Cards of Virulence and the Global Virulome for Humans

Cards of Virulence and the Global Virulome for Humans -...

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Pathogenic microbes can be put into two general categories: those acquired from the environment and those acquired from other humans and animals. Microbes that are acquired from the environment typically can survive in environmental niches and also in their hosts. In contrast, microbes acquired from other human and animal hosts usually have complex relationships with the originating host, and may utterly depend on their hosts to replicate and survive. Furthermore, diseases caused by environmentally acquired microbes may not be communicable, whereas those caused by host-derived microbes may depend on host-to-host spread for survival of the microbe. For example, endemic pathogenic fungi such as Histoplasma capsulatum , Cryptococcus neoformans , and Coccidioides spp. are found in soils and have no apparent requirement for animal hosts. Yet, these organisms can cause disease in numerous animal hosts and are generally considered nonspecific pathogens. At the other extreme, viruses completely depend on their hosts to replicate and survive, and often are specially adapted to particular hosts. Somewhere between these two extremes one finds microbes such as Bacillus anthracis , which lives in soils for prolonged periods but amplifies greatly when it causes anthrax in mammalian herbivores. The virulence in microbes acquired from other animal and human hosts can be understood as an interaction that damages the new host. For such microbes, infecting new hosts may be essential for their survival, whereas causing damage is not. For example, because all viruses depend on host cell machinery to replicate, they absolutely require a susceptible host to survive. Similarly, many parasites require one or more hosts to complete their respective life cycles, but do not immediately damage their new hosts. Many free-living bacteria, protozoa, and fungi with pathogenic capacities do not require eukaryotic hosts to survive and face no obvious selection pressures to cause or avoid causing damage to human and animal hosts, with the caveat that killing these hosts will likely return these microbes sooner to their ordinary environmental niches. The potential for soil microbes becoming virulent thus raises some fundamental questions about the origin of virulence. Recent studies of interactions of soil microbes with other microorganisms, such as fungi and amoebae, suggest ways in which virulence can emerge in microbes that otherwise do not require animal hosts. Microbes Play a Metaphorical Card Game within an Extreme Environment Soils teem with countless bacteria, fungi, phage, viruses, protists, and small animals--all competing for nutrients in an environment that is subject to extremes of humidity, oxygen tension, temperature, electromagnetic radiation, ionic strength, and pH, because of diurnal cycles, seasons, and climates. For example, 1 g of soil may contain 109 microorganisms from more than 4,000 microbial species. Many of them produce powerful enzymes to break down dead organic matter, while some devour other microbes as nutrient sources.
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This note was uploaded on 04/07/2008 for the course MBI 111 taught by Professor Clark during the Fall '07 term at Miami University.

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Cards of Virulence and the Global Virulome for Humans -...

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