The effects of temperature on the minimum inhibitory concentrations (MICs) of ciprofloxacin, ofloxacin or DR-3355 and on the ability of Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and Staphylococcus epidermidis to mutate to resist 5 times the MIC of these 4-quinolones were studied. Reducing the temperature of incubation from 37 to 30°C and then to 25°C only slightly affected MIC values. With Staph, epidermidis temperature reduction always reduced its mutation frequency to resist 4-quinolones. The effect of temperature on mutation frequency with the other three species was more variable and in some instances temperature reduction even elevated their mutation rates. The effect of temperature on the bactericidal activity of ciprofloxacin or DR-3355 against the four bacterial species was also investigated. In contrast to the MIC results it was found that bacterial kill was greatly lessened by temperature reduction. When the values of the apparent activation energies for bacterial death were calculated they were found to be consistent with inhibition of enzymes being the rate-limiting step in the killing of bacteria by these 4-quinolones. The effect of temperature on the multiplication of the four bacterial species in drug-free media was also studied. It was found that Staph, epidermidis multiplied more rapidly at 30°C than at 25 or at 37°C, and it did not divide at all at 20°C. However, the other three species divided more rapidly as the temperature of incubation was increased. In agreement with clinical findings it seems that 4-quinolone therapy of infectious sites which are at temperatures less than 37°C may be more prone to failure due to increased mutational resistance or to reduced bactericidal activity or to a combination of both these factors. In addition fomites which are at room temperature and contaminated with bacteria and 4-quinolone residues could act as sources of resistant mutants, particularly in hospitals
