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
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
, 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.