Chapter 114. Molecular Mechanisms of Microbial Pathogenesis (Part 11) 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 Further Readings Camilli A, Bassler BL: Bacterial small- molecule signaling pathways. Science 311:1113, 2006 [PMID: 16497924] Finlay BB, McFadden G: Anti- immunology: Evasion of the host immune system by bacterial and viral pathogens. Cell 124:767, 2006 [PMID: 16497587] Han J, Ulevitch RJ: Limiting inflammatory responses during activation of innate immunity. Nat Immunol 6:1198, 2005 [PMID: 16369559] Kawai T, Akira S: Innate immune recognition of viral infection. Nat Immunol 7:131, 2006 [PMID: 16424890] Knirel YA et al: Structural features and structural variability of the lipopolysaccharide of Yersinia pestis, the cause of plague. J Endotoxin Res 12:3, 2006 [PMID: 16420739] Mendes-Giannini MJ et al: Interaction of pathogenic fungi wi th host cells: Molecular and cellular approaches. FEMS Immunol Med Microbiol 45:383, 2005 [PMID: 16087326] Pizarro- Cerda J, Cossart P: Bacterial adhesion and entry into host cells. Cell 124:715, 2006 [PMID: 16497583] Spear PG et al: Different receptors binding to distinct interfaces on herpes simplex virus gD can trigger events leading to cell fusion and viral entry. Virology 344:17, 2006 [PMID: 16364731] Takahashi K et al: The mannose- binding lectin: A prototypic pattern recognition molecule. Curr Opin Immunol 18:16, 2006 [PMID: 16368230] Bibliography Abe A et al: Type- III effectors: Sophisticated bacterial virulence factors. C R Biol 328:41, 2005 Kobasa D, Kawaoka Y: Emerging influenza viruses: Past and present. Curr Mol Med 5:791, 2005 [PMID: 16375713] O'Donnell RA, Blackman MJ: The role of malaria merozoite proteases in red blood cell invasion. Curr Opin Microbiol 8:422, 2005 [PMID: 16019257] . Chapter 114. Molecular Mechanisms of Microbial Pathogenesis (Part 11) Transmission to New Hosts As part of the pathogenic process, most microbes are shed from the host, often 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. 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