An Improved Agar Medium for the Detection of Proteolytic Organisms in Total Bacterial Counts

Summary. An improved medium for the detection of proteolytic organisms has been developed by adding caseinate (1% w/v), citrate (0·015 m) and Ca 2+ (0·02 m) to Standard Methods Agar. Its greater sensitivity compared with existing milk agar media is related to its ability to detect the first stage of casein breakdown as shown by the formation of a white zone of precipitation. This results from the deposition of insoluble para‐caseins, mainly para‐ k ‐casein, which is readily detected in the transparent medium. The extent of proteolysis exhibited by an organism is reflected both by the size and type of precipitation zone forming round the colony. No significant difference was found between total counts of raw milk samples measured on the new medium and standard methods agar. The new medium can thus be used for the simultaneous determination of proteolytic and total bacterial counts.