Over the past four 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
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., needle-stick injury) routes. A few pathogens, such as Schistosoma species, can penetrate unbroken skin. The conjunctiva can serve as an entry point for pathogens of the eye, which occasionally spread systemically from that site.
Microbial entry usually relies on the presence of specific factors needed for persistence and growth in a tissue. Fecal-oral spread via the alimentary tract requires a biologic profile 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 in the warm moist environment of the urogenital mucosa and have restricted host ranges (e.g., Neisseria gonorrhoeae, Treponemapallidum, and HIV).
The biology of microbes entering through the skin is highly varied. Some of these 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 of the organism 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 injected by bite into a susceptible host. Trypanosomes are first ingested from infected hosts by reduviid bugs; the pathogens then multiply in the gastrointestinal tract of the insects 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 the 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.
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 120-1). Most microbes produce multiple adhesins specific for multiple host receptors. These adhesins are often redundant, are serologically variable, 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.
Table 120–1. Examples of Microbial Ligand-Receptor Interactions
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Table 120–1. Examples of Microbial Ligand-Receptor Interactions
|Microorganism||Type of Microbial Ligand||Host Receptor|
|Influenza virus||Hemagglutinin||Sialic acid|
|Wild-type strains||Hemagglutinin||Signaling lymphocytic activation molecule (SLAM)|
|Human herpesvirus type 6||?||CD46|
|Herpes simplex virus||Glycoprotein C||Heparan sulfate|
|HIV||Surface glycoprotein||CD4 and chemokine receptors (CCR5 and CXCR4)|
|Epstein-Barr virus||Envelope protein||CD21 (CR2)|
|Adenovirus||Fiber protein||Coxsackie-adenovirus receptor (CAR)|
|Coxsackievirus||Viral coat proteins||CAR and major histocompatibility class I antigens|
|Neisseria spp.||Pili||Membrane co-factor protein (CD46)|
Pili and flagella
Cystic fibrosis transmembrane conductance regulator (CFTR)
|Escherichia coli||Pili||Ceramides/mannose and digalactosyl residues|
|Streptococcus pyogenes||Hyaluronic acid capsule||CD44|
|Yersinia spp.||Invasin/accessory invasin locus||β1 Integrins|
|Bordetella pertussis||Filamentous hemagglutinin||CR3|
|Legionella pneumophila||Adsorbed C3bi||CR3|
|Mycobacterium tuberculosis||Adsorbed C3bi||CR3; DC-SIGNa|
|Blastomyces dermatitidis||WI-1||Possibly matrix proteins and integrins|
|Candida albicans||Int1p||Extracellular ...|
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