High occurrence of high-level mupirocin & chlorhexidine resistant genes in methicillin resistant staphylococcal isolates from dialysis unit of a tertiary care hospital

Sir,

Chronic hemodialysis (CHD) patients are vulnerable to infections, including infections by methicillin-resistant staphylococci (MRS) because they are repeatedly exposed to the hospital environment and often receive prolonged courses of antibiotics, besides being immunocompromised1. Nasal carriage of MRS in hospital personnel also adds to the colonization pressure in healthcare facilities, acting as reservoirs for transmission to these patients1. Routine use of mupirocin and chlorhexidine in healthcare settings has contributed to acquisition of resistance to these antimicrobial agents among microbes which cause outbreaks in these settings2. Resistance to mupirocin is of low-level (mutations in the chromosomal ileS gene) or high-level [by a plasmid-mediated mupA (ileS2) gene, encoding a novel IleS]3. Chlorhexidine resistance is conferred by the plasmid- mediated qacA/B genes which encode proton-dependent multidrug efflux pumps4. We conducted a cross-sectional study to detect the presence of mupirocin and chlorhexidine resistance among methicillin resistant staphylococcal isolates obtained from the dialysis unit of a tertiary care hospital.

A total of 83 non-duplicate methicillin resistant coagulase negative staphylococcal (MRCoNS) isolates from anterior nares of CHD patients (n=124) and hospital personnel (n=30) from dialysis unit of Billroth Hospital, a tertiary care centre in Chennai, Tamil Nadu, India, were included in this study. Phenotypic detection of low- and high-level mupirocin resistance was carried out using mupirocin discs [5 and 200 μg (Hi-Media, Mumbai)] and minimum inhibitory concentration (MIC) for mupirocin was determined by agar dilution method56. Staphylococcus aureus ATCC 25923 was used as quality control strain and results were interpreted as per Clinical and Laboratory Standards Institute (CLSI) guidelines and British Society for Antimicrobial Chemotherapy (BSAC) guidelines56. Isolates resistant to 5 and 200 μg mupirocin discs were further subjected to mupA gene detection7. All isolates were screened for the presence of chlorhexidine resistance gene (qacA/B) by PCR8.

Of the 83 MRCoNS isolates, 68 (81.9%) were from CHD patients and 15 (18%) from dialysis unit staff members. Mupirocin resistance was observed in 26 (31.3%) isolates, of which, 22 (26.5%) exhibited high-level mupirocin resistance (HLMR) and were also positive for mupA gene. In our study, mupirocin resistance was slightly higher than that reported from another study from south India9. Majority of the isolates showing HLMR (n=16, 19.2%) were isolated from CHD patients. Six of 22 (27.3%) isolates with HLMR displayed qacA/B. The distribution of chlorhexidine resistance genes among high- and low-level mupirocin resistant and mupirocin sensitive isolates are shown in the Table. In this study, mupirocin sensitive isolates (12/83, 14.4%) were found to harbour higher percentage of qacA/B genes compared to mupirocin resistant isolates (8/83, 9.6%).

Table Distribution of chlorhexidine resistance genes in high and low-level mupirocin resistant and mupirocin sensitive isolates

In conclusion, our findings indicate that the routine use of chlorhexidine and mupirocin prophylaxis may increase the prevalence of chlorhexidine- and mupirocin-resistance genes in staphylococci in a hospital setting.