Microbes rapidly evolve characteristics that enable them to overcome the host’s defences

Microorganisms faced with the antimicrobial defences of the host species have evolved and developed a variety of characteristics that enable them to bypass or overcome these defences and carry out their obligatory steps for survival (Table 12.1). Unfortunately, microorganisms evolve with extraordinary speed in comparison with their hosts. This is partly because they multiply much more rapidly, the generation time of an average bacterium being 1    h or less compared with about 20     years for the human host. Rapid evolutionary change is also favoured in bacteria that can hand over genes (carried on plasmids) directly to other bacteria, including unrelated bacteria. Antibiotic resistance genes, for instance, can then be transferred rapidly between species. This rapid rate of evolution ensures that microbes are always many steps ahead of the host’s antimicrobial defences. Indeed, if there are possible ways around the established defences, microorganisms are likely to have discovered and taken advantage of them. Infectious microorganisms therefore owe their success to this ability to adapt and evolve, exploiting weak points in the host’s defences, as outlined in Table 12.2 and Figures 12.1, 12.2. The host, in turn, has had to respond to such strategies by slowly improving defences, adding extra features, and having multiple defence mechanisms with overlap and a good deal of duplication.

Table 12.1 Successful infectious microorganisms must take certain obligatory steps

a The last step, causing damage in the host, is not strictly necessary, but a certain amount of damage may be essential for shedding. The outpouring of infectious fluids in the common cold or diarrhea, for instance, or the trickle from vesicular or pustular lesions, is required for transmission to fresh hosts.

Figure 12.1 Every infection is a race. Delays in mobilizing host adaptive defences can lead to disease or death.

Figure 12.2 Myxomatosis is the best-studied example of the appearance of a highly lethal microbe in a host population that gradually settles down to a state of more balanced pathogenicity. Vibrio cholerae has progressed in this direction, and perhaps HIV is destined to tread the same path.

The speed with which host adaptive responses can be mobilized is crucial

Every infection is a race between the capacity of the microorganism to multiply, spread and cause disease and the ability of the host to control and finally terminate the infection (Fig. 12.1). For instance, a 24-h delay before an important host response comes into operation can give a decisive advantage to a rapidly growing microorganism. From the host’s point of view, it may allow enough damage to cause disease. More importantly, from the microbe’s point of view, it may give the microbe the opportunity to be shed from the body in larger amounts or for an extra day or two. A microbe that achieves this will be rapidly selected for in evolution.

Adaptation by both host and parasite leads to a more stable balanced relationship

The picture of conflict between host and parasite, usually and appropriately described in military terms, is central to an understanding of the biology of infectious disease. As with military conflicts, adaptation on both sides (Box 12.1) tends to lessen the damage and incidence of death in the host population, leading to a more stable and balanced relationship. The successful parasite gets what it can from the host without causing too much damage, and in general, the more ancient the relationship, the less the damage. Many microbial parasites, not only the normal flora (see Ch. 8), but also polioviruses, meningococci and pneumococci and others, live for the most part in peaceful coexistence with their human host.