Activation of phenotypic subpopulations in response to ciprofloxacin treatment in Acinetobacter baumannii

• Online Access: http://hdl.handle.net/2047/d20003082

The desiccation-resistant pathogen, Acinetobacter baumannii, has spread swiftly through hospitals worldwide. Favoring chromosomal insertions and rearrangements, A. baumannii isolates are quickly becoming pandrug resistant. We have recently demonstrated that A. baumannii regulates the expression of multiple copies of genes encoding error-prone DNA polymerases, which seem to be responsible for DNA damage- or desiccation-induced mutagenesis, leading to antibiotic resistance. Here, we explore expression of the DNA damage-inducible system in A. baumannii, using transcriptional reporters fused to green fluorescent protein for selected DNA damage response genes. The expression of these genes in response to treatment with the antibiotic, ciprofloxacin, was then analyzed by single cell analyses. We found the genes are regulated in a bimodal fashion, forming two phenotypic subpopulations: induced and uninduced. The subpopulations are formed as a result of a phenotypic switch, rather than a genetic change; each subpopulation returns to a starting state and differentiates with repeated treatment. This mode of regulation seems to be specific to A. baumannii, since the non-pathogenic Acinetobacter baylyi dinB gene responds in a unimodal fashion to the same antibiotic treatment. We have identified two separate regulatory motifs upstream of selected DNA damage-inducible genes, as well as a bimodal deficient strain with deregulated expression of the umuDC genes. The bistable induction of DNA damage response genes differs greatly from what is known about such systems in other organisms and could define an alternate route to pathogenic success. In creating subpopulations, A. baumannii increases the probability that it will be able to confront variable environments and ensure survival of the species. Lastly, we have also begun to explore the pathogenicity of A. baumannii within a host, using Caenorhabditis elegans as a model and have created a transposon library in A. baumannii for use in future experiments.