Characterization of Chlorhexidine Resistance Among Clinical Isolates of Coagulase-Negative Staphylococcus Species in a Tertiary Care Center, Chennai

Background Coagulase-negative <i>Staphylococci </i>(CoNS) are potential pathogens and are often associated with healthcare-associated infections (HAIs). Chlorhexidine (CHX) is the most widely used antiseptic to reduce colonization and infection by all <i>Staphylococci</i>, including CoNS. Resistance to CHX among CoNS has been observed over the past few years, consequent to its widespread use. Phenotypic tolerance or reduced susceptibility to CHX is conferred by plasmid-mediated <i>qac</i> group of genes, mainly <i>qacA/B</i> and <i>smr</i>, which cause activation of efflux pumps over the bacterial cell wall. This study aims to characterize the phenotypic and genotypic resistance exhibited by CoNS species against CHX. Methods After ethical approval, 148 consecutive, non-repetitive isolates of clinically significant CoNS species of hospitalized patients, isolated from blood samples and exudative specimens, were included in the study. Speciation was performed by conventional biochemical identification and automated methods. Antimicrobial susceptibility testing was performed by disc diffusion technique and for vancomycin by minimum inhibitory concentration (MIC) determination, as per Clinical Laboratory Standards Institute (CLSI) M-100 2023 guidelines. Methicillin resistance was detected using a cefoxitin disc. MIC for CHX was performed by agar dilution method; reduced susceptibility was considered when MIC to CHX ≥4 µg/mL. The simplex polymerase chain reaction (PCR) was carried out with suitable controls to detect <i>qacA/B</i> and <i>smr</i>. Statistical analysis was conducted to determine the association of <i>qacA/B</i> and <i>smr</i> genes with MIC of CHX in the study isolates. Results Fifteen different species of CoNS were obtained from clinical samples. A high percentage of resistance was observed against various classes of antibiotics. Methicillin resistance was observed in 69.6% (103/148) of isolates. Of 148 CoNS, 52.7% (78/148) of isolates exhibited reduced susceptibility to CHX with an MIC ≥4 µg/mL. These isolates exhibited a higher percentage of methicillin resistance (75.6%, 59/78). By PCR, 34.5% (51/148) of isolates carried either or both genes. Gene <i>qacA/B</i> was solely detected in 27.02% (40/148) of isolates, of which 14 were CHX-tolerant and the remaining 26 were CHX-susceptible. Gene <i>smr</i> was solely detected in 4.1% (6/148) of isolates comprising three isolates each in CHX-tolerant and susceptible categories<i>. </i>There were 3.4% (5/148) of isolates that harbored both genes, of which only one isolate was CHX-susceptible, while the other four were CHX-tolerant. A proportion of isolates that were phenotypically tolerant to CHX did not carry either or both genes. A significant statistical association was found between reduced susceptibility to CHX and the presence of antiseptic resistance genes in the study isolates (p-value=0.033942). Conclusion To our knowledge, this is the first study from South India to investigate CHX resistance among CoNS using phenotypic and genotypic methods. The rise of antiseptic resistance among CoNS is an emerging threat to current infection control practices. The presence of <i>qacA/B </i>and<i> smr</i> genes, especially in CHX susceptible isolates, is concerning since these resistance genes are located on transferable plasmids, and the isolates can develop resistance eventually upon exposure to CHX.