Estimation of Gram‐negative bacteria in milk: A comparison of inhibitor systems for preventing Gram‐positive bacterial growth

A number of inhibitor systems which have been reported to allow selection of Gram-negative bacteria were tested against Gram-positive and Gram-negative isolates of dairy origin. No one system worked perfectly. A mixture of crystal violet-penicillin-nisin or monensin had least inhibitory effect on Gram-negative isolates whereas Selectocult (a commercially available mixture of Benzalkon A 50% and crystal violet) and sodium deoxycholate were the most effective inhibitors of Gram-positive bacteria. Cetrimide-fucidin-cephaloridine solutions, which have been reported as allowing selective growth of pseudomonads, were not so specific when applied to milk systems.