The Influence of the Nature of the Recovery Medium on the Apparent Viability of Phenol-Treated Bacteria

Abstract

Abstract Suspensions of Escherichia coli and Staphylococcus aureus, untreated and treated by exposure to phenol, o-cresol or p-chloro-m-cresol, were counted on various types of media. While the counts from untreated suspensions were not affected by the nature of the medium, those from treated suspensions were influenced markedly. There was no correlation between the nutritional status of a medium and its suitability for the cultivation of the treated organisms. Although both bacterial species gave similar patterns of response and ranked the media in roughly the same order of suitability, E. coli was the more responsive species. Generally, the order of suitability was: meat extracts, yeast extracts, meat extract-peptone broth, peptones (all the single ingredients 1 per cent, pH 7; media solidified with New Zealand agar). Although supplementation of the meat extract-peptone control medium, especially with yeast extracts, led to higher counts from treated suspensions, dilution of the extract broth also gave similar results. Further, a fresh infusion broth was superior both to the extract broth and to a digest medium, for counting treated bacteria. The deficiencies of the extract broth were due partly to the phosphate precipitation procedure used in its preparation. The results indicate that when media suitable for the growth of damaged bacteria are required, they should be selected without recourse to preconceived ideas about composition, concentration or method of preparation.