STUDIES ON VIRULENCE : III. INFLUENCE OF HYDROGEN ION CONCENTRATION AND OF INGREDIENTS OF PLAIN BROTH ON THE VIRULENCE OF PNEUMOCOCCI

The Effect on the Virulence of Bact. aertrycke of Cultivation in Atmospheres containing Varying Proportions of Oxygen

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.

VARIATIONS IN SPECIFICITY AND VIRULENCE OF PNEUMOCOCCI DURING GROWTH IN VITRO

When pneumococci are grown repeatedly in broth containing immune serum or bile, they become less virulent and less type-specific. These changes are apparently due to the fact that under the conditions mentioned a number of organisms appear which have lost certain properties. These variant organisms form colonies differing in appearance from the colonies of the typical organisms, and cultures made from the atypical colonies are avirulent and are not type-specific, The degree of modification of the original culture is directly related to the relative number of organisms that have undergone variation. After prolonged growth in bile or homologous immune serum variation may have become manifest in all the bacteria of the cultures and the change is then irreversible. The significance of these findings as regards the epidemiology of pneumococcus infections is noted.

STUDIES ON VIRULENCE : II. THE INCREASE IN VIRULENCE IN VITRO OF A STRAIN OF PNEUMOCOCCUS

1. An automatic apparatus is described by which a new supply of food can be furnished actively growing organisms at any desired interval of time-an automatic transferring device. 2. A single strain of pneumococcus, Type I, Neufeld, which had become avirulent for mice, acquired virulence of maximal degree when grown in the described apparatus with skimmed milk sterilized in an Arnold sterilizer as the medium. Transfers were made at intervals of 2, 4, and 8 hours. It is difficult to determine the best interval of transfer, but the 8 hour interval apparently is most suitable, with the 4 and 2 hour following in preference in the order named. 3. Pasteurized skimmed milk from a single dairy, but obtained on different days, when used as medium at a 2 hour interval, varied in effect on the virulence of pneumococci, the results showing that the virulence might be either increased, decreased, or maintained. 4. Skimmed milk heated for varying lengths of time, 30 minutes at 17 pounds pressure, or 60 or 90 minutes at the same pressure, lost its suitability for maintaining virulence of a pneumococcus when transferred every 2 hours, the effect being in direct proportion to the length of time the milk was heated. 5. The H ion concentration of milk had slight effect on virulence. A virulent strain of pneumococcus was grown in milk adjusted to pH = 5, 6, 7, 8, and 9, transfers being made every 2 hours. Virulence was maintained to a like degree on milk titrated to pH = 5, 6, and 7, but when the organism was grown in milk of pH = 8 and 9, virulence decreased, more rapidly at pH = 9 than at pH = 8. 6. A pure line practically avirulent strain of pneumococcus picked by the Barber method, when grown in milk at a 4 hour interval of transfer, increased in virulence 10 million fold; that is to say, until one diplococcus would kill a mouse. Prior to this study, no record has been found in which the virulence of any microorganism has been increased to such a degree by an in vitro method.