, bites, burns, trauma) along with injection via natural (i.e., vector-borne) or artificial (i.e., needlestick) routes. A few pathogens, such as Schistosoma spp., can penetrate unbroken skin. The conjunctiva can serve as an entry point for pathogens of the eye. Microbial entry usually relies on the presence of specific microbial factors needed for persistence and growth in a tissue. Fecal-oral spread via the alimentary tract requires a biology consistent with survival in the varied environments of the gastrointestinal tract (including the low pH of the stomach and the high bile content of the intestine) as well as in contaminated food or water outside the host. Organisms that gain entry via the respiratory tract survive well in small moist droplets produced during sneezing and coughing. Pathogens that enter by venereal routes often survive best on the warm moist environment of the urogenital mucosa and have restricted host ranges (e.g., Neisseria gonorrhoeae,Treponema pallidum, and HIV). The biology of microbes entering through the skin is highly varied. Some organisms can survive in a broad range of environments, such as the salivary glands or alimentary tracts of arthropod vectors, the mouths of larger animals, soil, and water.A complex biology allows protozoan parasites such as Plasmodium, Leishmania, and Trypanosoma spp. to undergo morphogenic changes that permit transmission to mammalian hosts during insect feeding for blood meals. Plasmodia are injected as infective sporozoites from the salivary glands during mosquito feeding. Leishmania parasites are regurgitated as promastigotes from the alimentary tract of sandflies and are injected by bite into a susceptible host. Trypanosomes are ingested from infected hosts by reduviid bugs, multiply in the insects’ gastrointestinal tract, and are released in feces onto the host’s skin during subsequent feedings. Most microbes that land directly on intact skin are destined to die, as survival on the skin or in hair follicles requires resistance to fatty acids, low pH, and other antimicrobial factors on skin. Once it is damaged (and particularly if it becomes necrotic), the skin can be a major portal of entry and growth for pathogens and elaboration of their toxic products. Burn wound infections and tetanus are clear examples. After animal bites, pathogens resident in the animal’s saliva gain access to the victim’s tissues through the damaged skin. Rabies is the paradigm for this pathogenic process; rabies virus grows in striated muscle cells at the site of inoculation.

 

MICROBIAL ADHERENCE

Once in or on a host, most microbes must anchor themselves to a tissue or tissue factor; the possible exceptions are organisms that directly enter the bloodstream and multiply there. Specific ligands or adhesins for host receptors constitute a major area of study in the field of microbial pathogenesis. Adhesins comprise a wide range of surface structures, not only anchoring the microbe to a tissue and promoting cellular entry where appropriate, but also eliciting host responses critical to the pathogenic process (Table 2-1). Most microbes produce multiple adhesins specific for multiple host receptors. These adhesins are often redundant, are serologically variable, 11 and act additively or synergistically with other microbial factors to promote microbial sticking to host tissues. In addition, some microbes adsorb host proteins onto their surface and utilize the natural host protein receptor for microbial binding and entry into target cells.

Viral Adhesins
All viral pathogens must bind to host cells, enter them, and replicate within them.Viral coat proteins serve as the ligands for cellular entry, and more than one ligand-receptor interaction may be needed; for example, HIV uses its envelope glycoprotein (gp) 120 to enter host cells by binding to both CD4 and one of two receptors for chemokines (designated CCR5 and CXCR4). Similarly, the measles virus H glycoprotein binds to both CD46 and the membrane-organizing protein moesin on host cells. The gB and gC proteins on herpes simplex virus bind to heparan sulfate; this adherence is not essential for entry, but rather serves to concentrate virions close to the cell surface. This step is followed by attachment to mammalian cells mediated by the viral gD protein. Herpes simplex virus can use a number of eukaryotic cell surface receptors for entry, including the herpesvirus entry mediator (related to the tumor necrosis factor receptor); members of the immunoglobulin superfamily; two proteins called nectin-1 and nectin-2; and modified heparan sulfate.

