Determination of susceptibility of Staphylococcus aureus to methicillin by luciferin-luciferase assay of bacterial adenosine triphosphate.

Susceptibility of 50 strains of Staphylococcus aureus to methicillin was determined by disc diffusion, minimum inhibitory concentration (MIC) test and luciferin-luciferase assay of bacterial adenosine triphosphate (ATP). ATP concentration time curves calculated on eight strains (four sensitive, four resistant) incubated at 30 degrees C indicated 2.5 h as the optimum time for determination of methicillin susceptibility. The ATP concentration and relative light unit (RLU) value of each test broth (antibiotic-containing), expressed as a percentage of its own control broth (antibiotic-free) indicated values of less than 30% for ATP and less than 40% for RLU to be indicative of methicillin sensitivity. Single time point ATP assays carried out on the 50 strains after 2.5 h at 30 degrees C correlated exactly with disc diffusion and MIC. Luciferin-luciferase assay of bacterial ATP appeared to be a reliable, rapid technique for determining the susceptibility of Staph. aureus to methicillin.