The Effect on the Virulence ofBact. aertryckeof Cultivation in Atmospheres Containing Varying Proportions of Oxygen

Abstract

1. A given strain ofBact. aertryckewas sub-cultured daily, except on Sundays, in 5–7 c.c. of casein broth,pH 7·4–7·6, under different partial pressures of oxygen. Most experiments were continued till about ninety sub cultures had been made. Virulence determinations were made from time to time by intraperitoneal inoculation of twenty mice with about 100 living organisms, the control and the experimental cultures generally being tested simultaneously. The results showed that under anaerobic conditions there was no appreciable change in virulence. Under the usual aerobic conditions, and in partial pressures of oxygen varying from 1 to 21 per cent., the virulence declined, the extent and rapidity of the fall increasing with the pressure of oxygen supplied. In high partial pressures of oxygen, varying from 40 to 100 per cent., there was a slight increase in virulence, the increase being greatest in cultures incubated in pure oxygen.2. Some of the literature dealing with incitants to microbic dissociation is discussed, particularly that dealing with variations in oxygen pressure.3. In explanation of the experimental results obtained, a tentative hypothesis is put forward with reference to the dissociation ofBact. aertrycke. It is suggested that under aerobic conditions, and under low partial pressures of oxygen generally, substances are formed in the medium, as the result of growth, which favour dissociation, and lead to the appearance of weakly virulent or completely avirulent variants. Under anaerobic conditions these substances either are not formed at all, or appear in amounts too small to be of significance. In cultures incubated in high partial pressures of oxygen, these substances are formed, probably in large quantity, but owing to the high alkalinity of the medium and the abundant oxygen present, they undergo rapid destruction. This explanation is based on the fact that many protein degradation products, particularly those with a phenolic grouping, prove markedly unstable in an alkaline medium in the presence of free oxygen.As an alternative hypothesis it is suggested that the maintenance of the organisms in the virulent state depends on the intracellularpH. Under anaerobic conditions it is supposed that the metabolism is such as to lead to no marked change in the H-ion concentration within the cell. In the presence of air and of low pressures of oxygen generally, owing to the breakdown of the protein in the medium and the production of ammonia, the intracellular reaction becomes more alkaline; whereas in high pressures of oxygen the increased amount of CO2produced is sufficient to compensate for the production of ammonia, and the intracellularpH remains more or less at its original value. This hypothesis rests on the observation of Stephenson and Whetham (1924) that in high pressures of oxygenBact. colimay produce considerably larger quantities of CO2from lactic acid than under ordinary aerobic conditions; and on the demonstration by Jacobs (1920b) that, owing to the ease with which CO2is able to penetrate the living cell, the intracellularpH may differ markedly from that of the medium to which the cell is exposed.4. It is of interest to note thatBact. aertrycke, when incubated under partial pressures of 40 to 75 per cent. of oxygen, rapidly passed into the nonmotile O form.