Growth of Bacillus subtilis and of Escherichia coli following treatment with oxygen at 10 atmospheres for 18 hr was synchronous. Treatment at lower pressures resulted in a much lower degree of synchrony. After exposure to high-pressure oxygen to induce synchronous growth of B. subtilis the resistance of various stages of the cell cycle to further trea.tment with oxygen at I o atm. was studied. Maximum resistance occurred immediately following division of the bacilli and then fell to a low value. The region of maximum resistance in B. subtilis compared fairly closely with the time of maximum catalase activity. I N T R O D U C T I O NPritchard & Hudson (1967) showed that induction of catalase occurred in cultures of Mucor racernosus and Candida utilis when exposed to high-pressure oxygen (HPO) for periods of up to 20 days. It was suggested that this might provide a mechanism for preventing peroxide accumulation, thus inducing resistance to high pressure oxygen toxicity. The results presented in the present paper describe the changes in catalase activity of Bacillus subtilis when exposed to 10 atm. oxygen and the relation between resistance and physiological state at the time of pressure increase. METHODS Bacterial strains. Bacillus subtilis NCIB 3610and Escherichia coli B I 3 were obtained from the culture collection of the Department.Media and cultivation. Bacteria were maintained on malt agar at 2 5 ' . Cultures for HPO treatment were grown in 25 ml. liquid glucose glutamate medium (Pritchard, 1965) in IOO ml. Erlenmeyer flasks at 37" on a rotary shaker. For Escherichia coli the medium was supplemented with 0.5 yo (w/v) casein hydrolysate.Catalase determination. Bacteria for catalase determination were sedimented by centrifugation, resuspended in 4 ml. cold 0.05 M-phosphate buffer (pH 6.8) and disrupted by ultrasonic treatment. The material was centrifuged for 10 min. at 30,ooog and the catalase activity of the supernatant fluid determined by the method of Herbert (1954). Enzyme activity is expressed per g. extracted protein.Protein determination. Soluble protein was determined by the Folin phenol method of Lowry, Rosebrough, Farr & Randall (I 95 I).Bacterial counts. Total counts were made by using a bacterial counting chamber, 0.02 mm. deep, with Thoma ruling.Viable counts. Survival following HPO treatment was determined by dispensing in

SUMMARYExponential cultures of Saccharomyces cerevisiae and Candida utilis exposed to oxygen at ten atmospheres did not undergo any further cell division during treatment. All cells ultimately died when the treatment was prolanged for several days. Populations survived considerably longer with ethanol as a carbon source than with glucose. Stationary-phase populations were more resistant than exponentially-growing populations. The cell cycle of S. cerevisiae shows two points of resistance to oxygen toxicity which corresponded to two periods of catalase synthesis in air. These results are attributed to periodic synthesis of protective enzymes.

No abstract

No abstract