Diabetes Drug Sitagliptin Shows Promise Against Drug-Resistant Enterococcus

Enterococcus faecalis is one of the most stubborn opportunistic pathogens in modern hospitals, a bacterium that lives quietly in the human gut yet becomes a formidable adversary once it gains access to wounds, the bloodstream, or the urinary tract. A new study from researchers at Al-Muthanna University and collaborating institutions in Iraq has now added an intriguing twist to the fight against this organism, reporting that sitagliptin, a widely prescribed anti-diabetic medication, can modulate the expression of two of the most consequential resistance genes the bacterium carries. The work, published in Molecular Biology Reports, combined classical microbiology with molecular sequencing and computational modeling to characterize the vanA and blaTEM genes in clinical isolates, and to test whether an approved drug might be repurposed as a resistance-modifying agent rather than a conventional antibiotic.

The research team began with a substantial collection of clinical specimens gathered from patients at Al-Hussein Teaching Hospital in Al-Muthanna governorate. Out of 350 samples analyzed, 83 isolates, or 23.71 percent, were confirmed as E. faecalis. The statistical analysis of the clinical data revealed a significant association between these isolates and patients with urinary catheters and pyelonephritis, with a p-value of 0.045, underscoring the organism’s particular affinity for the urinary tract environment where catheterization and compromised kidney function create favorable conditions for colonization and infection. This epidemiological grounding matters because it ties the molecular findings directly to the kinds of infections clinicians actually encounter on the ward.

Antimicrobial susceptibility testing painted a sobering picture of the resistance landscape. The isolates showed their highest level of resistance to amoxicillin/clavulanate, with 95.2 percent of strains unaffected by this beta-lactam combination, a figure that reflects the widespread dissemination of beta-lactamase enzymes among local enterococcal populations. Resistance to vancomycin, the glycopeptide antibiotic long reserved as a last-line treatment for serious gram-positive infections, was present in 13.3 percent of the isolates, a level that remains lower than rates reported in many European surveillance studies but is nonetheless clinically significant. Notably, the isolates showed almost no intrinsic resistance to sitagliptin itself, with only 2.4 percent affected, which positioned the drug as a candidate for combination strategies rather than monotherapy.

Molecular analysis confirmed the genetic basis of these phenotypes. Polymerase chain reaction screening detected the vanA gene, the archetypal determinant of high-level vancomycin resistance, and the blaTEM gene, which encodes a beta-lactamase enzyme capable of degrading penicillin-class antibiotics. The researchers then went a step further and sequenced selected isolates to probe the fine structure of these genes. In vanA, they identified two single nucleotide polymorphisms, G521A and C560T, while blaTEM carried a G244A substitution. Each of these point mutations translates into an amino acid change in the corresponding protein, and such substitutions can alter enzyme kinetics, substrate affinity, or the structural conformation of the resistance machinery. Documenting these variants in a local clinical population provides a molecular fingerprint that can be compared with sequences deposited from around the world.

The quantitative gene expression experiments formed the heart of the study. Using quantitative reverse transcription PCR, the team measured how the two resistance genes responded to different chemical environments. When the bacteria were exposed to vancomycin, vanA expression was up-regulated 1.47-fold, consistent with the known biology of the vancomycin resistance system, in which the VanS sensor kinase detects the presence of the glycopeptide and activates the VanR response regulator to switch on the resistance operon. Exposure to sitagliptin produced a different and more surprising pattern, with vanA expression reaching 3.66-fold, indicating that the drug perturbs the regulatory circuitry of the resistance system in ways that are not yet fully understood. The blaTEM gene told a complementary story: its expression was strongly induced to 1.95-fold under antibiotic pressure, but fell to 0.75-fold, a clear down-regulation, when sitagliptin was present instead.

This differential modulation is precisely what makes the findings interesting from a therapeutic standpoint. A resistance-modifying agent does not need to kill bacteria on its own; it needs to disarm the defenses that make conventional antibiotics ineffective. The down-regulation of blaTEM under sitagliptin exposure suggests the drug could sensitize resistant strains to beta-lactam therapy, and the checkerboard assays performed by the team support this interpretation, revealing a synergistic effect when sitagliptin was combined with amoxicillin. In such assays, fractional inhibitory concentrations of each agent are plotted against one another, and synergy indicates that the combination achieves more than the simple sum of its parts. For a pathogen in which 95.2 percent of isolates resist amoxicillin/clavulanate, restoring susceptibility even partially would carry real clinical weight.

