Effect of temperature on inoculum as a potential source of error in agar dilution plate count bactericidal measurements.

The effect of increased temperature on Staphylococcus aureus during the inoculation step of the agar dilution plate count method was investigated as a possible cause of artificially high persister counts. For some isolates, exposure of the inoculum to increased temperature resulted in higher persister counts and diminution or loss of the paradoxical effect. The persister patterns for three representative S. aureus isolates are presented to illustrate the strain- and temperature-dependent nature of the phenomenon. For any isolate, the net effect appears to be caused by an interplay of temperature-induced inoculum loss and temperature-induced cell division cycle blockage. A modification of the agar dilution plate count inoculation step to circumvent such problems is described.