Helicobacter pylori susceptibility testing by disc diffusion

The bacterium Helicobacter pylori is found in c. 40% of the population and is responsible for the development of duodenal disease. Triple treatment with a proton-pump inhibitor or bismuth salt plus two antibiotics is now commonplace in all patients diagnosed. As antibiotic resistance reduces treatment efficacy, it is time to consider routine susceptibility testing to guide individual patient treatment and surveillance of antibiotic resistance. There are no published nationally agreed standards for disc diffusion testing of H. pylori. After reviewing the literature, we recommend the following method for disc diffusion tests. A suspension of cultures < or = 4 days old equivalent to McFarland Standard no. 4 (10(8) cfu/mL) should be used on Mueller-Hinton or Columbia agar base with 5-10% blood, using a metronidazole disc strength of 5 Ig and a clarithromycin disc strength of 2 microg. Anaerobic pre-incubation of plates is unnecessary. A H. pylori control susceptible to metronidazole (e.g. NCTC 12822) should be used. Zone sizes with the Mueller-Hinton agar base for metronidazole testing are <16 mm resistant, 16-21 mm intermediate and >21 mm susceptible. We suggest that isolates in the intermediate zone should be re-tested by Etest. Zone sizes with the Columbia agar base for metronidazole testing are <10 mm resistant and > or = 10 mm susceptible. Co-infection with two strains, which may be a mixture of isolates susceptible and resistant to metronidazole leading to conflicting susceptibility results, occurs in 5-10% of patients. Zone sizes with Mueller-Hinton agar and Columbia blood agar for clarithromycin testing are resistant no zone and susceptible any zone.