Transcription of the dinB gene, encoding DNA polymerase IV, is induced by the inhibition of cell wall synthesis at different levels. Using the ␤-lactam antibiotic ceftazidime, a PBP3 inhibitor, as a model, we have shown that this induction is independent of the LexA/RecA regulatory system. Induction of dinB transcription mediated by ceftazidime produces an increase in the reversion of a ؉1 Lac frameshift mutation.

SummaryAdaptive evolution depends on both the genetic variability in a population of organisms and the selection of the better adapted genotypes. However, for the fittest variants to be selected they must survive over a sufficient period under the new conditions. Bacteria are often exposed to different types of stress in nature, including antibiotics. We analysed the global expression profiles of the opportunistic pathogen Pseudomonas aeruginosa in response to ceftazidime, a PBP3 inhibitor, at different concentrations and times. PBP3 inhibition exerts a global impact on the transcription of a large number of genes. From an adaptive perspective, it is noteworthy the induction of several SOS genes, as well as adaptation, protection and antibiotic resistance genes. Intriguingly, transcription of pyocin genes, previously described as SOS-regulated, was repressed upon PBP3 inhibition. Ciprofloxacin, an SOS inducer, produced transcriptional induction of pyocins. Our results indicate that: (i) the SOS responses resulting from treatments with these two antibiotics cause only partially overlapping transcription profiles; (ii) PBP3 and DNA-gyrase inhibition produce opposite effects on transcription of pyocin genes. Consequently, ceftazidime decreases ciprofloxacin toxicity; (iii) error-prone DNApolymerase DinB is induced by PBP3 inhibition but not by DNA-gyrase inhibition; (iv) PBP3 inhibition causes induced mutagenesis; (v) ceftazidime upregulates several antibiotic-resistance and adaptation genes; and (vi) ceftazidime concentrations thought previously to be lethal are not, as most cells treated with ceftazidime remain alive and recover their capacity to form colonies. Thus, transcriptional changes demonstrated in this work are likely to be adaptively relevant to cells that survive.

Background: Bacterial motility is a crucial factor in the colonization of natural environments. Escherichia coli has two flagella-driven motility types: swimming and swarming. Swimming motility consists of individual cell movement in liquid medium or soft semisolid agar, whereas swarming is a coordinated cellular behaviour leading to a collective movement on semisolid surfaces. It is known that swimming motility can be influenced by several types of environmental stress. In nature, environmentally induced DNA damage (e.g. UV irradiation) is one of the most common types of stress. One of the key proteins involved in the response to DNA damage is RecA, a multifunctional protein required for maintaining genome integrity and the generation of genetic variation.

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