The study also incorporated computational structural work to explore how sitagliptin might interact with the resistance proteins at the molecular level. Using homology modeling approaches of the kind implemented in servers such as SWISS-MODEL, the researchers built three-dimensional models of the mutant proteins and examined the structural context of the observed amino acid substitutions. This in silico component reflects a broader trend in antimicrobial research, where drug repurposing pipelines use virtual screening and docking to identify approved compounds that bind bacterial targets or regulatory proteins. Sitagliptin itself has a growing record in this arena, with previous studies reporting that it attenuates virulence in Serratia marcescens and Porphyromonas gingivalis, and that gliptins as a class exhibit anti-quorum-sensing activity against Pseudomonas aeruginosa and Staphylococcus aureus. The new findings extend this repurposing narrative into the enterococci, a genus where the therapeutic pipeline is desperately thin.

Phylogenetic analysis of 16S rRNA sequences added an evolutionary dimension to the work. The Iraqi isolates clustered closely with E. faecalis strains previously reported from China, Hungary, and Tunisia, a pattern consistent with the international dissemination of successful lineages through travel, food chains, and healthcare networks. Resistance genes such as vanA typically reside on mobile genetic elements, including the transposon Tn1546 and various plasmids, which facilitate horizontal transfer between strains and even between bacterial species. The close genetic relatedness of isolates from geographically distant countries is a reminder that antimicrobial resistance is inherently a global problem, and that local surveillance data, such as the SNP profiles documented in this study, contribute to a worldwide picture of how resistance determinants evolve and spread.

The authors are careful to frame sitagliptin as a potential adjunct rather than a standalone cure. The drug showed no clear bactericidal activity against the isolates, meaning it does not kill the bacteria directly, and the up-regulation of vanA expression under sitagliptin exposure raises questions that will need to be resolved before any clinical translation. Whether the elevated vanA transcript levels translate into functional glycopeptide resistance, or whether they reflect a stress response without phenotypic consequence, remains an open issue. What the study does establish is a proof of concept: an FDA-approved oral anti-diabetic agent, already taken daily by millions of patients with a well-characterized safety profile, can measurably alter the expression of the two genes that define resistance in multidrug-resistant E. faecalis, and can act synergistically with a beta-lactam antibiotic in vitro.

The implications reach beyond a single drug and a single pathogen. As vancomycin-resistant enterococci continue to pose an ongoing challenge in hospitals across Europe, the Middle East, and beyond, strategies that preserve the utility of existing antibiotics are increasingly viewed as essential complements to new drug development. Resistance modifiers, anti-virulence agents, and combination therapies all belong to this emerging toolkit, and repurposed medications offer the advantage of bypassing the slowest and most expensive phases of drug development. The Iraqi team’s work, grounded in local clinical isolates yet connected to global sequence data, illustrates how a modestly resourced laboratory can contribute meaningful evidence to this effort. The next steps will involve confirming the modulatory effects across larger and more diverse isolate collections, elucidating the mechanism by which sitagliptin interferes with resistance gene regulation, and testing whether the synergy observed in checkerboard plates holds up in animal models of infection. If those results are positive, clinicians may one day reach for a diabetes pill not to lower blood sugar, but to strip a deadly bacterium of its chemical armor.

Subject of Research: Repurposing sitagliptin to modulate vanA and blaTEM antibiotic resistance gene expression in Enterococcus faecalis

Article Title: Molecular Characterization of vanA and blaTEM Mutant Genes in Enterococcus faecalis: In silico and in vitro investigation of Sitagliptin as a potential Resistance Modulator

Article References: Alabdali, Y. A. J., Munahi, M. G., Dawood, Y. A., Kudhair, B. K., Al-Hejjaj, M. Y., & Aziz, Z. N. (2026). Molecular Characterization of vanA and blaTEM Mutant Genes in Enterococcus faecalis: In silico and in vitro investigation of Sitagliptin as a potential Resistance Modulator. Molecular Biology Reports, 53(1), Article 1676. https://doi.org/10.1007/s11033-026-12827-2

Image Credits: AI Generated

DOI: 10.1007/s11033-026-12827-2

Keywords: Enterococcus faecalis, antimicrobial resistance, vanA, blaTEM, sitagliptin, drug repurposing, vancomycin resistance, qRT-PCR, single nucleotide polymorphisms, checkerboard synergy assay, 16S rRNA phylogenetics, Iraq