Slowed Adsorption

Bohannan and Lenski (15) describe bacteria that have entered a "genetic" refuge (phage-resistant mutants) as "invulnerable prey.'' However, unless a phage's collision with a bacterium results in some degree of phage-host attachment, then a resistant bacterium is not potential prey but instead some relatively inert component of the environment from which phage "bounce." Wilkinson (75), on the other hand, has suggested a model in which completely resistant bacteria really are invulnerable "prey." Here the assumption is that the"predator"species (in this case a Bdellovibrio rather than a phage) may reversibly interact with non-prey bacteria by pausing following collision. This delay in detachment extends the Bdellovibrio's extracellular search. From the perspective of susceptible bacteria, this delay is equivalent to a reduction in effective predator density. Wilkinson's conclusion upon modeling such a system is that the presence of invulnerable-prey bacteria, even in the absence of metabolic competition with vulnerable-prey bacteria, will result in an increase in the stability of Bdellovibrio-sensitive bacterial densities.

Reductions in phage adsorption rates could similarly result in increased community stability. For instance, a partial reduction in host reception to phage adsorption (e.g., bacteria partially resistant to phageT2) should contribute to an increase in community stability by delaying phage attachment to sensitive bacteria (17). Figure 5-3B presents a simulated chemostat for which the phage adsorption constant has been reduced by one half, and bacteria and phage extinctions are thereby avoided. Again with phage T2, there apparently is a tendency for these phage to be temporarily adsorption-inhibited (up to weeks at room temperature) following release from infected bacteria (62). This phenomenon could also serve to increase community stability by delaying phage adsorption. By reducing phage numbers, mechanisms of phage decay, including outflow from chemo-stat growth chambers, should also have the effect of increasing community stability. Furthermore, phage evolution could result in a decrease in community stability by increasing rates of phage adsorption or by reducing rates of phage decay.

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