Stochastic activation of a family of TetR type transcriptional regulators controls phenotypic heterogeneity in Acinetobacter baumannii

Pathogenicity and virulence of Acinetobacter baumannii: Factors contributing to the fitness in healthcare settings and the infected host

Acinetobacter baumannii is a common cause of healthcare-associated infections and hospital outbreaks, particularly in intensive care units. Much of the success of A. baumannii relies on its genomic plasticity, which allows rapid adaptation to adversity and stress. The capacity to acquire novel antibiotic resistance determinants and the tolerance to stresses encountered in the hospital environment promote A. baumannii spread among patients and long-term contamination of the healthcare setting. This review explores virulence factors and physiological traits contributing to A. baumannii infection and adaptation to the hospital environment. Several cell-associated and secreted virulence factors involved in A. baumannii biofilm formation, cell adhesion, invasion, and persistence in the host, as well as resistance to xeric stress imposed by the healthcare settings, are illustrated to give reasons for the success of A. baumannii as a hospital pathogen.

Global analysis of the RNA-RNA interactome in Acinetobacter baumannii AB5075 uncovers a small regulatory RNA repressing the virulence-related outer membrane protein CarO

Acinetobacter baumannii is an opportunistic Gram-negative pathogen that infects critically ill patients. The emergence of antimicrobial resistant A. baumannii has exacerbated the need to characterize environmental adaptation, antibiotic resistance and pathogenicity and their genetic regulators to inform intervention strategies. Critical to adaptation to changing environments in bacteria are small regulatory RNAs (sRNAs), however, the role that sRNAs play in the biology of A. baumannii is poorly understood. To assess the regulatory function of sRNAs and to uncover their RNA interaction partners, we employed an RNA proximity ligation and sequencing method (Hi-GRIL-seq) in three different environmental conditions. Forty sRNAs were ligated to sRNA-RNA chimeric sequencing reads, suggesting that sRNA-mediated gene regulation is pervasive in A. baumannii. In-depth characterization uncovered the sRNA Aar to be a post-transcriptional regulator of four mRNA targets including the transcript encoding outer membrane protein CarO. Aar initiates base-pairing with these mRNAs using a conserved seed region of nine nucleotides, sequestering the ribosome binding sites and inhibiting translation. Aar is differentially expressed in multiple stress conditions suggesting a role in fine-tuning translation of the Aar-target molecules. Our study provides mechanistic insights into sRNA-mediated gene regulation in A. baumannii and represents a valuable resource for future RNA-centric research endeavours.

Dissecting cell heterogeneities in bacterial biofilms and their implications for antibiotic tolerance

Bacterial biofilms consist of large, self-formed aggregates where resident bacteria can exhibit very different physiological states and phenotypes. This heterogeneity of cell types is crucial for many structural and functional emergent properties of biofilms. Consequently, it becomes essential to understand what drives cells to differentiate and how they achieve it within the three-dimensional landscape of the biofilms. Here, we discuss recent advances in comprehending two forms of cell heterogeneity that, while recognized to coexist within biofilms, have proven challenging to distinguish. These two forms include cell heterogeneity arising as a consequence of bacteria physiologically responding to resource gradients formed across the biofilms and cell-to-cell phenotypic heterogeneity, which emerges locally within biofilm subzones among neighboring bacteria due to stochastic variations in gene expression. We describe the defining features and concepts related to both forms of cell heterogeneity and discuss their implications, with a particular focus on antibiotic tolerance.

Colony phase variation switch modulates antimicrobial tolerance and biofilm formation in Acinetobacter baumannii

Carbapenem-resistant Acinetobacter baumannii causes one of the most difficult-to-treat nosocomial infections. Polycationic drugs like polymyxin B or colistin and tetracycline drugs such as doxycycline or minocycline are commonly used to treat infections caused by carbapenem-resistant A. baumannii. Here, we show that a subpopulation of cells associated with the opaque/translucent colony phase variation by A. baumannii AB5075 displays differential tolerance to subinhibitory concentrations of colistin and tetracycline. Using a variety of microscopic techniques, we demonstrate that extracellular polysaccharide moieties mediate colistin tolerance to opaque A. baumannii at single-cell level and that mushroom-shaped biofilm structures protect opaque bacteria at the community level. The colony switch phenotype is found to alter several traits of A. baumannii, including long-term survival under desiccation, tolerance to ethanol, competition with Escherichia coli, and intracellular survival in the environmental model host Acanthamoeba castellanii. Additionally, our findings suggest that extracellular DNA associated with membrane vesicles can promote colony switching in a DNA recombinase-dependent manner.IMPORTANCEAs a WHO top-priority drug-resistant microbe, Acinetobacter baumannii significantly contributes to hospital-associated infections worldwide. One particularly intriguing aspect is its ability to reversibly switch its colony morphotype on agar plates, which has been remarkably underexplored. In this study, we employed various microscopic techniques and phenotypic assays to investigate the colony phase variation switch under different clinically and environmentally relevant conditions. Our findings reveal that the presence of a poly N-acetylglucosamine-positive extracellular matrix layer contributes to the protection of bacteria from the bactericidal effects of colistin. Furthermore, we provide intriguing insights into the multicellular lifestyle of A. baumannii, specifically in the context of colony switch variation within its predatory host, Acanthamoeba castellanii.

Evidence for Rho-dependent control of a virulence switch in Acinetobacter baumannii

IMPORTANCE

Acinetobacter baumannii is a significant cause of infections in the healthcare setting. More recently, A. baumannii has been a leading cause of secondary bacterial pneumonia in patients infected with SARS-CoV-2 and the overall frequency of A. baumannii infection increased 78% during the COVID-19 pandemic. A. baumannii can exist in virulent or avirulent subpopulations and this interconversion is mediated by the expression of a family of TetR-type transcriptional regulators. In this study, we demonstrate that Rho is a key regulatory component in the expression of these TetR regulators. Overall, this study is the first to address a role for Rho in A. baumannii and provides additional evidence for the role of Rho in regulating diversity in bacterial subpopulations.

The VBNC state: a fundamental survival strategy of Acinetobacter baumannii

IMPORTANCE

Currently, the viable but non-culturable (VBNC) state is an underappreciated niche for pathogenic bacteria which provides a continuous source for recurrent infections and transmission. We propose the VBNC state to be a global persistence mechanism used by various A. baumannii strains to cope with many stresses it is confronted with in the clinical environment and in the host. This requires a novel strategy to detect viable cells of this pathogen that is not only based on plating assays.