ANTIMICROBIAL SUSCEPTIBILITY TESTING OF 230 HELICOBACTER-PYLORI STRAINS - IMPORTANCE OF MEDIUM, INOCULUM, AND INCUBATION-TIME

No standardized method of susceptibility testing for Helicobacter pylo ri is currently available, so before a large agar dilution study compr ising 230 H. pylori strains belonging to more than 80 genetically diff erent groups was initiated, we performed a relatively small preliminar y study to determine the influences of medium, inoculum density, and i ncubation time. Seven media were investigated and were primarily evalu ated on the basis of their abilities to support growth both semiquanti tatively and qualitatively; Iso-Sensitest agar supplemented with 10% h orse blood was found to be well suited for the purpose; this was close ly followed by Mueller-Hinton agar with 10% horse blood, Mueller-Hinto n with 10% sheep blood, and finally, 7% lysed horse blood agar. invest igations of two inoculum densities and two incubation times resulted i n recommendations for the use of 10(9) CFU/ml (10(6) CFU/spot) as the inoculum and 72 h as the incubation time. A modest inoculum effect was noted for amoxicillin and metronidazole. By the methodology derived f rom our preliminary study, the susceptibilities of 230 H. pylori strai ns to six antibiotics were subsequently determined. The results were g enerally in accord with those of others, and apart from metronidazole, the MIC of which for approximately 25% of the strains tested was >8 m u g/ml, resistance was low in Denmark The situation might, however, qu ickly change when and if the number of indications for antibiotic ther apy for H. pylori infections increase. Consequently, susceptibility te sting of all H. pylori strains is recommended in order to survey the d evelopment of resistance, and in our hands the described methodology w as relatively easy to perform and the results were easy to read.