Effect of Growth Conditions on Chlorine Sensitivity of Escherichia coli

Escherichia coli grows luxuriantly on rich media like nutrient broth or nutrient agar; however, minimal media fully satisfy all its nutritional demands as the organism can use glucose and ammonium salts as sole sources of carbon and nitrogen.During studies on the disinfective power of chlorine, we found considerable differences in susceptibility to the disinfectant between cells growing in minimal media and cells from nutrient agar.In this report the nature of the differences is described and the mechanism involved is discussed.MATERIALS AND M'IETHODS AMedia.The following media were used:(1) Nutrient agar: Peptone (Difco),2 10 g; NaCl, 5 g; Na2HPO4-12H20, 2.5 g; Bovril,3 3 g; Bacto-agar (Difco), 20 g; distilled water, 1000 ml.(2) Minimal agar (according to Davis and Mingioli in 1950): K2HPO4, 7 g; KH2PO4, 3 g; Na3-citrate.3H20, 0.5 g; MgSO4*7H20, 0.1 g; (NH4)2SO4, 1 g; Bacto-agar (Difco), 20 g; distilled water, 1]000 ml.Glucose was sterilized separately and added aseptically to the medium to contain finally 0.2 per cent.(2) Minimal agar + 1 per cent peptone (Difco).(4) Minimal agar + 0.3 per cent Bovril (Bovril).(5) Minimal agar + 2 per cent yeast extract (Difco).Bacteria.Cultures of E. coli strain B/r4 grown for at least three consecutive passages on the above men- tioned media were taken.Bacteria grown on nutrient agar were designated N and those grown on minimal agar as MG.The inoculum consisted of cells obtained from 18-to 24-hr-old cultures.The cells were washed 3 times with bi-distilled water and diluted to give a barely visible suspension in a Coleman Junior spectro- photometer5 (optical density (OD) 0.02 at 500 m,u).This suspension was further diluted 5-fold in the disinfection mixture.For determination of SH groups, suspensions of an OD of 0.8 were prepared and these were diluted 2-fold in the disinfection mixture.