Haemophilus influenzae with Non-Beta-Lactamase-Mediated Beta-Lactam Resistance: Easy To Find but Hard To Categorize.
Haemophilus influenzae is a major pathogen, and beta-lactams are first-line drugs. Resistance due to altered penicillin-binding protein 3 (rPBP3) is frequent, and susceptibility testing of such strains is challenging. A collection of 154 beta-lactamase-negative isolates with a large proportion of rPBP3 (67.5%) was used to evaluate and compare Etest (Haemophilus test medium [HTM]) and disk diffusion (EUCAST method) for categorization of susceptibility to aminopenicillins and cefuroxime, using MICs generated with broth (HTM) microdilution and clinical breakpoints from CLSI and EUCAST as the gold standards. In addition, the proficiency of nine disks in screening for the rPBP3 genotype (N526K positive) was evaluated. By Etest, both essential and categorical agreement were generally poor (<70%), with high very major errors (VME) (CLSI, 13.0%; EUCAST, 34.3%) and falsely susceptible rates (FSR) (CLSI, 87.0%; EUCAST, 88.3%) for ampicillin. Ampicillin (2 μg) with adjusted (+2 mm) zone breakpoints was superior to Etest for categorization of susceptibility to ampicillin (agreement, 74.0%; VME, 11.0%; FSR, 28.3%). Conversely, Etest was superior to 30 μg cefuroxime for categorization of susceptibility to cefuroxime (agreement, 57.1% versus 60.4%; VME, 2.6% versus 9.7%; FSR, 7.1% versus 26.8%). Benzylpenicillin (1 unit) (EUCAST screening disk) and cefuroxime (5 μg) identified rPBP3 isolates with highest accuracies (95.5% and 92.2%, respectively). In conclusion, disk screening reliably detects rPBP3 H. influenzae, but false ampicillin susceptibility is frequent with routine methods. We suggest adding a comment recommending high-dose aminopenicillin therapy or the use of other agents for severe infections with screening-positive isolates that are susceptible to aminopenicillins by gradient or disk diffusion.
Department of Microbiology, Vestfold Hospital Trust, Tønsberg, Norway Department of Medical Biology, Faculty of Health Sciences, University of Tromsø-The Arctic University of Norway, Tromsø, Norway dagfinn.skaare@siv.no.
Dagfinn
D
Skaare
Department of Microbiology, Vestfold Hospital Trust, Tønsberg, Norway.
Astrid
A
Lia
Department of Microbiology, Vestfold Hospital Trust, Tønsberg, Norway.
Anja
A
Hannisdal
Department of Medical Biochemistry, Telemark Hospital Trust, Skien, Norway.
Yngvar
Y
Tveten
Clinical Microbiology, Central Hospital, Växjö, Sweden.
Erika
E
Matuschek
Clinical Microbiology, Central Hospital, Växjö, Sweden.
Gunnar
G
Kahlmeter
Department of Medical Biology, Faculty of Health Sciences, University of Tromsø-The Arctic University of Norway, Tromsø, Norway.
Bjørn-Erik
BE
Kristiansen
D000667:Ampicillin / Q000494:pharmacology; D000668:Ampicillin Resistance / Q000235:genetics*; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D002444:Cefuroxime / Q000494:pharmacology; D052978:Disk Diffusion Antimicrobial Tests / Q000379:methods*; D006192:Haemophilus Infections / Q000382:microbiology; D006193:Haemophilus influenzae / Q000187:drug effects* / Q000201:enzymology / Q000235:genetics* / Q000302:isolation & purification; D006801:Humans; D010397:Penicillanic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D046915:Penicillin-Binding Proteins / Q000235:genetics*; D001618:beta-Lactamases / Q000235:genetics
D000900:Anti-Bacterial Agents; D046915:Penicillin-Binding Proteins; D000667:Ampicillin; D010397:Penicillanic Acid; D001618:beta-Lactamases; D002444:Cefuroxime; C004408:aminopenicillanic acid