GigA and GigB are Master Regulators of Antibiotic Resistance, Stress Responses, and Virulence in Acinetobacter baumannii

Abstract

A critical component of bacterial pathogenesis is the ability of an invading organism to sense and adapt to the harsh environment imposed by the host's immune system. This is especially important for opportunistic pathogens, such as Acinetobacter baumannii, a nutritionally versatile environmental organism that has recently gained attention as a life-threatening human pathogen. The emergence of A. baumannii is closely linked to antibiotic resistance, and many contemporary isolates are multidrug resistant (MDR). Unlike many other MDR pathogens, the molecular mechanisms underlying A. baumannii pathogenesis remain largely unknown. We report here the characterization of two recently identified virulence determinants, GigA and GigB, which comprise a signal transduction pathway required for surviving environmental stresses, causing infection and antibiotic resistance. Through transcriptome analysis, we show that GigA and GigB coordinately regulate the expression of many genes and are required for generating an appropriate transcriptional response during antibiotic exposure. Genetic and biochemical data demonstrate a direct link between GigA and GigB and the nitrogen phosphotransferase system (PTS Ntr ), establishing a novel connection between a novel stress response module and a well-conserved metabolic-sensing pathway. Based on the results presented here, we propose that GigA and GigB are master regulators of a global stress response in A. baumannii, and coupling this pathway with the PTS Ntr allows A. baumannii to integrate cellular metabolic status with external environmental cues. IMPORTANCE Opportunistic pathogens, includingAcinetobacter baumannii, encounter many harsh environments during the infection cycle, including antibiotic exposure and the hostile environment within a host. While the development of antibiotic resistance in A. baumannii has been well studied, how this organism senses and responds to environmental cues remain largely unknown. Herein, we investigate two previously identified virulence determinants, GigA and GigB, and report that they are required for in vitro stress resistance, likely comprising upstream elements of a global stress response pathway. Additional experiments identify a connection between GigA/GigB and a widely conserved metabolic-sensing pathway, the nitrogen phosphotransferase system. We propose that coordination of these two pathways allows A. baumannii to respond appropriately to changing environmental conditions, including those encountered during infection.

Parallel evolution of tobramycin resistance across species and environments

“…The ptsP gene encodes phosphoenolpyruvate phosphotransferase protein, which is part of the nitrogen phosphotransferase system and has previously been identified as a target of aminoglycoside resistance for P. aeruginosa , but not A. baumannii (Sanz-García et al, 2018; Schurek et al, 2008). The mechanism by which mutations in ptsP may confer resistance to TOB is also unknown, although the nitrogen phosphotransferase system has been shown to regulate expression of genes responsible for coordinating the antibiotic stress response, suggesting an indirect link between the nitrogen phosphotransferase system and antibiotic resistance (Gebhardt and Shuman, 2017). Therefore, despite as many as 135 genes in which mutations confer reduced susceptibility, mutations in these two genes fusA1 and ptsP appear to be most fit in the presence of TOB across species and environments.…”

<p>Discovery of a Novel Hypervirulent <em>Acinetobacter baumannii</em> Strain in a Case of Community-Acquired Pneumonia</p>

“…Increased virulence in A. baumannii has been shown to be attributed to porins, capsular polysaccharides, protein secretion systems, biofilm formation, and metal acquisition systems. 2,12 The thick capsular polysaccharide of A. baumannii reduces the susceptibility to complement-mediated killing, while the ability to form biofilms promotes persistent growth in the healthcare setting. 1 A. baumannii actively pumps out chlorhexidine, a common hospital antiseptic that targets cell membranes, using the Acinetobacter chlorhexidine (AceI) efflux protein, rendering the disinfectant less effective.…”

“…Infection of G. mellonella was performed as previously described. 11,12 A. baumannii strains were grown overnight in an orbital shaker (37°C, 200 rpm), and overnight cultures were then diluted 100-fold into fresh medium and grown for 4 h. Cells were collected by centrifugation (5 min, 5000×g), washed once in phosphate-buffered saline solution (PBS), and resuspended in PBS to a final OD 600 of 1.0. Further dilutions were done in PBS.…”

Regulation of σ factors by conserved partner switches controlled by divergent signalling systems

“…Numerous PSS have been discovered in many bacteria including Gram-negative bacteria whereas they were initially thought to be restricted to Gram-positive bacteria (Shi et al, 1999;Mittenhuber, 2002;Kozak et al, 2005;Morris and Visick, 2010;Houot et al, 2012;Morris and Visick, 2013b;Eshghi et al, 2014;Mercer and Lang, 2014;Lambert et al, 2015;Thompson et al, 2015;Bouillet et al, 2016;Gebhardt and Shuman, 2017). Although PSS comprises four components (an anti-r factor, an anti-r factor antagonist, a phosphatase and a r factor), the domain organization of the partners is highly diversified as depicted in Fig.…”