Susceptibility of Catalase-negative Bacteria to Ultra-violet Irradiation

Abstract

IN the tribe Lactobacteriacea there are two closely related genera—Lactobacillus, which are catalase-negative, and Microbacterium, which are catalase-positive. Representative members of these genera were selected, namely, Lactobacillus pastorianus (ATC 8291) as the catalase-negative bacteria and Microbacterium flavum (ATC 10340) as the catalase-positive bacteria. Both species form distinct colonies and have an optimum temperature near 30° C. The bacteria were reared on Difco's stock culture broth media and buffered with calcium carbonate, and incubated for seven to nine days at 30° C. L. pastorianus was diluted 1–10,000 and M. flavum 1–50,000 with physiological salt solution, and 1 ml. of the solution placed in the centre of a sterile Petri plate. The aliquot was exposed to ultra-violet radiation from a 40-watt tube, model HUVM-30 (American Sterilizer Co., Erie, Pa.), with about 90 per cent of the radiation at 2537 A. The distance from the tube to the bacteria was 43 cm. and time of exposure was at various intervals from 1 to 14 sec. Thirty plates were run for each time of irradiation, and average results are presented in the accompanying graph. Immediately after irradiation, 15–20 ml. of tomato agar was added to the plate and agitated to distribute the bacteria uniformly around the plate. The plates are incubated for seven days at 30° C. and the colonies counted. Controls were those bacteria which were not exposed to ultra-violet radiation.