Effect of inoculum size on antimicrobial susceptibility ofHelicobacter pylori

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An agar dilution assay was used to assess the effect of inoculum size and culture period on the susceptibility of 15 clinical isolates ofHelicobacter pylori to ampicillin, erythromycin, tetracycline, chloramphenicol, metronidazole and tinidazole. The mean MIC of the isolates increased 2.2- to 21.2-fold as the inoculum size progressed from 103 to 107 cfu/spot. Identical results were noted when isolates were maintained for two or four days prior to testing. Inoculum size should be carefully controlled when assessing the in vitro susceptibility ofHelicobacter pylori.