BACTERICIDAL ACTION OF CHLORAMPHENICOL ON ESCHERICHIA COLI

Abstract

The antibiotics in common use have been described as being bacteriostatic in low concentrations and bactericidal in high concentrations (Waksman and Reilly, 1944; Hamre et al., 1946). However, measuring bactericidal action of antibiotics is not always simple, because it may be difficult with conventional methods to discriminate between static and cidal activity. It would be desirable at times to be able to ascertain Whether a particular antibiotic, at a given concentration, exerts a static or a cidal action on a particular pathogen. Such information might be useful not only in fundmental studies of mode of action but also in clinical investigations. The present study describes a paper electrophoresis procedure with which the bactericidal activity of antibiotics can be determined. Applying this procedure, the action of chloramphenicol, which usually has been considered only weakly bactericidal, was investigated using Escherichia coli as the test organism. MATERIALS AND METHODS Test organism. E. coli strain no. 60, the growth of which is inhibited by a chloramphenicol concentration of 1 jig per ml, was used. The organisms were grown on nutrient agar slants for 24 hr at 37 C, washed twice by resuspending and centrifuging in distilled water, and then were diluted to give a suspension containing approximately 200 million viable cells per ml. Agar disc. The agar medium contained NaCl, 0.8 per cent; Na2HPO4. 12H20, 0.8 per cent; KH2PO4, 0.25 per cent; and agar, 1.5 per cent. It was melted and then cooled to 48 C. Ten ml