Salt tolerance of EMRSA-16 and its effect on the sensitivity of screening cultures

The salt (NaCl) tolerance of methicillin-resistant Staphylococcus aureus (EMRSA)-16 was compared with 18 other MRSA isolates by an agar incorporation technique. The NaCl minimum inhibitory concentration (MIC) of EMRSA-16 was 7% which compared with an MIC(50) of 7%, MIC(90) of 10%, range (55-105%) for the other isolates. Study of the growth kinetics in broth containing NaCl at concentrations up to 10% indicated complete inhibition of growth by 7 and 10% NaCl and partial inhibition by 5%. Addition of EMRSA-16 at inocula of less than or equal to 1 cfu/mL into salt broths revealed lower than expected EMRSA recovery from broths containing 5, 7.5 and 10% NaCl. Two and a half per cent NaCl broths were not inhibitory. Selective broth containing 2.5% NaCl should be considered for use when screening for EMRSA-16.