Influence of adding Salts to the Medium on Counts of Phenol-treated Escherichia coli on Membrane Filters

Abstract

IT has been shown¹ that suspensions of Escherichia coli which had been treated with phenol gave lower counts on membrane filters than on the surface of nutrient agar. Many workers² have observed that bacteria which have been exposed to bactericides or heat exhibit a marked sensitivity to the nature of the subculture medium, and both Flett et al.² and Jacobs and Harris³ have shown that when ferric chloride was added to the medium it exerted a beneficial effect, demonstrated by longer extinction times than in control media, on the growth of phenol-treated organisms. The latter workers found that the rate and extent of the death of phenol-treated cells of E. coli and Staphylococcus aureus which occurred in nutrient broth⁴ was reduced by the inclusion of 5 × 10⁻³M of magnesium chloride, and that the inclusion of ferric chloride and other metal chlorides in nutrient agar gave surface-viable counts of such cells greater than were obtained on nutrient agar without metals⁵. In view of the present widespread use of membrane filters, and suggestions that these filters should be used to recover damaged cells from a variety of harmful environments, for example, in sterility tests, it was thought useful to examine the effect of adding salts to the nutrient broth on which the membranes are incubated.