PROBLEMS WITH CURRENT RECOMMENDATIONS FOR SUSCEPTIBILITY TESTING OF HAEMOPHILUS-INFLUENZAE

We compared results of MIC and disk susceptibility tests on Haemophilus test medium (HTM) and those on comparative media. Ampicillin MICs were determined with seven ampicillin-resistant, non-β-lactamase-producing (Amp(r)NBLP) isolates by using HTM and supplemented brain heart infusion (sBHI) agar. Ampicillin and amoxicillin-clavulanate disk tests with 16 Amp(r)NBLP strains, 18 ampicillin-susceptible (Amp(s)) isolates, and 17 ampicillin-resistant, β-lactamase-producing (Amp(r)BLP) strains were performed by using five media: laboratory-prepared HTM (PHTM), commercial HTM (CHTM), sBHI, enriched chocolate agar, and Mueller-Hinton chocolate agar. We observed that five of seven and three of seven Amp(r)NBLP strains were misclassified as susceptible with PHTM (MIC, < 2 μg/ml) with inocula of 103 and 105 CFU, respectively, but were resistant with sBHI (MIC, ≥ 2 μg/ml). Whereas Mueller-Hinton chocolate agar and enriched chocolate agar plates supported the growth of all 51 strains by the disk tests, 37% (19 of 51) and 8% (4 of 51) of strains did not grow on PHTM and CHTM, respectively. Lack of growth on PHTM was observed for all three phenotypes; 7 or 18 Amp(s), 4 of 17 Amp(r)BLP, and 8 of 16 Amp(r)NBLP strains did not grow. The four strains that did not grow on CHTM were all Amp(r)NBLP isolates. Zone sizes were significantly larger on PHTM than on the other media. Of the strains that were evaluable by the new National Committee for Clinical Laboratory Standards guidelines with either PHTM or CHTM, all Amp(s) strains were classified as susceptible. Among the Amp(r)BLP strains, CHTM correctly identified all as resistant, whereas PHTM detected two isolates to beintermediate. Among the Amp(r)NBLP strains, CHTM and PHTM misclassified four (33%) and five (62%) isolates, respectively, as susceptible; an additional isolate was identified as intermediate on both media. We conclude that there is strain-dependent growth on HTM, that adoption of this medium for routine Haemophilus susceptibility testing is problematic due to this growth variability, and that detection of Amp(r)NBLP isolates would be unreliable.