VIRULENCE AND MORPHOLOGICAL CHARACTERISTICS OF MAMMALIAN TUBERCLE BACILLI

Antituberculous immunity induced by methanol extracts of tubercle bacilli; its enhancement by adjuvants

It is possible to prepare from tubercle bacilli a fraction soluble in methanol which is capable of eliciting in mice a marked degree of resistance against virulent tuberculous infection. The immunity was evident whether the infective dose was large and caused a disease with a rapid course, or was very small and caused a disease of many months duration. Active material has been obtained by extraction with methanol at 55 degrees C. of bacterial cells killed with 2 per cent phenol, and washed with acetone. The methanol extracts used in the present study have been prepared from the phenol-killed cells of a culture of BCG, and of the avirulent culture H37Ra. Vaccination of mice bas been carried out by the intraperitoneal route, and the challenge infection (with a highly virulent bovine culture), by the intravenous route. Weight for weight, the protective activity of the methanol extract is smaller than that of the bacterial cells from which it is extracted, but its primary toxicity for mice is also considerably lower. The protective activity can be increased, and the immunity prolonged, by using certain adjuvants as vehicle for injection of the vaccine. An oil adjuvant mixture, and small amounts of a highly purified preparation of the somatic antigen of typhoid bacilli, have been found capable of enhancing and prolonging the antituberculous immunity induced by the methanol extract. Under appropriate conditions the resistance resulting from intraperitoneal injection of the methanol extract is of the same order as that which follows vaccination with whole killed tubercle bacilli or with living BCG.

Cultivation and visualization of mycobacteria on molecular filter membranes

Images

Multiplication and survival of tubercle bacilli in the organs of mice

Cultures of tubercle bacilli (typical bovine and human strains) known to differ in the severity of the lesions they induce in experimental animals, were injected in various doses into the cerebrum, peritoneal cavity, or blood stream of mice. Quantitative determinations of the numbers of living bacilli present in the tissues at different intervals of time after infection led to the following classification of the cultures tested:- (a) Certain well known variant forms of tubercle bacilli were found to be unable to multiply in vivo, although they could survive for many weeks in the tissues of mice. These organisms proved to be truly avirulent. (b) Other variant forms underwent multiplication in vivo, even when extremely small infective doses were used, but could not give rise to progressive disease. It is proposed to designate these strains, which produce only abortive infections, as "attenuated." Different levels of attenuation could be detected. The maximum numbers of living bacilli that were recovered from the tissues corresponded directly to the severity and duration of the abortive lesions that could be produced by the strain in guinea pigs or in mice and were characteristic for each strain tested. The two BCG substrains tested were found to differ markedly in their level of attenuation. (c) The cultures virulent for guinea pigs were also capable of establishing a progressive infection in mice even when small infective doses were used. In the case of the attenuated and virulent strains, the population of living bacilli present in the lungs was at first much lower than that in the spleen, but it continued to increase in the former organs throughout the period of observation. This was notably true in the case of the virulent cultures. In contrast, the numbers of living bacilli in the spleen rapidly reached a maximum in the case of all cultures and then decreased progressively. For a given infective dose, and a given interval of time after inoculation, the maximum levels of living bacterial population attained in the spleen and in the lungs proved to be a direct expression of the virulence of the strain.

Antituberculous immunity in mice vaccinated with killed tubercle bacilli

The resistance of white mice to tuberculous infection could be increased by preliminary vaccination with small amounts of tubercle bacilli killed by contact with 2 per cent phenol. Vaccine prepared from a variant strain of human tubercle bacilli unable to multiply in vivo (H37Ra) proved as active as vaccines prepared from either virulent or attenuated strains. The immunity induced by phenol-killed bacilli persisted for several weeks. Under the conditions of the experiments, however, it was never able to bring about the death of the virulent bacilli used for the challenge infection, even when the infective inoculum was very small. Its protective effect could be detected (a) by the increased survival time of mice infected with a very large dose of virulent bacilli, and particularly (b) by the lower numbers of bacilli present in the organs of mice sacrificed at various periods of time after injection of sublethal infective doses. Under the proper conditions of vaccination the immunity produced in mice by phenol-killed cells of avirulent bacilli was of the same order as that produced by BCG. The protective antigen proved to be susceptible to heat, particularly at acid reactions. It retained its activity when the bacilli were disintegrated and rendered non-acid-fast by grinding with concentrated phenol. It remained in the insoluble cellular debris when the bacilli were extracted with 88 per cent phenol. Reasons are presented to support the view that the antigenic components present in the tubercle bacilli (avirulent as well as virulent) killed with phenol play a significant part in several manifestations of increased resistance to tuberculosis.

Studies on the virulence of tubercle bacilli; variations in virulence effected by tween 80 and thiosemicarbazone

Tubercle bacilli were grown in the presence of different concentrations of tween 80, ranging from 0.05 to 2.1 per cent. Equal numbers of viable bacteria from these cultures were compared in infection experiments in the mouse. The average survival time of the mice was used as a criterion for the virulence of the bacilli. High tween concentrations in the culture medium caused a reduction of the bacterial virulence. The reduction was slight in bacterial suspensions from cultures with tween 80 ranging from 0.05 to 1.0 per cent, but considerable in cultures with 2.1 per cent tween. Bacteria grown in the presence of 2.1 per cent tween gave rise to the same number of colonies, in vitro, as bacteria grown in ordinary media. Their oxygen uptake was increased as compared with that of bacilli grown in media containing less tween. Virulent bacteria grown in the presence of high amounts of tween 80 decolorized methylene blue in a test in which organisms from the same virulent strain but cultured without tween, or with only small proportions of the detergent in the medium, did not reduce the dye. A positive methylene blue test is typical of non-virulent tubercle bacilli and of saprophytic mycobacteria. Essentially the same changes occurred when virulent tubercle bacilli were grown in the presence of 0.5 microg./ml. of para-formacetanilide thiosemicarbazone (TBI). This small amount of the substance was not sufficient to prevent the growth of bacteria, or to reduce the number of viable cells in a culture, but it reduced the virulence of the bacteria considerably and rendered them capable of decolorizing methylene blue. Cord factor, a lipid constituent of virulent bacteria which is toxic for mice, was shown to be present in filtrates from cultures of virulent bacteria when the media contained 2 per cent tween 80, but no such material could be recovered from culture filtrates containing the usual 0.05 per cent tween. On the other hand, no toxic material could be extracted from bacteria grown in the presence of 0.5 microg./ml. TBI.