Effect of pH, inoculum size, and incubation time on the susceptibility of Ureaplasma urealyticum to erythromycin in vitro.

Determinations of the susceptibility of Ureaplasma urealyticum to erythromycin in vitro as measured by the broth dilution method have shown wide variations with minimal inhibitory concentrations (MICs) from 0.04 to > or = 8 micrograms/mL, indicating a need for standardization. The effects of pH, inoculum size, and incubation were studied. In the broth dilution test, pH had an important effect. The apparent MIC was 4- to 16-fold higher at pH 6.0 than at pH 7.0, with the MIC falling progressively from values of 8 to > or = 8 micrograms/mL to values of 0.25 to 1 microgram/mL as the pH of the medium was increased in steps to pH 7.0. Large inocula also inflated the MICs 2- to 4-fold. In addition, the time of incubation influenced the apparent MIC, with increases of 4- to 16-fold between days 1 and 5. In agar dilution assays, MICs decreased from 2 micrograms/mL at pH 6.2 to 0.5 microgram/mL at pH 6.6. Since pH, inoculum level, and incubation time appear to be responsible for most of the variation in results, we propose that susceptibility testing for ureaplasmas can be improved by using medium with a more neutral pH than that usually used (pH 6) and by standardizing the inoculum size and incubation period.