Bacterial Adhesins
Among the microbial adhesins studied in greatest detail are bacterial pili and flagella (Fig. 2-1). Pili or fimbriae are commonly used by gram-negative and gram-positive bacteria for attachment to host cells and tissues. In electron micrographs, these hairlike projections (up to several hundred per cell) may be confined to one end of the organism (polar pili) or distributed more evenly over the surface. An individual cell may have pili with a variety of functions. Most pili are made up of a major pilin protein subunit (molecular weight, 17,000-30,000) that polymerizes to form the pilus. Many strains of Escherichia coli isolated from urinary tract infections express mannose- binding type 1 pili, whose binding to the integral membrane glycoproteins called uroplakins that coat the cells in the bladder epithelium is inhibited by D-mannose. Other strains produce the Pap (pyelonephritis-associated) or P pilus adhesin that mediates binding to digalactose (gal-gal) residues on globosides of the human P blood groups. Both of these types of pili have proteins located at the tips of the main pilus unit that are critical to the binding specificity of the whole pilus unit. It is interesting that, although immunization with the mannose-binding tip protein (FimH) of type 1 pili prevents experimental E. coli bladder infections in mice and monkeys, a trial of this vaccine in humans was not successful. E. coli cells causing diarrheal disease express pilus-like receptors for enterocytes on the small bowel, along with other receptors termed colonization factors. The type IV pilus, a common type of pilus found in Neisseria spp., Moraxella spp., Vibrio cholerae, Legionella pneumophila, Salmonella enterica serovar typhi, enteropathogenic E. coli, and Pseudomonas aeruginosa, mediates adherence of these organisms to target surfaces.These pili tend to have a relatively conserved amino-terminal region and a more variable carboxyl-terminal region. For some species (e.g., N. gonorrhoeae, N. meningitidis, and enteropathogenic E. coli), the pili are critical for attachment to mucosal epithelial cells. For others, such as P. aeruginosa, the pili only partially mediate the cells’ adherence to host tissues.Whereas interference with this stage of colonization would appear to be an effective antibacterial strategy, attempts to develop pilus-based vaccines for human diseases have not been highly successful to date. Flagella are long appendages attached at either one or both ends of the bacterial cell (polar flagella) or distributed over the entire cell surface (peritrichous flagella). Flagella, like pili, are composed of a polymerized or aggregated basic protein. In flagella, the protein subunits form a tight helical structure and vary serologically with the species. Spirochetes such as T. pallidum and Borrelia burgdorferi have axial filaments similar to flagella running down the long axis of the center of the cell, and they “swim” by rotation around these filaments. Some bacteria can glide over a surface in the absence of obvious motility structures. Other bacterial structures involved in adherence to host tissues include specific staphylococcal and streptococcal proteins that bind to human extracellular matrix proteins such as fibrin, fibronectin, fibrinogen, laminin, and collagen. Fibronectin appears to be a commonly used receptor for various pathogens; a particular amino acid sequence in fibronectin (Arg-Gly-Asp, or RGD) is critical for bacterial binding. Binding of the highly conserved Staphylococcus aureus surface protein clumping factor A (ClfA) to fibrinogen has been implicated in many aspects of pathogenesis.The conserved outer-core portion of the lipopolysaccharide (LPS) of P. aeruginosa mediates binding to the cystic fibrosis transmembrane conductance regulator (CFTR) on airway epithelial cells-an event that appears to be critical for normal host resistance to infection. A number of bacterial pathogens, including coagulase-negative staphylococci, S. aureus, and uropathogenic E. coli as well as Yersinia pestis, Y. pseudotuberculosis, and Y. enterocolitica, express a surface polysaccharide composed of poly-N-acetylglucosamine. One function of this polysaccharide is to promote binding to materials used in catheters and other types of implanted devices; poly-N-acetylglucosamine may be a critical factor in the establishment of device-related infections by pathogens such as staphylococci and E. coli. High-powered imaging techniques (e.g., atomic force microscopy) have revealed that bacterial cells have a nonhomogeneous surface that is probably attributable to different concentrations of cell surface molecules, including microbial adhesins, at specific places on the cell surface (Fig 2-1D).

Fungal Adhesins
Several fungal adhesins have been described that mediate colonization of epithelial surfaces, particularly adherence to structures like fibronectin, laminin, and collagen. The product of the Candida albicans INT1 gene, Int1p, bears similarity to mammalian integrins that bind to extracellular matrix proteins. Transformation of normally nonadherent Saccharomyces cerevisiae with this gene allows these yeast cells to adhere to human epithelial cells. The agglutinin-like sequence (ALS) adhesins are large cell-surface glycoproteins mediating adherence of pathogenic Candida to host tissues. These adhesins are expressed under certain environmental conditions (often associated with stress) and are crucial for pathogenesis of fungal infections. For several fungal pathogens that initiate infections after inhalation, the inoculum is ingested by alveolar macrophages, in which the fungal cells transform to pathogenic phenotypes.

Eukaryotic Pathogen Adhesins
Eukaryotic parasites use complicated surface glycoproteins as adhesins, some of which are lectins (proteins that bind to specific carbohydrates on host cells). For example, Plasmodium vivax binds (via Duffy-binding protein) to the Duffy blood group carbohydrate antigen Fy on erythrocytes. Entamoeba histolytica expresses two proteins that bind to the disaccharide galactose/N-acetylgalactosamine. Reports indicate that children with mucosal IgA antibody to one of these lectins are resistant to reinfection with virulent E. histolytica. A major surface glycoprotein (gp63) of Leishmania promastigotes is needed for these parasites to enter human macrophages—the principal target cell of infection. This glycoprotein promotes complement binding but inhibits complement lytic activity, allowing the parasite to use complement receptors for entry into macrophages; gp63 also binds to fibronectin receptors on macrophages. In addition, the pathogen can express a carbohydrate that mediates binding to host cells. Evidence suggests that, as part of hepatic granuloma formation, Schistosoma mansoni expresses a carbohydrate epitope related to the Lewis X blood group antigen that promotes adherence of helminthic eggs to vascular endothelial cells under inflammatory conditions.

