Comparison of the susceptibilities of Burkholderia pseudomallei to meropenem and ceftazidime by conventional and intracellular methods.
The effect of the two antibiotics ceftazidime and meropenem on a collection of 46 Burkholderia pseudomallei isolates representing clinical and environmental sources across northern Australia was investigated by using a series of in vitro test methods. The susceptibility testing methods used included Kirby-Bauer disk diffusion, Etest MIC, broth microdilution MIC, and a modification of the microdilution method in which Acanthamoeba cells were added to simulate the effect of a professional phagocytic cell on test outcome. In a semiquantitative validation coculture series, the majority of bacteria were intracellular up to a multiplicity of infection of 10 bacteria to one ameba. The optical density and bacterial count (log10 CFU/ml) correlated across the range tested (r2 = 0.77; P < 0.0001). Susceptibility test results were compared against clinical outcomes. The MICs of ceftazidime were consistently higher than those of meropenem by all three methods. The MICs of both agents were significantly higher when Acanthamoeba trophozoites were added to the broth microdilution method. Conventional and intracellular MIC results were consistent for clinical isolates from the Western Australian outbreak cluster despite the wide variety of clinical outcomes. Further development of the intracellular MIC method is expected to help assess the efficacy of antimicrobial agents on this bacterial species in an intracellular setting.
The Division of Microbiology and Infectious Diseases, Western Australian Centre for Pathology and Medical Research, Nedlands, Western Australia, Australia. tim.inglis@health.wa.gov.au
T J J
TJ
Inglis
F
F
Rodrigues
P
P
Rigby
R
R
Norton
B J
BJ
Currie
D000048:Acanthamoeba / Q000382:microbiology; D000818:Animals; D016957:Burkholderia pseudomallei / Q000187:drug effects*; D002442:Ceftazidime / Q000494:pharmacology* / Q000627:therapeutic use; D002511:Cephalosporins / Q000494:pharmacology* / Q000627:therapeutic use; D004196:Disease Outbreaks; D006801:Humans; D007201:Indicator Dilution Techniques; D007202:Indicators and Reagents; D008554:Melioidosis / Q000188:drug therapy / Q000382:microbiology; D000077731:Meropenem; D008826:Microbial Sensitivity Tests; D015203:Reproducibility of Results; D013845:Thienamycins / Q000494:pharmacology* / Q000627:therapeutic use; D016896:Treatment Outcome; D014904:Western Australia
D002511:Cephalosporins; D007202:Indicators and Reagents; D013845:Thienamycins; D002442:Ceftazidime; D000077731:Meropenem