Selective Preliminary Incubation for Gram-Negative Psychrotrophic Bacteria in Milk

Milk samples containing crystal violet, benzalkonium chloride, crystal violet: benzalkonium chloride, or alkylaryl sulfonate were inoculated with each of Pseudomonas fluorescens , Pseudomonas fragi , Pseudomonas putida , Pseudomonas aeruginosa , Escherichia coli , Bacillus cereus , Staphylococcus aureus , and Streptococcus sp. Samples were incubated at 21°C for 18 h, and growth of the bacteria monitored to evaluate the inhibitory effects of the added reagents. Benzalkonium chloride proved to be the preferred inhibitor for selective incubation. It inhibited growth of Bacillus cereus , Staphylococcus aureus , and Streptococcus sp. at initial cell concentrations of 10¹, 10², and 10⁴ per ml, while allowing uninhibited growth of all Pseudomonas species tested with an average increase of 3 logs. This finding is most important for the selective preliminary incubation necessary prior to milk quality assessment by plating and rapid instrumentation methods.