The role of penicillin binding protein 4 in β-lactam resistance in Staphylococcus aureus

Abstract

The role of penicillin binding protein 4 in β-lactam resistance in Staphylococcus aureus
Ahang H. Mawlood
Abstract
Staphylococcus aureus possesses four native penicillin binding proteins PBPs (PBP1-4). Because of the structural resemblance between their natural substrate, the D-Ala-D-Ala end of the stem pentapeptide precursor, and penicillin-type antibiotics, the late stage peptidoglycan synthesizing enzymes are sensitive to penicillin. Resistance to penicillin-type antibiotics in S. aureus arises due to acquisition of an alternative PBP (PBP2A, encoded by mecA), expression of β-lactamase, or point mutations in genes encoding endogenous penicillin binding proteins, which reduces penicillin binding to the enzyme active site. The reduced sensitivity to penicillin G and cefotaxime in vitro selected mutant RS1/19 cells is likely related to PBP4 overproduction, because antibiotics that selectively inhibit PBP4 activity cause the greatest reduction in penicillin G MIC (minimum inhibitory concentration), when used in combination.

To test whether PBP4 overproduction is sufficient and necessary for the penicillin resistance observed in RS1/19, the pbp4 gene was cloned into a high copy number plasmid and introduced into different genetic backgrounds of S. aureus. Antibiotic sensitivity testing showed that there was no difference in the minimum inhibitory concentration (MIC) of penicillin G or oxacillin, this means that increased PBP4 activity is not sufficient for resistance. Allelic replacement using double cross-over event was used to delete pbp4 in the background of RS1/19 strain. RS1/19∆ pbp4 tested in this study have a moderately increased sensitivity to cefoxitin and cefsoludin, but a marked increase in sensitivity to cefotaxim, oxacillin, and penicillin G compared to RS1/19. These data lead to the conclusion that PBP4 is necessary for the penicillin and methicillin resistance phenotype in RS1/19.

In addition, PBP4 must work with a network of genes that collectively associates with the resistance observed in RS1/19 strain. The data presented here clearly shows that PBP4 is not the only factor involved in resistance in RS1/19, but that it is necessary for the full expression of resistance. It has been concluded that deletion of monofunctional trasglycosylase mgt is not essential for viability and expression of resistance in both wild-type and RS1/19strains. Future studies of RS1/19 are likely to uncover other factors that are essential for full expression of methicillin resistance in mecA negative strains of S. aureus.