Aerobic Sporeforming Bacteria Capable of Growth at High Temperatures.

The collection of 621 strains of aerobic mesophilic sporeforming bacteria studied by Smith, Gordon, and Clark (1946) included 8 strains that grew slowly, if at all, at 28 C and required a higher temperature than the others for maximum growth. They were closely related and as a group were distinguishable in several respects from the other species. Pending the investigation of more strains the 8 were assigned to the speciesBacillus coagulans Hammer (1915), withwhichBacillus thermoacidurans Berry (1933) is in synonymy. Our interest in acquiring and examining additional strains of this poorly represented species was twofold. In the first place, a larger number was needed to give a more complete characterization of the species, for only through the study of an adequate number of strains can a reliable description of a species be prepared. Also, as an apparent intermediate between the mesophilic and thermophilic sporeformers, Bacillus coagulans seemed from the data available to be an inviting preliminary step toward the study of the so-called thermophilic group. Thermophilic bacteria have been characterized by different workers in a variety of ways. Prickett (1928) ably summarized the various definitions and accepted the following one of Cameron and Esty (1926): Cultures growing at 55 C but not at 37 C were classified as obligate thermophiles, and those growing at both 55 C and 37 C, as facultative thermophiles. According to this definition none of the strains studied by Smith, Gordon, and Clark (1946) were obligate thermophiles; but 7 of the 8 strains of Bacllus coagulans and a few of Bacillus subtilis were facultative. Approximately 60 different species of aerobic thermophilic sporeforming bacteria have been named and described. In the sixth edition of Bergey's Manual (Breed et al., 1948, p. 707), Smith offered this apology: "The data on the species of this group are so meager that it is not possible to offer a rational system of classification. Many of the characters used for separating the various species are probably as variable in this group as they have been found to be in the mesophilic group. Lacking a knowledge of the limits of variability and lacking other pertinent data, the present arrangement is regarded as temporary only." It was the purpose of thiswork, therefore, to studyBacillus coagulans more thoroughly, to characterize the species growing at still higher temperatures, and,