Pilus Production in Acinetobacter baumannii Is Growth Phase Dependent and Essential for Natural Transformation

Abstract

Acinetobacter baumannii is a severe threat to human health as a frequently multidrug-resistant hospital-acquired pathogen. Part of the danger from this bacterium comes from its genome plasticity and ability to evolve quickly by taking up and recombining external DNA into its own genome in a process called natural competence for transformation. This mode of horizontal gene transfer is one of the major ways pathogens can acquire new antimicrobial resistances and toxic traits. Because these processes in A. baumannii are not well studied, we herein characterized new aspects of natural transformability in this species that include the species’ competence window. We uncovered a strong correlation with a growth–phase-dependent synthesis of a type IV pilus (TFP), which constitutes the central part of competence-induced DNA-uptake machinery. We used bacterial genetics and microscopy to demonstrate that the TFP is essential for the natural transformability and surface motility of A. baumannii, whereas pilus-unrelated proteins of the DNA-uptake complex do not impact the motility phenotype. Furthermore, TFP biogenesis and assembly is subject to input from two regulatory systems that are homologous to Pseudomonas aeruginosa, namely the PilSR two-component system and the Pil-Chp chemosensory system. We demonstrated that these systems not only impact the piliation status of cells but also their ability to take up DNA for transformation. Importantly, we report on discrepancies between TFP biogenesis and natural transformability within the same genus by comparing A. baumannii to data reported for A. baylyi, the latter of which served for decades as a model for natural competence. IMPORTANCE Rapid bacterial evolution has alarming negative impacts on animal and human health, which can occur when pathogens acquire antimicrobial resistance traits. As a major cause of antibiotic-resistant opportunistic infections, A. baumannii is a high priority health threat, which has motivated renewed interest in studying how this pathogen acquires new, dangerous traits. In this study, we deciphered a specific time window in which these bacteria can acquire new DNA, and correlated that with its ability to produce the external appendages that contribute to the DNA acquisition process. These cell appendages function doubly for motility on surfaces and for DNA uptake. Collectively, we showed that A. baumannii is similar in its TFP production to Pseudomonas aeruginosa, though differs from the well-studied species A. baylyi.

Involvement of H-NS in <em>Acinetobacter baumannii’s</em> Natural Transformation

“…Furthermore, A. baumannii DNA uptake by A. baumannii occurs while moving across wet surfaces [12]. Further studies refined our understanding of natural competency, which shows a correlation with growth phase-dependent synthesis of a type IV pilus [26]. The studies briefly described above conclusively showed that motility and natural competence are intimately associated.…”

“…To determine the effect of the H-NS global regulator in natural transformation, we deleted the gene in two experimentally validated A. baumannii strains and determined the expression of competence-associated genes. Pilus-related genes and twitching motility are essential for A. baumannii's transformability [8,26]. Therefore, we compared the expression levels of pilA, pilT, pilQ, comEA, comEC, comF, and drpA in the wild type, the Δhns, and a complemented strains.…”

“…Natural transformation assays were performed as were previously described [25,26]. Briefly, 20 ul of A. baumannii cells grown overnight in LB medium at 37ºC were mixed with 1 μg of pMBLe-OA-ArK (apramycin resistance) plasmid and the mixture was spotted onto twitching motility plates (10).…”

Natural Transformation as a Mechanism of Horizontal Gene Transfer in Aliarcobacter butzleri

“…This profile was similar to that observed for other related bacteria, such as C. jejuni , C. coli or H. pylori , with the natural transformation occurring at all the growth phases as well, although for these species, the frequency of transformation reaches a peak at the mid-exponential phase [ 18 , 19 , 41 ]. However, the regulatory pathways of natural transformation are divergent according to bacterial species/genera [ 13 , 42 ], and the most efficient process can occur at different growth phases, either when DNA is added prior to the exponential phase [ 41 ], during the exponential phase [ 18 , 19 , 43 , 44 ] or during the late-log to mid-stationary phase [ 42 ].…”

Coming from the Wild: Multidrug Resistant Opportunistic Pathogens Presenting a Primary, Not Human-Linked, Environmental Habitat

“…In addition, recent works indicate that ARGs-containing bacteriophages might contribute to AR spread in this microorganism [ 153 , 154 ]. Finally, it has been recently found that this organism can be naturally competent [ 155 , 156 ], opening the possibility that direct transformation could be a relevant mechanism triggering the acquisition of ARGs by this bacterium.…”