HOST RECEPTORS
Host receptors are found both on target cells (e.g., epithelial cells lining mucosal surfaces) and within the mucous layer covering these cells. Microbial pathogens bind to a wide range of host receptors to establish infection (Table 2-1). Selective loss of host receptors for a pathogen may confer natural resistance to an otherwise susceptible population. For example, 70% of individuals in West Africa lack Fy antigens and are resistant to P. vivax infection. S. enterica serovar typhi, the etiologic agent of typhoid fever, uses CFTR to enter the gastrointestinal submucosa after being ingested. As homozygous mutations in CFTR are the cause of the life-shortening disease cystic fibrosis, heterozygote carriers (e.g., 4–5% of individuals of European ancestry) may have had a selective advantage due to decreased susceptibility to typhoid fever. Numerous virus–target cell interactions have been described, and it is now clear that different viruses can use similar host-cell receptors for entry.The list of certain and likely host receptors for viral pathogens is long. Among the host membrane components that can serve as receptors for viruses are sialic acids, gangliosides, glycosaminoglycans, integrins and other members of the immunoglobulin superfamily, histocompatibility antigens, and regulators and receptors for complement components. A notable example of the effect of host receptors on the pathogenesis of infection comes from comparative binding studies of avian influenza A virus subtype H5N1 and influenza A virus strains expressing hemagglutinin subtype H1. The H1-subtype strains, which tend to be highly pathogenic and transmissible from human to human, bind to a receptor composed of two sugar molecules: sialic acid linked á-2-6 to galactose. This receptor is highly expressed in the airway epithelium.When virus is shed from this surface, its transmission via coughing and aerosol droplets is readily facilitated. In contrast, H5N1 avian influenza virus binds to sialic acid linked á-2-3 to galactose, and this receptor is highly expressed in pneumocytes in the alveoli. Alveolar infection is thought to underlie not only the high mortality rate associated with avian influenza but also the low human-to-human transmissibility rate of this strain, which is not readily transported to the airways (from which it could be expelled by coughing).

MICROBIAL GROWTH AFTER ENTRY
Once established on a mucosal or skin site, pathogenic microbes must replicate before causing full-blown infection and disease.Within cells, viral particles release their nucleic acids, which may be directly translated into viral proteins (positive-strand RNA viruses), transcribed from a negative strand of RNA into a complementary mRNA (negative-strand RNA viruses), or transcribed into a complementary strand of DNA (retroviruses); for DNA viruses, mRNA may be transcribed directly from viral DNA, either in the cell nucleus or in the cytoplasm.To grow, bacteria must acquire specific nutrients or synthesize them from precursors in host tissues. Many infectious processes are usually confined to specific epithelial surfaces— e.g., H1-subtype influenza to the respiratory mucosa, gonorrhea to the urogenital epithelium, and shigellosis to the gastrointestinal epithelium. Although there are multiple reasons for this specificity, one important consideration is the ability of these pathogens to obtain from these specific environments the nutrients needed for growth and survival. Temperature restrictions also play a role in limiting certain pathogens to specific tissues. Rhinoviruses, a cause of the common cold, grow best at 33°C and replicate in cooler nasal tissues, but not as well in the lung. Leprosy lesions due to Mycobacterium leprae are found in and on relatively cool body sites. Fungal pathogens that infect the skin, hair follicles, and nails (dermatophyte infections) remain confined to the cooler, exterior, keratinous layer of the epithelium. Many bacterial, fungal, and protozoal species grow in multicellular masses referred to as biofilms.These masses are biochemically and morphologically quite distinct from the free-living individual cells referred to as planktonic cells. Growth in biofilms leads to altered microbial metabolism, production of extracellular virulence factors, and decreased susceptibility to biocides, antimicrobial agents, and host defense molecules and cells. P. aeruginosa growing on the bronchial mucosa during chronic infection, staphylococci and other pathogens growing on implanted medical devices, and dental pathogens growing on tooth surfaces to form plaques represent several examples of microbial biofilm growth associated with human disease. Many other pathogens can form biofilms during in vitro growth, and it is increasingly accepted that this mode of growth contributes to microbial virulence and induction of disease.

