Effect of Variable Incubation Temperatures and Salt Concentration Susceptibility Testing of Methicillin Resistant Staphylococcus Aureus (MRSA)

Abstract

The effect of variable incubation temperatures and variable salt concentration in the sensitivity agar on susceptibility testing of MRSA was evaluated. For this purpose 58 MRSA strains were tested. For each MRSA strain nine Mueller-Hinton agar plates, three plates each with 2%, 5% and 7.5% sodium chloride (NaCI) supplement were inoculated. One plate from each of the salt concentration was incubated at 30°C, 37°C and 40°C. It was found that with increase in temperature of incubation the size of inhibition zones increased. In the presence of 2% NaCI supplement in the sensitivity agar, 5 MRSA appeared falsely intermediate sensitive on incubation at 40°C. While on incubation at 37°C although all MRSA remained methicillin resistant, statistical analysis revealed the effect on inhibition zone size to be approaching significance level (0.1>P>0.05). The effect of incubation temperature on inhibition zone size was statistically insignificant (P>O.05) if 5% or 7.5% NaCI was incorporated ¡n the sensitivity agar. Therefore we recommend the use of 5% NaCI supplemented sensitivity agar and incubation temperature of 37°C for the accurate detection of methicillin resistance in Staphylococcus aureus.