Title : Comparison of susceptibility testing methods with “MECA” and “CFR” gene prevalence among Coagulase-negative Staphylococcus for determining methicillin and linezolid resistance- a prospective study from Delhi.
CoNS, a major commensal of skin, now a notable cause of nosocomial infections, especially in patients undergoing surgical procedures. In previous decades, the β-lactams, most efficient weapons to control CoNS infection, have now blunted progressively. CoNS have been reported to show a continuous loss of susceptibility toward most of the available antibiotics. Linezolid serves as an alternate for methicillin resistant CoNS, Although, linezolid resistance is still a rare phenomenon, but concerns are growing.
Prospective study. All clinical samples were tested for CoNS and antibiotic susceptibility testing was then performed as per CLSI guidelines. This study highlighting the prevalence of methicillin and linezolid resistance in CoNS, focussing on a comparative analysis of various phenotypic methods (Oxacillin : Microbroth Dilution – MBD, Disc Diffusion – DD, Automated- VITEK 2) (Cefoxitin- DD, VITEK2) (Linezolid – DD, VITEK2) correlating them with mec A and cfr genes (by PCR) respectively.
Among 380 Staphylococcus isolates, prevalence of CoNS was 36.84 % (42.14% -males and 57.85% - females). Sample type : Blood was the most common sample (49%), followed by urine (28.5%) and pus (22.1%).
Nearly 72.3% of isolates were Novobiocin sensitive (S.epidermidis group).
Cefoxitin resistance was reported to be 87.14 % and 72.14% respectively by DD and VITEK 2. The number of isolates resistant to oxacillin were in synchronization by all the methods- 80.71% by DD and MBD and 80% by VITEK 2.
However, mecA was only present in 62.85% of all the total isolates. Of these, 53 isolates had MICs value of 1 μg/mL. 12 isolates had 0.5 μg/mL. 7 isolates had 2 μg/mL, 10 isolates had 4 μg/mL and 4 had 16 μg/mL and above. Further, 5 of these 88 were misreported as cefoxitin sensitive and 8 were misreported as oxacillin sensitive phenotypic methods. Linezolid resistance was low- 2.85 % and 2.14 % respectively by DD and VITEK 2 methods. None of the isolates harbored cfr gene.
Cefoxitin is a better marker of mec A gene than oxacillin. The cefoxitin DD method is considered a better predictor than oxacillin for the detection of heterogeneous methicillin resistance. There is a need to modify MIC breakpoints of oxacillin. Considering the fact that methicillin resistance has gravely escalated in CoNS, the need for new treatment options is now a necessity.
Audience Take Away:
- This will help them to understand which phenotypic method is more sensitive while testing for oxacillin, cefoxitin and linezolid.
- Will aid in understanding the distribution of CoNS among clinical samples.
- The association of mecA with cefoxitin and oxacillin – to decide which better predicts the mecA gene.
- Prevalence of linezolid resistance among CoNS.