No New Antibiotic Needed: Small-Molecule SagA Blockade Re-Exposes a Weakness in Drug-Resistant Enterococci

When bacteria build defenses against important antibiotics, developing another new drug may not be the only answer. Teams at Cold Spring Harbor Laboratory and Scripps Research have proposed another approach: first use a small molecule to disrupt cell-wall engineering in drug-resistant bacteria, then allow the existing antibiotic vancomycin to once again find a site it can attack. This peer-reviewed study focuses on an important pathogen in healthcare-associated infections—vancomycin-resistant Enterococcus faecium (VREfm).

The study targeted an enzyme called SagA. SagA belongs to the NlpC/P60 family of peptidoglycan hydrolases and helps bacteria cleave and remodel their cell walls, enabling cells to separate and grow normally. After the team deleted the sagA gene, bacterial cell-wall remodeling and division were disrupted, increasing sensitivity to vancomycin. Restoring SagA eliminated this effect, supporting a causal relationship between the two.

The key is not simply that the bacteria become weaker. Deleting SagA had limited effects on the activity of ampicillin, daptomycin, and ceftriaxone, indicating that its effect was more specific to vancomycin. Imaging and cell-wall analyses showed that inhibiting SagA increased the accumulation of peptidoglycan fragments with D-Ala-D-Ala termini, re-exposing targets to which vancomycin can bind. Following treatment, the binding signal between fluorescently labeled vancomycin and the bacterial cells also increased markedly.

To translate the genetic experiments into a practical drug strategy, the researchers screened 616 electrophilic compounds and identified the first small molecules capable of covalently inhibiting NlpC/P60 hydrolases. The most potent, pghi-4, reduced the minimum inhibitory concentration of vancomycin in a concentration-dependent manner, by as much as eightfold. This potentiating effect was also observed in multiple VREfm clinical isolates with different genetic backgrounds, rather than being limited to a single laboratory strain.

In a mouse model of VREfm sepsis, combining pghi-4 with vancomycin reduced bacterial loads in organs and mitigated weight loss. Under the same treatment regimen, neither drug alone showed a significant effect. This makes pghi-4 more akin to an “antibiotic potentiator” than a standalone antibiotic: it does not necessarily need to kill bacteria directly, but instead dismantles part of the resistance mechanism, extending the useful life of an existing drug.

However, pghi-4 remains an early-stage research tool, not a therapeutic candidate ready for direct clinical use. Each group in the mouse experiments included only six animals, the treatment period was short, and the pharmacokinetic, safe-dose, and toxicity data required for use in humans have not yet been established. The study also found that pghi-4 may inhibit related hydrolases other than SagA, and its complete target profile and selectivity remain to be clarified. A reduction of up to eightfold in the minimum inhibitory concentration also does not mean that every clinical strain has returned to a treatable susceptibility range.

The team is developing more potent second-generation derivatives and exploring designs that link the inhibitor to vancomycin. If subsequent studies can demonstrate that such a combination has a sufficient safety window and does not readily induce new resistance, a potentiation strategy targeting cell-wall-remodeling enzymes could give established antibiotics another period of clinical usefulness.