THE CORRELATION BETWEEN THE HISTOLOGICAL CHANGES AND THE FATE OF LIVING TUBERCLE BACILLI IN THE ORGANS OF TUBERCULOUS RABBITS

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Abstract
It has been found that although there is some parallelism between the quantity of tubercle bacilli demonstrable histologically and the number of colonies that can be isolated from a given tissue, the culture method is far the more efficient in indicating quantitative relations. Tubercle bacilli were not perceived in the organs of rabbits 1 day after infection with the modified BCG although as many as 1,500 colonies were isolated from one of them. This may be solely because it is difficult to see widely dispersed single minute acid-fast rods in the diffuse infiltrations of mononuclears with their hyperchromatic nuclei and sparse cytoplasm. Later, with the formation of tubercle, the parallelism is much closer. The culture method gives evidence concerning the number of living tubercle bacilli in the tissue. The significance of the accumulation of acid-fast particles in the tissues has been discussed. It has been seen that from the beginning this accumulation is greater in the Kupffer cells of the liver, in the macrophages of the spleen and in the reticular cells of the bone marrow than within the mononuclears of the lung, the organ where the bacilli grow with the greatest rapidity and are destroyed with the greatest difficulty. Acid-fast particles are more prominent with the bovine than with the human bacillus or the BCG, the microorganism that is destroyed with the greatest difficulty thus leaving more incompletely digested bacillary debris at a given time within the cells. Thus it seems permissible to conclude from the presence of acid-fast material that some tubercle bacilli are undergoing destruction even 24 hours after infection. The initial accumulation of polynuclear leucocytes corresponds with the subsequent severity of the infection. Despite the greater primary localization of bacilli in the liver, this initial inflammatory reaction with all three infections is much greater in the lung than in the liver. In each organ it is more intense with the bovine than with the less virulent strains. The multiplication of the bacillus and its accumulation within large mononuclear and young epithelioid cells is accompanied by an intense formation of new mononuclears by mitosis. The more rapid the growth of the bacillus, the more conspicuous the regeneration of these cells. Thus with all strains mitosis is more intense in the more susceptible organ, as in the lung compared with the liver; with the most virulent strain the most extensive and diffuse accumulation of these new cells corresponds with the greater rise in the numbers of bovine bacilli after the lag of the 1st week. With the maturation of the epithelioid cells and the formation of tubercles the bacilli have already been greatly reduced numerically and the speed of this process diminishes with the virulence of the three strains used. The faster the development of tubercle the faster the destruction of the bacillus and the earlier the resorption of the tubercle. Tubercle bacilli never accumulate in such large numbers in the mononuclears of the liver as they do in the lung. Though at first the tubercles in the liver may be more numerous than those in the lung they never attain the same size. The formation of new mononuclears by mitosis is restricted and Langhans' giant cells appear very early (1st and 2nd weeks). In the lung, giant cells are not found until much later with the BCG and the human bacillus (4th week); they were not noted in the interstitial tubercles with the bovine type, but the extension of these tubercles was accompanied by an unabated mitosis of mononuclears until the death of the animal. The liver tubercles are resorbed early even with the bovine infection. Associated with these histological differences are the slow initial growth and the early and complete destruction of the tubercle bacilli even of bovine type in the liver, and the more rapid initial growth in the lung, with the later destruction of the BCG and the human bacillus and the unabated growth of the bovine bacillus. Similar differences were observed between the splenic pulp and corpuscle. In the former the accumulation of acid-fast particles was much greater and the tubercles developed earlier. Mitosis of mononuclears was less frequent and giant cells appeared earlier. Tubercle bacilli, always intracellular, disappeared from the tubercles in the pulp sooner than from those in the corpuscle, and the tubercles themselves first disappeared from the pulp. Consequently with the persistence of bacilli mitosis continued in the tubercles of the corpuscle and these attained a much larger size. Moreover individual resistance is linked with the ability to form mature tubercles early. In two animals simultaneously infected with the same strain and killed at the same time, the destruction or retardation of the bacillus is greater in that rabbit in which maturation of the tubercle and of epithelioid cells has proceeded further (Figs. 15 and 16). These observations indicate that the mononuclears of different organs or even of the same organ, as in the different parts of the spleen, have a different capacity to destroy the tubercle bacillus, and that the transformation of the mononuclear into the mature epithelioid cell follows its destruction of the tubercle bacilli. In the lung the more virulent types of bacillus are destroyed within the epithelioid cells of interstitial tubercles but persist in foci of tuberculous pneumonia. In this organ in rabbits infected with the human strain and to a lesser degree in rabbits infected with the bovine strain, the parasite largely disappears from the epithelioid cells of interstitial tubercles. But with both strains tubercle bacilli in large numbers may accumulate within epithelioid cells lying free in the alveoli. With the human type they are numerous within the cells and free in caseous material in the localized foci of caseous pneumonia. With the bovine infection, this caseous pneumonia is more often widespread and in the...