Sensor Domain of Histidine Kinase VxrA of Vibrio cholerae- A Hairpin-swapped Dimer and its Conformational Change

Disrupting Central Carbon Metabolism Increases β-Lactam Antibiotic Susceptibility in Vibrio cholerae

Antibiotic tolerance, the ability of bacteria to sustain viability in the presence of typically bactericidal antibiotics for extended time periods, is an understudied contributor to treatment failure. The Gram-negative pathogen Vibrio cholerae, the causative agent of cholera, becomes highly tolerant to β-lactam antibiotics (penicillin and related compounds) in a process requiring the two-component system VxrAB. VxrAB is induced by exposure to cell wall damaging conditions, which results in the differential regulation of >100 genes. While the effectors of VxrAB are relatively well known, VxrAB environment-sensing and activation mechanisms remain a mystery. Here, we used transposon mutagenesis to screen for mutants that spontaneously upregulate VxrAB signaling. This screen was answered by genes known to be required for proper cell envelope homeostasis, validating the approach. Unexpectedly, we also uncovered a new connection between central carbon metabolism and antibiotic tolerance in Vibrio cholerae. Inactivation of pgi (vc0374, coding for glucose-6-phosphate isomerase) resulted in an intracellular accumulation of glucose-6-phosphate and fructose-6-phosphate, concomitant with a marked cell envelope defect, resulting in VxrAB induction. Deletion of pgi also increased sensitivity to β-lactams and conferred a growth defect on salt-free LB, phenotypes that could be suppressed by deleting sugar uptake systems and by supplementing cell wall precursors in the growth medium. Our data suggest an important connection between central metabolism and cell envelope integrity and highlight a potential new target for developing novel antimicrobial agents. IMPORTANCE Antibiotic tolerance (the ability to survive exposure to antibiotics) is a stepping stone toward antibiotic resistance (the ability to grow in the presence of antibiotics), an increasingly common cause of antibiotic treatment failure. The mechanisms promoting tolerance are poorly understood. Here, we identified central carbon metabolism as a key contributor to antibiotic tolerance and resistance. A strain with a mutation in a sugar utilization pathway accumulates metabolites that likely shut down the synthesis of cell wall precursors, which weakens the cell wall and thus increases susceptibility to cell wall-active drugs. Our results illuminate the connection between central carbon metabolism and cell wall homeostasis in V. cholerae and suggest that interfering with metabolism may be a fruitful future strategy for the development of antibiotic adjuvants.

The role of sensory kinase proteins in two-component signal transduction

Two-component systems (TCSs) are modular signaling circuits that regulate diverse aspects of microbial physiology in response to environmental cues. These molecular circuits comprise a sensor histidine kinase (HK) protein that contains a conserved histidine residue, and an effector response regulator (RR) protein with a conserved aspartate residue. HKs play a major role in bacterial signaling, since they perceive specific stimuli, transmit the message across the cytoplasmic membrane, and catalyze their own phosphorylation, and the trans-phosphorylation and dephosphorylation of their cognate response regulator. The molecular mechanisms by which HKs co-ordinate these functions have been extensively analyzed by genetic, biochemical, and structural approaches. Here, we describe the most common modular architectures found in bacterial HKs, and address the operation mode of the individual functional domains. Finally, we discuss the use of these signaling proteins as drug targets or as sensing devices in whole-cell biosensors with medical and biotechnological applications.

Diversity in Sensing and Signaling of Bacterial Sensor Histidine Kinases

Two-component signal transduction systems (TCSs) are widely conserved in bacteria to respond to and adapt to the changing environment. Since TCSs are also involved in controlling the expression of virulence, biofilm formation, quorum sensing, and antimicrobial resistance in pathogens, they serve as candidates for novel drug targets. TCSs consist of a sensor histidine kinase (HK) and its cognate response regulator (RR). Upon perception of a signal, HKs autophosphorylate their conserved histidine residues, followed by phosphotransfer to their partner RRs. The phosphorylated RRs mostly function as transcriptional regulators and control the expression of genes necessary for stress response. HKs sense their specific signals not only in their extracytoplasmic sensor domain but also in their cytoplasmic and transmembrane domains. The signals are sensed either directly or indirectly via cofactors and accessory proteins. Accumulating evidence shows that a single HK can sense and respond to multiple signals in different domains. The underlying molecular mechanisms of how HK activity is controlled by these signals have been extensively studied both biochemically and structurally. In this article, we introduce the wide diversity of signal perception in different domains of HKs, together with their recently clarified structures and molecular mechanisms.