Selection of a reference lot of Mueller-Hinton agar

A collaborative study was undertaken to evaluate the performance of currently marketed Mueller-Hinton agars from seven manufacturers by replicate disk diffusion tests with standard quality control strains. Identification of the manufacturers was concealed, and the resulting data were evaluated for the selection of a physical reagent standard against which the performance of future production lots would be tested and made to conform. A medium was selected which was sufficiently close to existing National Committee for Clinical Laboratory Standards quality control limits that current interpretive criteria would require minimum modification. Two of the seven lots were eliminated from further consideration because the final pHs were outside acceptable limits. The remaining four lots had 96% of mean zone diameters less than or equal to 2 mm from those of the chosen lot and 65% of the means were less than or equal to 1 mm from those of the chosen lot for all 28 antimicrobial agent-organism combinations. Manufacturers then attempted to produce new lots of Mueller-Hinton agar which performed within the prescribed limits of the chosen lot. One lot performed in close conformity with the selected standard, but the overall performance of the media was essentially the same as that of the randomly chosen lots in the initial study. It was concluded that one of the original seven lots demonstrated properties which made it a tentative candidate for a physical reagent standard and that the use of a physical reagent standard in evaluating production lots might aid in stabilizing the performance of Mueller-Hinton agar.