This prospective study was conducted in the Department of Microbiology over a period of 9 months from April 2021 to December 2021 after obtaining clearance from the Ethical Committee. Fifty non-duplicate isolates of Methicillin Resistant S. aureus strains isolated from various clinical samples were included in the study. Isolates which showed gram positive cocci in clusters on grams stain and gave positive results on catalase, slide and tube coagulase where identified as S. aureus.
Screening for methicillin resistance was done by modified Kirby Bauer disc diffusion method using cefoxitin (30 µg) discs. A zone size of ≥22 mm was interpreted as methicillin sensitive and ≤21 mm was interpreted as methicillin resistant as per Clinical and Laboratory Standards Institute (CLSI) guidelines (23 ). S. aureus American Type Culture Collection (ATCC) strain 25923 and S. aureus ATCC strain 43300 were used as susceptibility and resistance controls respectively. Isolates which were Methicillin sensitive were excluded from the study. Antimicrobial susceptibility of the isolates was done by Kirby Bauers disc diffusion
Testing for ceftaroline susceptibility was done by E- test strip method. The ceftaroline E-test strips (0.002–32 µg/mL) was obtained from Biomerieux, France. The E- test strips were placed on the lawn culture of the organism and the plates were incubated at 37°C for 18–24 hours. MIC's were read where the ellipse intersects the MIC (Minimum Inhibitory Concentration) scale. Since E-test strip has continuous gradient, MIC values “in-between” two-fold dilutions can be obtained. These values were rounded up to next two-fold dilution before categorisation. MICs were interpreted according to EUCAST version 11.0 and CLSI 2021.
EUCAST version 11.0 for Ceftaroline (In pneumonia) - ≤1- Susceptible, > 1- Resistant
EUCAST version 11.0 for Ceftaroline (For conditions other than pneumonia)-≤1- Susceptible, >2- Resistant
CLSI 2021 - ≤1- Susceptible, 2–4(Susceptible Dose Dependent), ≥8- Resistant.
Screening for methicillin resistance was done by modified Kirby Bauer disc diffusion method using cefoxitin (30 µg) discs. A zone size of ≥22 mm was interpreted as methicillin sensitive and ≤21 mm was interpreted as methicillin resistant as per Clinical and Laboratory Standards Institute (CLSI) guidelines (23 ). S. aureus American Type Culture Collection (ATCC) strain 25923 and S. aureus ATCC strain 43300 were used as susceptibility and resistance controls respectively. Isolates which were Methicillin sensitive were excluded from the study. Antimicrobial susceptibility of the isolates was done by Kirby Bauers disc diffusion
Testing for ceftaroline susceptibility was done by E- test strip method. The ceftaroline E-test strips (0.002–32 µg/mL) was obtained from Biomerieux, France. The E- test strips were placed on the lawn culture of the organism and the plates were incubated at 37°C for 18–24 hours. MIC's were read where the ellipse intersects the MIC (Minimum Inhibitory Concentration) scale. Since E-test strip has continuous gradient, MIC values “in-between” two-fold dilutions can be obtained. These values were rounded up to next two-fold dilution before categorisation. MICs were interpreted according to EUCAST version 11.0 and CLSI 2021.
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