Although the virulence of Bacterium tularense may be maintained for prolonged periods of cultivation on artificial media, a considerable reduction in v-irulence has been frequently observed. As early as 1922 Francis reported the loss of vTirulence of certain strains of B. tularense following serial transfer on seium glucose agar. Foshay (1932) described the loss of virulence of two strains isolated from fatal human cases following prolonged cultivation on coagulated egg yolk medium. Ransmeier (1943) reported a reduction of virulence after serial transfer of an originally highly virulent strain on glucose cystine blood agar. It is also well known that repeated passage of cultures through a given animal species may result in a marked increase in virulence for the particular host (Philip and Davis, 1935). Thus, changes in virulence have been amply demonstrated; however, it has not been possible to correlate these changes with any other easily detectable characteristics. All attempts to correlate cellular morphology with virulence (Hesselbrock and Foshay, 1945; Eigelsbach, Chambers, and Coriell, 1946) have met with failure. Because various bacterial species display a high degree of correlation between colonial morphology, pathogenicity, and immunogenic properties (Braun, 1947), an attempt was made to determine whether such correlations may also exist for B. tularense. As far as the authors are aware, studies on the colonial morphology of B. tularense have not been reported previously in the literature. MATERIALS AND METHODS The majority of experiments were initiated with either a highly virulent culture, Schu, originally isolated by Foshay in 1941 from a human ulcer, or with an avirulent culture, 38, which was originally isolated by Francis in 1920 from a human lymph node and subsequently became totally avirulent (Hesselbrock and Foshay, 1945). These cultures, as well as variant types subsequently isolated, were maintained on glucose cysteine blood agar (GCBA) at 5 to 10 C following a 24-hour growth period at 37 C. The solid transparent medium used for viable count and colony type observations consisted of 2 per cent Difco peptone, 1 per cent NaCl, 0.1 per cent glucose, 0.1 per cent cysteine hydrochloride, and 2 per cent agar with a final reaction of pH 6.8 (Snyder et al., 1946). The basal liquid medium used was identical in composition except for the absence of agar. Vliable counts were made by plating serial dilutions on GCBA; total growth was determined by packed cell volume in modified hematocrit tubes after centrifugation at 4,000 rpm for 1 hour. Colony types wvere inspected under a dissecting microscope with the help of obliquely transmitted light, achieved by placing the mirror, concave side up, 557
The pathogenesis of tularemia was studied in groups of rhesus monkeys ( Macaca mulatta ) that inhaled graded 10-fold doses ranging from 10 through 10 6 organisms of Francisella tularensis 425, a strain highly virulent for the white mouse but of reduced virulence for the domestic rabbit. Mean incubation periods ranged from 3 to 6 days followed by acute illness lasting 5 to 11 days with subsequent recovery of most animals. The higher inhaled doses resulted in shorter incubation periods, longer and more severe acute illnesses, and 18% mortality at the highest dose. Strain 425 multiplied in the lungs, disseminated to the regional lymph nodes, and became systemic. Maximal bacterial populations in tissues were reached by the 7th day after exposure of the animals regardless of the number of organisms inhaled. F. tularensis was no longer recoverable from any of six tissues examined 2 months after exposure. The most significant tissue changes occurred in the lungs; these consisted of foci of liquefaction necrosis, lobular consolidation, and pleural effusion and adhesions. The data indicate that the inhaled dose of strain 425 determined the maximal growth of the organism in the lungs which in turn influenced the severity of the usually self-limiting pneumonia and systemic infection. Although the monkey is less resistant to tularemia than is man, this laboratory animal when infected with F. tularensis 425 provides a useful model for the self-limiting type of human pulmonary tularemia usually observed in Europe and Asia but to a lesser extent in North America.
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