Performance of Haemophilus Test Media prepared with 12 different lots of Mueller-Hinton agar from four manufacturers.

Haemophilus Test Media (HTM) were prepared from 12 different lots of Mueller-Hinton agar. When tested with Haemophilus influenzae ATCC 49247, most lots were initially rejected because of small zones of inhibition for cefaclor, cefuroxime, and cefamandole disks, whereas five other drugs performed satisfactorily on the 11 lots that supported growth of the control strain. At the same time, tests of Escherichia coli ATCC 25922 and Staphylococcus aureus ATCC 25923 documented the acceptability of these agar media, with or without HTM supplements. The current control limits for cefaclor, cefuroxime, and cefamandole appear to be unrealistic. Because the beta-lactamase-negative, ampicillin-resistant control strain of H. influenzae (ATCC 49247) has altered penicillin-binding proteins, it is resistant to ampicillin and is probably also resistant to cefaclor, cefuroxime, and cefamandole. Consequently, media that produce no or very small zones of inhibition with those three drugs might be clinically correct and should not be rejected. One manufacturer provided three lots of Mueller-Hinton agar that gave unusually large zones of inhibition with all beta-lactams. The three other manufacturers provided eight Mueller-Hinton agars that were satisfactory for the preparation of HTM agar, provided that small zones of inhibition with cefaclor, cefuroxime, and cefamandole disks are accepted as the preferred result.