In vitro chloramphenicol susceptibility testing of Haemophilus influenzae: disk diffusion procedures and assays for chloramphenicol acetyltransferase.

The activity of chloramphenicol against 100 different strains of Haemophilus influenzae was assessed by a macrotube broth dilution technique and by a standardized disk diffusion method using both enriched chocolate agar (CHOC) and Mueller-Hinton agar containing 1.0% hemoglobin and 1.0% IsoVitaleX (BBL Microbiology Systems, Cockeysville, Md.) supplement (CHOC-MHA). Filter disks containing 30 micrograms of chloramphenicol were used with the disk diffusion procedure. The following zone diameter interpretive criteria were defined: CHOC-MHA, less than or equal to 25 mm = resistant [corrected] and greater than or equal to 26 mm = susceptible [corrected]; CHOC, less than or equal to 28 mm = resistant [corrected] and greater than or equal to 29 mm = susceptible [corrected]. all of the H. influenzae strains examined were also characterized by using two rapid assays for chloramphenicol acetyltransferase (CAT) activity: a 1-h tube method (t-CAT) and a 30-min procedure which used commercially available reagent-impregnated disks (d-CAT). The t-CAT procedure was found to be significantly more accurate than the d-CAT procedure as a means for demonstrating production of CAT.