Discrepancies in the Enumeration of Escherichia coli

Stationary-phase cells ofEscherichia coli were enumerated by the pour plate method on Trypticase soy agar containing 0.3% yeast extract (TSYA), violet red-bile agar, and desoxycholate-lactose agar, and by the most-probable-number method in Brilliant Green-bile broth and lauryl sulfate broth. Maximum counts were assumed to be those on TSYA. In general, numbers detected were lower with the selective solid media and higher with the selective liquid media. Inhibitory effects, especially on selective solid media varied with the strains ofE. coli . The lower detection on selective solid media was partly due to the stress induced in some cells by the temperature of the melted media used in the pour plate method. These cells apparently failed to repair and form colonies in the selective media. Improved detection on the selective solid media was achieved by using 1% nonfat milk solids, 1% peptone, or 1% MgSO4 .7H2 O in the dilution blanks. Higher detection on selective agar media was effected by surface plating or by surface-overlay plating of the cells. The surface-overlay method appeared to be superior for the direct enumeration ofE. coli in foods.