Temperature effect on cephalothin sensitivity of methicillin-resistant Staphylococcus aureus.

Nineteen methicillin-resistant Staphylococcus aureus isolates from clinical specimens were tested for cephalothin sensitivity by disk diffusion and tube-dilution methods using incubation temperatures of 35 C and 30 C. Resistant colonies were observed to grow within the zones of antibiotic inhibition for 17 of the 19 isolates at 30 C but not at 35 C incubation. Twenty control strains known to be methicillin-sensitive did not show these resistant colonies at 30 C. Five of the methicillin-resistant isolates were sensitive, one of intermediate sensitivity to cephalothin, and the rest were resistent by tube-dilution testing at 35 C. At 30 C, however, two isolates were of intermediate sensitivity, and the rest were resistant to cephalothin by tube-dilution testing. These observations strongly suggest that the 30 C incubation temperature is more reliable in detecting in-vitro resistance of methicillin-resistant strains of Staphylococcus aureus to cephalothin in agar as well as in liquid media.