Bacteria owe their ability to thrive in diverse and ever-changing conditions to their extraordinary faculty of adaptation. They have developed many strategies to adjust to new environments. These mechanisms include random modifications within their genome, such as point mutation, duplication, deletion, insertion, and acquisition of new DNA (e.g., lateral gene transfer). These multiple events generate a heterogeneous microbial population containing numerous novel phenotypes. Whenever the environment changes, a subpopulation more apt to survive in these new conditions emerges, thus allowing bacteria to thrive, for example, by acquiring resistance to antibiotics. However, as the intensity, duration, and nature of stress are extremely variable, the optimal response to new environmental conditions may be unpredictable. The means by which bacteria either respond to stress, such as exposure to toxic/inhibitory compounds or starvation, or avoid detection by the host's immune system are crucial for their survival. The spontaneous mutation rate is usually insufficient for allowing an efficient adaptation to these changes. However, certain bacterial populations contain hypermutator strains exhibiting highly increased rates of spontaneous mutations, therefore promoting adaptation to changing environments (Taddei et al., 1997). Nevertheless, this benefit may disappear once adaptation is achieved because the evolved genotype may have accumulated irreversible mutations that are detrimental in other conditions (Giraud et al., 2001a, 2001b).
Alternatively, bacteria have developed adaptation strategies based on DNA rearrangement events restricted to specific genomic regions. These defined loci allow bacteria to generate an array of phenotypic variants, whereas minimizing detrimental effects of random mutations on fitness.