AVOIDANCE OF INNATE HOST DEFENSES
Because microbes have probably interacted with mucosal/ epithelial surfaces since the emergence of multicellular organisms, it is not surprising that multicellular hosts have a variety of innate surface defense mechanisms that can sense when pathogens are present and contribute to their elimination. The skin is acidic and is bathed with fatty acids toxic to many microbes. Skin pathogens such as staphylococci must tolerate these adverse conditions. Mucosal surfaces are covered by a barrier composed of a thick mucous layer that entraps microbes and facilitates their transport out of the body by such processes as mucociliary clearance, coughing, and urination. Mucous secretions, saliva, and tears contain antibacterial factors such as lysozyme and antimicrobial peptides as well as antiviral factors such as interferons. Gastric acidity is inimical to the survival of many ingested pathogens, and most mucosal surfaces—particularly the nasopharynx, the vaginal tract, and the gastrointestinal tract—contain a resident flora of commensal microbes that interfere with the ability of pathogens to colonize and infect a host. Pathogens that survive these factors must still contend with host endocytic, phagocytic, and inflammatory responses as well as with host genetic factors that determine the degree to which a pathogen can survive and grow. The growth of viral pathogens entering skin or mucosal epithelial cells can be limited by a variety of host genetic factors, including production of interferons, modulation of receptors for viral entry, and age- and hormone-related susceptibility factors; by nutritional status; and even by personal habits such as smoking and exercise.

ENCOUNTERS WITH EPITHELIAL CELLS
Over the past decade, many bacterial pathogens have been shown to enter epithelial cells (Fig. 2-2); the bacteria often use specialized surface structures that bind to receptors, with consequent internalization. However, the exact role and the importance of this process in infection and disease are not well defined for most of these pathogens. Bacterial entry into host epithelial cells is seen as a means for dissemination to adjacent or deeper tissues or as a route to sanctuary to avoid ingestion and killing by professional phagocytes. Epithelial cell entry appears, for instance, to be a critical aspect of dysentery induction by Shigella. Curiously, the less virulent strains of many bacterial pathogens are more adept at entering epithelial cells than are more virulent strains; examples include pathogens that lack the surface polysaccharide capsule needed to cause serious disease.Thus, for Haemophilus influenzae, Streptococcus pneumoniae, Streptococcus agalactiae (group B Streptococcus). and Streptococcus pyogenes, isogenic mutants or variants lacking capsules enter epithelial cells better than the wild-type, encapsulated parental forms that cause disseminated disease. These observations have led to the proposal that epithelial cell entry may be primarily a manifestation of host defense, resulting in bacterial clearance by both shedding of epithelial cells containing internalized bacteria and initiation of a protective and nonpathogenic inflammatory response. However, a possible consequence of this process could be the opening of a hole in the epithelium, potentially allowing uningested organisms to enter the submucosa.This scenario has been documented in murine S. enterica serovar typhimurium infections and in experimental bladder infections with uropathogenic E. coli. In the latter system, bacterial pilus–mediated attachment to uroplakins induces exfoliation of the cells with attached bacteria. Subsequently, infection is produced by residual bacterial cells that invade the superficial bladder epithelium, where they can grow intracellularly into biofilm-like masses encased in an extracellular polysaccharide-rich matrix and surrounded by uroplakin. This mode of growth produces structures that have been referred to as bacterial pods. At low bacterial inocula, epithelial cell ingestion and subclinical inflammation are probably efficient means to eliminate pathogens; in contrast, at higher inocula, a proportion of surviving bacterial cells enter host tissue through the damaged mucosal surface and multiply, producing disease. Alternatively, failure of the appropriate epithelial cell response to a pathogen may allow the organism to survive on a mucosal surface where, if it avoids other host defenses, it can grow and cause a local infection. Along these lines, as noted above, P. aeruginosa is taken into epithelial cells by CFTR, a protein missing or nonfunctional in most severe cases of cystic fibrosis. The major clinical consequence is chronic airwaysurface infection with P. aeruginosa in 80–90% of patients with cystic fibrosis. The failure of airway epithelial cells to ingest and promote the removal of P. aeruginosa via a properly regulated inflammatory response has been proposed as a key component of the hypersusceptibility of these patients to chronic airway infection with this organism.


