Evaluation of the Etest for detection of tetracycline resistance in Mycoplasma hominis

The Etest was compared with microbroth dilution for performing in vitro susceptibility tests in 38 isolates of Mycoplasma hominis chosen to represent a wide range of MIC values. MIC50s were 4 micrograms/ml for both methods; whereas, MIC90s were 64 and > 256 micrograms/ml for broth and Etest, respectively. Etest MICs determined on SP 4 agar were usually two or more dilutions greater than microbroth MICs in SP 4 broth, and values were prone to change by one to two dilutions when different inoculum densities were used. Inocula of 10(5) to 10(6) color-changing units/ml gave the most consistently readable MICs, with discrete colony formation surrounding the ellipse. Etest MICs for 5 isolates tested a second time at the same inoculum on SP 4 agar agreed with the original value within 1 dilution. For 12 isolates tested on A 8 agar simultaneously with SP 4 agar, MICs for 10/12 agreed within one dilution; whereas, in the other 2 isolates, MICs varied by two dilutions. These findings suggest that the Etest, when properly controlled, can be used to determine tetracycline MICs for M. hominis.