Comparison of agar versus broth dilution techniques for determining antibiotic susceptibilities of Ureaplasma urealyticum

Abstract

We determined minimal inhibitory concentrations (MIC) for tetracycline and erythromycin for 72 clinical isolates using broth and agar dilution methods. Erythromycin MIC ranges were <0.125–8 μg/ml and 1–64 μg/ml in broth and agar, respectively. The erythromycin MIC 50 and MIC 90 as determined by broth were two dilutions (fourfold) lower than those for agar. Tetracycline MIC ranges in broth and agar were <0.125– >64 and 0.25–>64 μg/ml, respectively. The tetracycline broth MIC 50 was one dilution lower than that for agar. The tetracycline broth MIC 90 was 64 μg/ml and that for agar was >64 μg/ml. Of the strains tested, 98.6% using broth were susceptible or moderately susceptible to erythromycin as compared with 75% using agar, representing a significant difference ( p < 0.001). For tetracycline, 80.6% of strains were susceptible or moderately susceptible using broth and 73.6% using agar. MICs were determined by agar dilution after 72 and 96 hr of incubation in 32 strains. There was an increase in the erythromycin MIC by one dilution in 16 strains and two dilutions in one strain with the longer incubation. The tetracycline MIC increased by one dilution in nine strains between readings. Broth MICs were reproducible with one dilution for both drugs in 10 of 12 strains tested twice. Agar MICs were reproducible within a maximum of two dilutions (fourfold). Different interpretations of susceptibility versus resistance may be made depending on which assay is utilized, thus influencing conclusions regarding spectrum of activity of investigational drugs as well as treatment options. The technique employed should always be considered whenever apparently differing drug susceptibility patterns are reported.