ENCOUNTERS WITH PHAGOCYTES
Phagocytosis and Inflammation
Phagocytosis of microbes is a major innate host defense that limits the growth and spread of pathogens. Phagocytes appear rapidly at sites of infection in conjunction with the initiation of inflammation. Ingestion of microbes by both tissue-fixed macrophages and migrating phagocytes probably accounts for the limited ability of most microbial agents to cause disease.A family of related molecules called collectins, soluble defense collagens, or pattern-recognition molecules are found in blood (mannose-binding lectins), in lung (surfactant proteins A and D), and most likely in other tissues as well and bind to carbohydrates on microbial surfaces to promote phagocyte clearance. Bacterial pathogens seem to be ingested principally by polymorphonuclear neutrophils (PMNs), whereas eosinophils are frequently found at sites of infection with protozoan or multicellular parasites. Successful pathogens, by definition, must avoid being cleared by professional phagocytes. One of several antiphagocytic strategies employed by bacteria and by the fungal pathogen Cryptococcus neoformans is to elaborate large-molecular-weight surface polysaccharide antigens, often in the form of a capsule that coats the cell surface.Most pathogenic bacteria produce such antiphagocytic capsules. On occasion, proteins or polypeptides form capsule-like coatings on organisms such as Bacillus anthracis. Because activation of local phagocytes in tissues is a key step in initiating inflammation and migration of additional phagocytes into infected sites, much attention has been paid to microbial factors that initiate inflammation. Encounters with phagocytes are governed largely by the structure of the microbial constituents that elicit inflammation, and detailed knowledge of these structures for bacterial pathogens has contributed greatly to our understanding of molecular mechanisms of microbial pathogenesis (Fig. 2-3). One of the beststudied systems involves the interaction of LPS from gram-negative bacteria and the glycosylphosphatidylinositol (GPI)-anchored membrane protein CD14 found on the surface of professional phagocytes, including migrating and tissue-fixed macrophages and PMNs. A soluble form of CD14 is also found in plasma and on mucosal surfaces. A plasma protein, LPS-binding protein (LBP), transfers LPS to membrane-bound CD14 on myeloid cells and promotes binding of LPS to soluble CD14. Soluble CD14/LPS/LBP complexes bind to many cell types and may be internalized to initiate cellular responses to microbial pathogens. It has been shown that peptidoglycan and lipoteichoic acid from gram-positive bacteria and cell-surface products of mycobacteria and spirochetes can interact with CD14 (Fig. 2-3). Additional molecules, such as MD-2, also participate in the recognition of bacterial activators of inflammation. GPI-anchored receptors do not have intracellular signaling domains. Instead, the mammalian Toll-like receptors (TLRs) transduce signals for cellular activation due to LPS binding. It has recently been shown that binding of microbial factors to TLRs to activate signal transduction occurs not on the cell surface, but rather in the phagosome of cells that have internalized the microbe. This interaction is probably due to the release of the microbial surface factor from the cell in the environment of the phagosome, where the liberated factor can bind to its cognate TLRs.TLRs initiate cellular activation through a series of signal-transducing molecules (Fig. 2-3) that lead to nuclear translocation of the transcription factor nuclear factor êB (NF-êB), a master-switch for production of important inflammatory cytokines such as tumor necrosis factor á (TNF-á) and interleukin (IL) 1. Inflammation can be initiated not only with LPS and peptidoglycan, but also with viral particles and other microbial products such as polysaccharides, enzymes, and toxins. Bacterial flagella activate inflammation by binding of a conserved sequence to TLR5. Some pathogens, including Campylobacter jejuni, Helicobacter pylori, and Bartonella bacilliformis, make flagella that lack this sequence and thus do not bind to TLR5.The result is a lack of efficient host response to infection. Bacteria also produce a high proportion of DNA molecules with unmethylated CpG residues that activate inflammation through TLR9.TLR3 recognizes double-strand RNA, a pattern-recognition molecule produced by many viruses during their replicative cycle.TLR1 and TLR6 associate with TLR2 to promote recognition of acylated microbial proteins and peptides. The myeloid differentiation factor 88 (MyD88) molecule is a generalized adaptor protein that binds to the cytoplasmic domains of all known TLRs and also to receptors that are part of the IL-1 receptor (IL-1Rc) family. Numerous studies have shown that MyD88- mediated transduction of signals from TLRs and IL-1Rc is critical for innate resistance to infection. Mice lacking MyD88 are more susceptible than normal mice to infection with group B Streptococcus, Listeria monocytogenes, and Mycobacterium tuberculosis. However, it is now appreciated that some of the TLRs (e.g., TLR3 and TLR4) can activate signal transduction via an MyD88- independent pathway.

Additional Interactions of Microbial Pathogens and Phagocytes
Other ways that microbial pathogens avoid destruction by phagocytes include production of factors that are toxic to phagocytes or that interfere with the chemotactic and ingestion function of phagocytes. Hemolysins, leukocidins, and the like are microbial proteins that can kill phagocytes that are attempting to ingest organisms elaborating these substances. For example, staphylococcal hemolysins inhibit macrophage chemotaxis and kill these phagocytes. Streptolysin O made by S. pyogenes binds to cholesterol in phagocyte membranes and initiates a process of internal degranulation, with the release of normally granule-sequestered toxic components into the phagocyte’s cytoplasm. E. histolytica, an intestinal protozoan that causes amebic dysentery, can disrupt phagocyte membranes after direct contact via the release of protozoal phospholipase A and pore-forming peptides. Microbial Survival inside Phagocytes Many important microbial pathogens use a variety of strategies to survive inside phagocytes (particularly macrophages) after ingestion. Inhibition of fusion of the phagocytic vacuole (the phagosome) containing the ingested microbe with the lysosomal granules containing antimicrobial substances (the lysosome) allows M. tuberculosis, S. enterica serovar typhi, and Toxoplasma gondii to survive inside macrophages. Some organisms, such as L. monocytogenes, escape into the phagocyte’s cytoplasm to grow and eventually spread to other cells. Resistance to killing within the macrophage and subsequent growth are critical to successful infection by herpes-type viruses, measles virus, poxviruses, Salmonella, Yersinia, Legionella, Mycobacterium, Trypanosoma, Nocardia, Histoplasma, Toxoplasma, and Rickettsia. Salmonella spp. use a master regulatory system, in which the PhoP/PhoQ genes control other genes, to enter and survive within cells; intracellular survival entails structural changes in the cell envelope LPS.

