抑制Stenotrophomonas maltophilia mrcA基因之功能可誘發L1和L2 β-lactamases 基本表現

Abstract

The cell wall of gram-negative bacteria is composed of a rigid murein layer, peptidoglycan, which determines cell shape. Bacteria generally own a set of unique penicillin binding proteins (PBPs) which have the DD-transpeptidase activity involved in the cross-linking of the glycan strands. In the ampR-β-lactamase-bearing gram-negative bacteria, such as Enterobacteriaceae, Pseudomonas, and Stenotrophomonas, the inducible expression of the chromosomal β-lactamase gene is the major mechanism of the β-lactam resistance. The role of PBPs in the peptidoglycan turnover and the binding affinity between PBPs and β-lactams are responsible for the induction of the chromosomal β-lactamases. This study includes two major sections. The first section points on the each PBP role of S. maltophilia KJ, except PBP3, in the bacterial growth, β-lactamase activity, and β-lactam susceptibility. Inactivation of mrcA, mrcB, pbpC, dacB, and dacC genes made little effect on the bacterial growth. However, the KJΔmrcB displayed a lower saturated optical density at 450nm than wild-type KJ strain. In addition, inactivation of different PBPs conferred to various phenotypes in β-lactamase activity and β-lactam resistance. Among them, KJΔmrcA exhibited a noteworthy phenotype of basal derepressed β-lactamase activity and further inducibility. Therefore, in the second section of this study, we focus on characterization of the KJΔmrcA mutant. The phenotype of KJΔmrcA is similar to that of Pseudomonas aeruginosa dacB mutant (PAOΔdacB). Nevertheless, the creBCD two-component regulation system is not involved in the phenotype of KJΔmrcA, unlike its role in the phenotype of PAOΔdacB. To elucidate the relationship between mrcA and the known ampNG-ampDI-ampR regulon of S. maltophilia β-lactamase induction, mutants KJΔRΔmrcA, KJΔDIΔmrcA, and KJΔNGΔmrcA were constructed. The β-lactamase activity assay and susceptibility test were comparatively determined between the mutant and its respective parent strain. The results demonstrate that the basal derepressed β-lactamase activity of KJΔmrcA is ampN-ampG- and ampR-dependent. The introduction of an ΔmrcA allele into the ΔampDI background augments the induced β-lactamase expression, signifying that ΔampDI and ΔmrcA have an addition effect on the β-lactam resistance.