Abstract
Acinetobacter baumannii is a serious threat to public health, and there is increasing attention to the development of antibiotic resistance in this bacterium. Natural transformation is a major horizontal gene transfer mechanism that can lead to antibiotic resistance. To better understand the mechanism of natural transformation in A. baumannii, we selected a clinical isolate that was transformable but had no visible extracellular type IV pili (T4P) filaments and then examined the effects of multiple single-gene knockouts on natural plasmid transformation. Among 33 candidate genes, 28 knockout mutants had severely or completely impaired transformability. Some of these genes had established roles in T4P biogenesis; DNA transfer across the outer membrane, periplasm, or inner membrane; and protection of intracellular single-stranded DNA (ssDNA). Other genes had no previously reported roles in natural transformation of A. baumannii, including competence activator cAMP receptor protein (CRP), a periplasmic protein that may function in T4P assembly (TonB2), a T4P secretin-associated protein (TsaP), and two type II secretion system (T2SS) minor pseudopilus assembly prime complex competent proteins (XcpV and XcpW). The deletion of the T2SS assembly platform protein X had no effect on transformation, and the minor pseudopilins were capable of initiating major pilin assembly. Thus, we speculate that XcpV and XcpW may function in DNA uptake with major pilin assembly, a non-T2SS-dependent mechanism and that a competence pseudopilus similar to T4P constituted the central part of the DNA uptake complex. These results may help guide future research on the alarming increase of antibiotic resistance in this pathogen.
Highlights
Acinetobacter baumannii is a Gram-negative coccobacillus that is a significant public health concern worldwide because of its remarkable ability to develop antibiotic resistance (Peleg et al, 2008)
We examined the process of DNA uptake in naturally competent A. baumannii W068 by selecting a series of candidate genes for knockout
The two remaining genes are crp, a competence regulator gene, and tonB2, which we considered possibly involved in natural transformation
Summary
Acinetobacter baumannii is a Gram-negative coccobacillus that is a significant public health concern worldwide because of its remarkable ability to develop antibiotic resistance (Peleg et al, 2008). Horizontal gene transfer (HGT) is mostly responsible for the alarming development of resistance in this species (Snitkin et al, 2011). There are three main mechanisms of HGT in bacteria: transduction, conjugation, and natural transformation. Natural transformation does not rely on mobile genetic elements but is driven solely by a developmental program in the recipient bacteria. It begins with the triggering of natural competence in recipient bacteria, a physiological state that allows them to take up genetic material from their surroundings. Most of our knowledge of natural transformation is from studies of several model organisms, such as Vibrio cholerae (Matthey and Blokesch, 2016)
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