TISSUE INVASION AND TISSUE TROPISM TISSUE INVASION
Most viral pathogens cause disease by growth at skin or mucosal entry sites, but some pathogens spread from the initial site to deeper tissues.Virus can spread via the nerves (rabies virus) or plasma (picornaviruses) or within migratory blood cells (poliovirus, Epstein-Barr virus, and many others). Specific viral genes determine where and how individual viral strains can spread. Bacteria may invade deeper layers of mucosal tissue via intracellular uptake by epithelial cells, traversal of epithelial cell junctions, or penetration through denuded epithelial surfaces. Among virulent Shigella strains and invasive E. coli, outer-membrane proteins are critical to epithelial cell invasion and bacterial multiplication. Neisseria and Haemophilus spp. penetrate mucosal cells by poorly understood mechanisms before dissemination into the bloodstream. Staphylococci and streptococci elaborate a variety of extracellular enzymes, such as hyaluronidase, lipases, nucleases, and hemolysins, that are probably important in 17 breaking down cellular and matrix structures and allowing the bacteria access to deeper tissues and blood. Organisms that colonize the gastrointestinal tract can often translocate through the mucosa into the blood and, under circumstances in which host defenses are inadequate, cause bacteremia. Y. enterocolitica can invade the mucosa through the activity of the invasin protein. Some bacteria (e.g., Brucella) can be carried from a mucosal site to a distant site by phagocytic cells (e.g., PMNs) that ingest but fail to kill the bacteria. Fungal pathogens almost always take advantage of host immunocompromise to spread hematogenously to deeper tissues.The AIDS epidemic has resoundingly illustrated this principle:The immunodeficiency of many HIV-infected patients permits the development of life-threatening fungal infections of the lung, blood, and brain. Other than the capsule of C. neoformans, specific fungal antigens involved in tissue invasion are not well characterized. Both fungal and protozoal pathogens undergo morphologic changes to spread within a host. Yeast-cell forms of C. albicans transform into hyphal forms when invading deeper tissues. Malarial parasites grow in liver cells as merozoites and are released into the blood to invade erythrocytes and become trophozoites. E. histolytica is found as both a cyst and a trophozoite in the intestinal lumen, through which this pathogen enters the host, but only the trophozoite form can spread systemically to cause amebic liver abscesses. Other protozoal pathogens, such as T. gondii, Giardia lamblia, and Cryptosporidium, also undergo extensive morphologic changes after initial infection to spread to other tissues.


TISSUE TROPISM
The propensity of certain microbes to cause disease by infecting specific tissues has been known since the early days of bacteriology, yet the molecular basis for this propensity is understood somewhat better for viral pathogens than for other agents of infectious disease. Specific receptor-ligand interactions clearly underlie the ability of certain viruses to enter cells within tissues and disrupt normal tissue function, but the mere presence of a receptor for a virus on a target tissue is not sufficient for tissue tropism. Factors in the cell, route of viral entry, viral capacity to penetrate into cells, viral genetic elements that regulate gene expression, and pathways of viral spread in a tissue all affect tissue tropism. Some viral genes are best transcribed in specific target cells, such as hepatitis B genes in liver cells and Epstein-Barr virus genes in B lymphocytes. The route of inoculation of poliovirus determines its neurotropism, although the molecular basis for this circumstance is not understood. The lesser understanding of the tissue tropism of bacterial and parasitic infections is exemplified by Neisseria spp. There is no well-accepted explanation of why N. gonorrhoeae colonizes and infects the human genital tract, whereas the closely related species N. meningitidis principally colonizes the human oropharynx. N. meningitidis expresses a capsular polysaccharide, whereas N. gonorrhoeae does not; however, there is no indication that this property plays a role in the different tissue tropisms displayed by these two bacterial species. N. gonorrhoeae can use cytidine monophosphate N-acetylneuraminic acid from host tissues to add N-acetylneuraminic acid (sialic acid) to its lipooligosaccharide (LOS) O side chain, and this alteration appears to make the organism resistant to host defenses. Lactate, present at high levels on genital mucosal surfaces, stimulates sialylation of gonococcal LOS. Bacteria with sialic acid sugars in their capsules, such as N. meningitidis, E. coli K1, and group B streptococci, have a propensity to cause meningitis, but this generalization has many exceptions. For example, all recognized serotypes of group B streptococci contain sialic acid in their capsules, but only one serotype (III) is responsible for most cases of group B streptococcal meningitis. Moreover, both H. influenzae and S. pneumoniae can readily cause meningitis, but these organisms do not have sialic acid in their capsules.

