Role of the PhoPQ two-component regulatory system in the β-lactam resistance of Stenotrophomonas maltophilia

Background: Stenotrophomonas maltophilia, an opportunistic pathogen, is intrinsically resistant to most beta-lactams except ceftazidime and ticarcillin/clavulanate, due to the inducibly expressed L1 and L2 beta-lactamases. A two-component regulatory system (TCS) allows organisms to sense and respond to changes in different environmental conditions. The PhoPQ TCS of S. maltophilia plays regulatory roles in antibiotic susceptibility, physiology, stress adaption and virulence. Inactivation of S. maltophilia phoPQ increases beta-lactamsusceptibility. Objectives: To elucidate the PhoPQ-regulating mechanism for beta-lactam resistance. Methods: The candidate genes responsible for the Delta phoPQ-mediated beta-lactam resistance compromise were identified by transcriptome analysis and verified by quantitative RT-PCR and complementation assay. Etest was used to assess beta-lactam susceptibility. The phosphorylation level of the PhoP protein was determined by Phos-tag SDS-PAGE and western blotting. A beta-lactaminflux assay was used to investigate the influx efficiency of a beta-lactam. Results: PhoPQ deletion down-regulated the expression of mltD1 and slt, attenuated the induced beta-lactamase activity and then compromised the beta-lactam resistance. Complementation of mutant phoPQ with mltD1 or slt genes partially reverted the induced beta-lactamase activity and beta-lactamresistance. The PhoPQ TCS was activated in logarithmically grown KJ cells and was further activated by low magnesium, but not by a beta-lactam. However, low-magnesium-mediated PhoPQ activation hardly made an impact on beta-lactam resistance enhancement. Furthermore, PhoPQ inactivation altered the outer membrane permeability and increased the influx of a beta-lactam. Conclusions: The PhoPQ TCS is activated to some extent in physiologically grown S. maltophilia. Inactivation of phoPQ attenuates the expression of mltD1 and slt, and increases beta-lactam influx, both synergically contributing to beta-lactam resistance compromise.

Automated summary

The PhoPQ TCS in S. maltophilia plays a key role in regulating beta-lactam resistance by down-regulating specific genes, attenuating beta-lactamase activity, and increasing beta-lactam influx. Inactivation of PhoPQ leads to compromised beta-lactam resistance through alterations in gene expression and increased drug influx.