Multidrug-resistant (MDR) P. aeruginosa (Pa) are a pressing threat in healthcare, so novel approaches to fight them are urgently needed. The cell envelope of Gram-negatives consists of an outer and inner membrane. The interjacent periplasm contains the peptidoglycan (PGN) sacculus. PGN is crucial for cell stability and shape, and is made of short overlapping glycan chains. Glycan consists of disaccharides connected to muropeptides, crosslinked to each other and thereby forming a mesh. These crosslinks can be cleaved by PGN hydrolases, to allow for cell growth and division. In E. coli (Ec) it was shown that three such hydrolases (EcMepM (YebA), EcMepH and EcMepS) are essential for the assembly of PGN into the cell wall¹. Pa encodes for three putative MepMs, namely PaMepM1, PaMepM2, PaMepM3, which are phylogenetically highly related to EcMepM. Using Tn-directed insertion sequencing of an MDR Pa bloodstream isolate, we screened for genes important for β-lactam resistance². Among others, 3 genes encoding PGN hydrolases, namely mepM1, mepM2, and mepH2, were identified, and we could corroborate their contribution to β-lactam resistance. We also tested which virulence-associated phenotypes are influenced by MepM proteins in this MDR clinical Pa isolate. Thus, we investigated the impact of mepM1, mepM2, mepM3 deletions on biofilm formation, salt sensitivity, motility and cell shape, and found especially for the triple deletion mutant Pa ΔmepM123 (beside the break of resistance to β-lactam antibiotics) a change in morphology (cells were more curved in the mutant compared to wildtype), and a significant reduction in biofilm formation, salt sensitivity and motility. Interestingly, reduced motility was not due to lowered FliC abundance or lack of flagella, but was at least partially caused by a higher abundance of the efflux pump MexEFOprN, indicating a more global function of MepM proteins for cell physiology. In summary, PGN endopeptidases of the MepM family are promising targets to develop novel drugs as antivirulence agents and to sensitize MDR pathogens to β-lactam treatment.

1Park et al. 2020; 10.3389/fmicb.2020.565767; 2Sonnabend et al. 2020; 10.1128/AAC.01771-19