TISSUE DAMAGE AND DISEASE
Disease is a complex phenomenon resulting from tissue invasion and destruction, toxin elaboration, and host response.Viruses cause much of their damage by exerting a cytopathic effect on host cells and inhibiting host defenses. The growth of bacterial, fungal, and protozoal parasites in tissue, which may or may not be accompanied by toxin elaboration, can also compromise tissue function and lead to disease. For some bacterial and possibly some fungal pathogens, toxin production is one of the best-characterized molecular mechanisms of pathogenesis, whereas host factors such as IL-1,TNF-á, kinins, inflammatory proteins, products of complement activation, and mediators derived from arachidonic acid metabolites (leukotrienes) and cellular degranulation (histamines) readily contribute to the severity of disease.

BACTERIAL TOXINS
Among the first infectious diseases to be understood were those due to toxin-elaborating bacteria. Diphtheria, botulism, and tetanus toxins are responsible for the diseases associated with local infections due to Corynebacterium diphtheriae, Clostridium botulinum, and Clostridium tetani, respectively. Enterotoxins produced by E. coli, Salmonella, Shigella, Staphylococcus, and V. cholerae contribute to diarrheal disease caused by these organisms. Staphylococci, streptococci, P. aeruginosa, and Bordetella elaborate various toxins that cause or contribute to disease, including toxic shock syndrome toxin 1 (TSST-1); erythrogenic toxin; exotoxins A, S,T, and U; and pertussis toxin.A number of these toxins (e.g., cholera toxin, diphtheria toxin, pertussis toxin, E. coli heat-labile toxin, and P. aeruginosa exotoxins A, S, and T) have adenosine diphosphate (ADP)-ribosyltransferase activity—i.e., the toxins enzymatically catalyze the transfer of the ADP-ribosyl portion of nicotinamide adenine diphosphate to target proteins and inactivate them. The staphylococcal enterotoxins,TSST-1, and the streptococcal pyogenic exotoxins behave as superantigens, stimulating certain T cells to proliferate without processing of the protein toxin by antigen-presenting cells. Part of this process involves stimulation of the antigen-presenting cells to produce IL-1 and TNF-á, which have been implicated in many of the clinical features of diseases like toxic shock syndrome and scarlet fever. A number of gramnegative pathogens (Salmonella, Yersinia, and P. aeruginosa) can inject toxins directly into host target cells by means of a complex set of proteins referred to as the type III secretion system. Loss or inactivation of this virulence system usually greatly reduces the capacity of a bacterial pathogen to cause disease.

ENDOTOXIN
The lipid A portion of gram-negative LPS has potent biologic activities that cause many of the clinical manifestations of gram-negative bacterial sepsis, including fever, muscle proteolysis, uncontrolled intravascular coagulation, and shock. The effects of lipid A appear to be mediated by the production of potent cytokines due to LPS binding to CD14 and signal transduction via TLRs, particularly TLR4. Cytokines exhibit potent hypothermic activity through effects on the hypothalamus; they also increase vascular permeability, alter the activity of endothelial cells, and induce endothelial-cell procoagulant activity. Numerous therapeutic strategies aimed at neutralizing the effects of endotoxin are under investigation, but so far the results have been disappointing. One drug, activated protein C (drotrecogin alfa, activated), was found to reduce mortality by ∼20% during severe sepsis—a condition that can be induced by endotoxin during gram-negative bacterial sepsis.

INVASION
Many diseases are caused primarily by pathogens growing in tissue sites that are normally sterile. Pneumococcal pneumonia is mostly attributable to the growth of S. pneumoniae in the lung and the attendant host inflammatory response, although specific factors that enhance this process (e.g., pneumolysin) may be responsible for some of the pathogenic potential of the pneumococcus. Disease that follows bacteremia and invasion of the meninges by meningitis-producing bacteria such as N. meningitidis, H. influenzae, E. coli K1, and group B streptococci appears to be due solely to the ability of these organisms to gain access to these tissues, multiply in them, and provoke cytokine production, leading to tissue-damaging host inflammation. Specific molecular mechanisms accounting for tissue invasion by fungal and protozoal pathogens are less well described. Except for studies pointing to factors like capsule and melanin production by C. neoformans and (possibly) levels of cell wall glucans in some pathogenic fungi, the molecular basis for fungal invasiveness is not well defined. Melanism has been shown to protect the fungal cell against death caused by phagocyte factors such as nitric oxide, superoxide, and hypochlorite. Morphogenic variation and production of proteases (e.g., the Candida aspartyl proteinase) have been implicated in fungal invasion of host tissues. If pathogens are effectively to invade host tissues (particularly the blood), they must avoid the major host defenses represented by complement and phagocytic cells. Bacteria most often avoid these defenses through their cell surface polysaccharides—either capsular polysaccharides or long O-side-chain antigens characteristic of the smooth LPS of gram-negative bacteria. These molecules can prevent the activation and/or deposition of complement opsonins or limit the access of phagocytic cells with receptors for complement opsonins to these molecules when they are deposited on the bacterial surface below the capsular layer. Another potential mechanism of microbial virulence is the ability of some organisms to present the capsule as an apparent self antigen through molecular mimicry. For example, the polysialic acid capsule of group B N. meningitidis is chemically identical to an oligosaccharide found on human brain cells. Immunochemical studies of capsular polysaccharides have led to an appreciation of the tremendous chemical diversity that can result from the linking of a few monosaccharides. For example, three hexoses can link up in more than 300 different and potentially serologically distinct ways, whereas three amino acids have only six possible peptide combinations. Capsular polysaccharides, which have been used as effective vaccines against meningococcal meningitis as well as against pneumococcal and H. influenzae infections, may prove to be of value as vaccines against any organisms that express a nontoxic, immunogenic capsular polysaccharide. In addition, most encapsulated pathogens become virtually avirulent when capsule production is interrupted by genetic manipulation; this observation emphasizes the importance of this structure in pathogenesis.

