New insights on targeting mycobacterial transpeptidases to boost β-lactam activity

β-Lactams demonstrate promising in vitro activity against Mycobacterium species, but their in vivo effectiveness against Mycobacterium tuberculosis remains limited. That gap has led researchers to ask how to make β-lactams or new inhibitors work more reliably against mycobacteria. In mycobacteria, the enzymes that build and remodel the cell wall include L,D-transpeptidases (Ldts) and penicillin-binding proteins (PBPs), and both are potential targets for β-lactam drugs. Reports have suggested that blocking both kinds of enzymes might be necessary for strong antibacterial effects, but the individual contributions of Ldts and PBPs to drug action were not clear. To address this question, a team led by corresponding author Christopher J. Schofield investigated how inhibition of these transpeptidases affects bacterial growth. They focused on Mycobacterium smegmatis ( Msm ) as a laboratory model and tested classical β-lactam compounds alongside reported Ldt Mt2 inhibitors. Using this focused approach, the researchers set out to measure enzyme activity inside living cells and to compare those measurements directly with how well the drugs suppressed Msm growth.

To probe enzyme activity inside living cells, the investigators used fluorogenic substrate mimics that report on transpeptidase function. They treated Mycobacterium smegmatis ( Msm ) with a panel of β-lactams and reported Ldt Mt2 inhibitors, then measured impacts on the transpeptidase activities of Ldts and PBPs in situ. The results revealed a statistically significant correlation between inhibition of both Ldts and PBPs and suppression of Msm growth, meaning drugs that reduced enzyme activity tended to slow bacterial growth. Under the tested conditions, the correlation between Ldt inhibition and Msm growth suppression was stronger than the correlation for PBPs. Importantly, all compounds that showed activity blocked both PBPs and Ldts to some extent, although β-lactams manifested increased potency of PBP inhibition. The team also tested combinations and found that combining the β-lactams meropenem and faropenem with selected Ldt Mt2 inhibitors produced an additive inhibitory effect against Msm.

Taken together, these findings clarify how targeting mycobacterial transpeptidases relates to antibacterial activity and point to priorities for drug development. The observation that active compounds inhibited both classes of enzymes, and that Ldt inhibition showed the stronger correlation with growth suppression under the study conditions, suggests that improving inhibition of Ldts could be particularly important for enhancing β-lactam efficacy versus mycobacteria. At the same time, the apparent potency of β-lactams against PBPs and the additive effects seen when combining meropenem or faropenem with Ldt Mt2 inhibitors indicate that dual targeting or combination strategies remain a promising route. The authors conclude that further optimisation of both PBPs and Ldt transpeptidase inhibition, especially of Ldts, is needed to develop β-lactam-based therapies with broader, clinically relevant potency against mycobacterial pathogens.