Prevalence of and detection of resistance to ampicillin and other beta-lactam antibiotics in Haemophilus influenzae in Denmark.
The susceptibility of Haemophilus influenzae to penicillin V and G, ampicillin and cefuroxime was investigated by MIC, disc and tablet diffusion methods, using chocolate agar as test medium, to determine the prevalence of ampicillin-resistant isolates and the optimal method for their detection. Eighty-six isolates were clinical isolates collected prospectively from July to September 1998 and 22 isolates were clinical isolates with decreased susceptibility to ampicillin previously referred to the reference laboratory. Eighty-seven isolates were ampicillin-susceptible and 16 were ampicillin-resistant. Thirteen produced beta-lactamases. Among the consecutive isolates 12.8% were resistant. With each of the Rosco Neo-sensitabs containing penicillin G, 2.5 microg ampicillin and 33 microg ampicillin, 3 very major errors occurred (resistant isolates misinterpreted as susceptible) and 5-13 major errors (susceptible isolates misinterpreted as resistant). The AB biodisk containing ampicillin (10 microg) was superior to the penicillin V and G discs, i.e. only 1 very major error occurred and major and minor errors were infrequent. The cefuroxime disc identified 4/8 beta-lactamase-negative ampicillin-resistant isolates. Thus, for susceptibility testing with chocolate agar as test medium, the use of an inoculum of 10(5) colony-forming units, 10 microg ampicillin discs and interpretative zone diameters of > or = 28 mm indicating susceptibility and < or = 25 mm indicating resistance was found to produce reliable identification of ampicillin-resistant isolates of H. influenzae.
Arendrup
M
M
Department of Clinical Microbiology, Statens Serum Institut, Copenhagen, Denmark.
Knudsen
J D
JD
Jensen
E T
ET
Jensen
I P
IP
Frimodt-Møller
N
N
D000667:Ampicillin / Q000494:pharmacology*; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D015169:Colony Count, Microbial; D003470:Culture Media; D003718:Denmark / Q000453:epidemiology; D004352:Drug Resistance, Microbial; D006192:Haemophilus Infections / Q000453:epidemiology / Q000382:microbiology*; D006193:Haemophilus influenzae / Q000145:classification / Q000187:drug effects*; D006801:Humans; D008826:Microbial Sensitivity Tests / Q000592:standards; D010406:Penicillins / Q000494:pharmacology*; D011446:Prospective Studies; D012703:Serotyping; D018440:beta-Lactam Resistance; D001618:beta-Lactamases / Q000378:metabolism
D000900:Anti-Bacterial Agents; D003470:Culture Media; D010406:Penicillins; D000667:Ampicillin; D001618:beta-Lactamases