HOST RESPONSE
The inflammatory response of the host is critical for interruption and resolution of the infectious process, but also is often responsible for the signs and symptoms of disease. Infection promotes a complex series of host responses involving the complement, kinin, and coagulation pathways. The production of cytokines such as IL-1,TNF-á, and other factors regulated in part by the NF-êB transcription factor leads to fever, muscle proteolysis, and other effects, as noted above. An inability to kill or contain the microbe usually results in further damage due to the progression of inflammation and infection. In many chronic infections, degranulation of host inflammatory cells can lead to release of host proteases, elastases, histamines, and other toxic substances that can degrade host tissues. Chronic inflammation in any tissue can lead to the destruction of that tissue and to clinical disease associated with loss of organ function; an example is sterility from pelvic inflammatory disease caused by chronic infection with N. gonorrhoeae. The nature of the host response elicited by the pathogen often determines the pathology of a particular infection. Local inflammation produces local tissue damage, whereas systemic inflammation, such as that seen during sepsis, can result in the signs and symptoms of septic shock. The severity of septic shock is associated with the degree of production of host effectors. Disease due to intracellular parasitism results from the formation of granulomas, wherein the host attempts to wall off the parasite inside a fibrotic lesion surrounded by fused epithelial cells that make up so-called multinucleated giant cells. A number of pathogens, particularly anaerobic bacteria, staphylococci, and streptococci, provoke the formation of an abscess, probably because of the presence of zwitterionic surface polysaccharides such as the capsular polysaccharide of Bacteroides fragilis. The outcome of an infection depends on the balance between an effective host response that eliminates a pathogen and an excessive inflammatory response that is associated with an inability to eliminate a pathogen and with the resultant tissue damage that leads to disease.

TRANSMISSION TO NEW HOSTS
As part of the pathogenic process, most microbes are shed from the host, often in a form infectious for susceptible individuals. However, the rate of transmissibility may not necessarily be high, even if the disease is severe in the infected individual, as transmissibility and virulence are not linked traits. Most pathogens exit via the same route by which they entered: respiratory pathogens by aerosols from sneezing or coughing or through salivary spread, gastrointestinal pathogens by fecal-oral spread, sexually transmitted diseases by venereal spread, and vector-borne organisms by either direct contact with the vector through a blood meal or indirect contact with organisms shed into environmental sources such as water. Microbial factors that specifically promote transmission are not well characterized. Respiratory shedding is facilitated by overproduction of mucous secretions, with consequently enhanced sneezing and coughing. Diarrheal toxins such as cholera toxin, E. coli heat-labile toxins, and Shigella toxins probably facilitate fecal-oral spread of microbial cells in the high volumes of diarrheal fluid produced during infection. The ability to produce phenotypic variants that resist hostile environmental factors (e.g., the highly resistant cysts of E. histolytica shed in feces) represents another mechanism of pathogenesis relevant to transmission. Blood parasites such as Plasmodium spp. change phenotype after ingestion by a mosquito-a prerequisite for the continued transmission of this pathogen. Venereally transmitted pathogens may undergo phenotypic variation due to the production of specific factors to facilitate transmission, but shedding of these pathogens into the environment does not result in the formation of infectious foci. In summary, the molecular mechanisms used by pathogens to colonize, invade, infect, and disrupt the host are numerous and diverse. Each phase of the infectious process involves a variety of microbial and host factors interacting in a manner that can result in disease. Recognition of the coordinated genetic regulation of virulence factor elaboration when organisms move from their natural environment into the mammalian host emphasizes the complex nature of the host-parasite interaction. Fortunately, the need for diverse factors in successful infection and disease implies that a variety of therapeutic strategies may be developed to interrupt this process and thereby prevent and treat microbial infections.