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Chapter 14 : Bacterial Adaptation
Many bacterial regulatory circuits are not digital but analogue devices. In other words, their responses are not all-or-none but are proportional to the stimulus to which they respond. Overlap and redundancy contribute to bacterial robustness. Bacterial populations benefit from nondeterministic, random variations in their molecular circuitry-called epigenetic variation because the processes are heritable but not due to mutational changes in the DNA. Extrachromosomal DNA has been traditionally viewed as the main toolbox for bacterial genome plasticity. Plasmids are crucial indeed for bacterial adaptation. Studies with clinical isolates have indicated that hypermutable bacterial lineages may adapt better to harsh environmental conditions. While it seems out of the question that such hypermutable lineages may enter an evolutionary dead end, their existence emphasizes the importance of mutation as an adaptive strategy. Population geneticists have predicted that variation of mutation rates in response to environmental circumstances might have selective value. Under comfortable circumstances, however, elevated mutation rates would be unnecessary and probably detrimental. An example of variation of mutation rates upon environmental influence is observed when E. coli is exposed to fluoroquinolones, a class of antibiotics that target DNA topoisomerases, thus blocking DNA replication. Increased mutation rates may produce additional mutations that can further facilitate survival. SOS induction associated with antibiotic challenge is not the only environmentally controlled mechanism known to modulate mutation rates. Bacteria are equipped with analogue devices that permit efficient adaptation to changing conditions. Bacterial populations often display bistable or multistable states, created either by built-in mechanisms or by random fluctuations.