GENOTYPIC AND PHENOTYPIC CORRELATION OF MULTIDRUG-RESISTANT PATHOGENS ISOLATED FROM POST-SURGICAL WOUND INFECTIONS AT A TERTIARY CARE INSTITUTE

Abstract

Background: Surgical site infection (SSI) is among the commonest healthcare-associated infections in Indian tertiary care hospitals, and the progressive accumulation of transferable resistance determinants in SSI pathogens has narrowed empirical treatment options. Phenotypic susceptibility testing remains the operational basis of therapy in most Indian laboratories, yet the extent to which it reflects the underlying resistance genotype in post surgical wound isolates is not well characterised in the western Indian setting. Objectives: To determine the bacteriological profile and antimicrobial resistance pattern of pathogens isolated from post-surgical wound infections, to characterise the principal β-lactam resistance determinants by polymerase chain reaction (PCR), and to quantify the agreement between phenotypic and genotypic detection of methicillin resistance, extended-spectrum β-lactamase (ESBL) production and carbapenemase production. Methods: A prospective, cross-sectional, observational study was conducted in the Department of Microbiology in collaboration with the Department of Surgery at NAMO Medical Education and Research Institute, Silvassa, from July 2025 to July 2026. Patients undergoing surgical procedures were monitored for SSI using CDC/NHSN case definitions. Isolates were identified and tested for antimicrobial susceptibility by automated broth microdilution and Kirby–Bauer disc diffusion, interpreted per CLSI M100. Phenotypic screening comprised the cefoxitin disc test, the combined-disc ESBL confirmatory test and the modified carbapenem inactivation method (mCIM/eCIM). All isolates underwent multiplex and singleplex PCR for mecA, blaCTX-M, blaTEM, blaSHV, blaNDM, blaOXA-48, blaKPC, blaVIM, blaIMP, blaOXA-23 and blaOXA-51. Agreement was expressed as Cohen's κ with 95% confidence intervals; associations were tested by Pearson's χ² test. Results: Of 2,184 surgical procedures monitored, 178 met the case definition for SSI (8.15%; 95% CI 7.05–9.37). Cultures were positive in 156 cases (87.6%), yielding 187 non-duplicate isolates; 29 infections (18.6%) were polymicrobial. Gram-negative bacilli accounted for 135 isolates (72.2%), led by Klebsiella pneumoniae (38; 20.3%) and Escherichia coli (34; 18.2%); Staphylococcus aureus was the commonest Gram-positive isolate (33; 17.6%). Overall, 117 isolates (62.6%) were multidrug-resistant. Cefoxitin resistance was present in 15 of 33 S. aureus (45.5%) and mecA in 16 (48.5%). Among 88 Enterobacterales, 54 (61.4%) were phenotypic ESBL producers and 56 (63.6%) carried an ESBL determinant, blaCTX-M predominating (47; 53.4%); 27 (30.7%) were carbapenem non-susceptible and 28 (31.8%) carried a carbapenemase gene, most frequently blaNDM (17; 63.0% of carbapenem-resistant isolates) and blaOXA-48 (11; 40.7%), with co-carriage in 6. blaOXA-23 was detected in 16 of 21 Acinetobacter baumannii (76.2%). Agreement between phenotype and genotype was almost perfect for all three comparisons: κ = 0.939 (95% CI 0.822–1.000) for cefoxitin versus mecA, κ = 0.903 (0.810–0.996) for the combined-disc test versus ESBL genes, and κ = 0.921 (0.832–1.000) for meropenem non-susceptibility versus carbapenemase genes. Emergency surgery (OR 3.09), preoperative stay beyond 48 hours (OR 3.06) and prior antimicrobial exposure (OR 3.71) were significantly associated with MDR SSI. Conclusions: Post-surgical wound infections at this centre were dominated by multidrug-resistant Gram-negative bacilli carrying blaCTX-M and blaNDM determinants. Routine phenotypic methods showed almost perfect agreement with PCR and remain a dependable operational basis for therapy, but they missed a small number of gene-carrying isolates — including an oxacillin-susceptible mecA-positive S. aureus and two carbapenemase carriers that tested meropenem-susceptible — that are of consequence for infection control. Targeted molecular confirmation, applied selectively to discordant and epidemiologically important isolates, is a rational adjunct to phenotypic testing in Indian tertiary care laboratories.