Although the elemental food requirements of all microorganisms may be considered identical, the form in which the essential elements can be utilized bylthe various species is subject to almost infinite variation. Literally hundreds of different culture media have been proposed, many of which find application because they favor or prevent the growth of particular species or related groups. That the individual organisms which make up a pure culture may also vary widely in their metabolic requirements is apparently not recognized by many bacteriologists. The published reports of many investigators dealing with bacterial resistance and the cultural methods employed by a large number of our control laboratories show how little this fundamental fact affecting the nutrition of bacteria is understood. It is the purpose of the present paper to show that bacterial spores which survive destructive physical or chemical influences are much more exacting in their nutritional requirements than spores not subjected to such treatment. Evidence is presented which indicates that this phenomenon is not restricted to spores but applies to vegetative forms as well. EXPERIMENTAL Cultures and methods Of the cultures used Bacillus subtilis, Bacillus cohaerens, and Bacillus albolactis were obtained from The American Type Culture Collection, CC is an unidentified species isolated from spoiled evaporated milk. The spores were produced on infusion agar
The survival of sporeforming bacteria in the heat processing of milk and the various mechanisms by which spores withstand severe heat treatment are discussed. Data are given concerning the spoilsage potential and heat resistance of psychrotropic sporeforming bacteria in milk. The types, numbers and heat resistance of spores in milk from countries with high average temperatures are considered in relation to processing. Laboratory experiments predicting thermal death of spores at temperatures within the UHT range are described and the many factors influencing the heat resistance of endospores, the most significan,t of which are the environmental conditions during sporulation, heating and recovery, are summarized.
Relatively mild heating of the spores of mesophilic aerobes has been shown to hasten their subsequent germination (Evans and Curran, 1943). Although effective in bringing about a more rapid germination, with one exception, such heating had no measurable influence upon the number of spores that germinated. This tangible evidence of pregermination stimulation led us to extend our observations to a group of thermotolerant aerobes isolated from commercially canned evaporated milk that had spoiled. With nearly all the latter types, preheating was found to exert a determining influence upon the number of spores that germinated. This phenomenon, which must be regarded as true heat activation, has not been reported previously for bacterial forms, although evidence of its operation may be found in the publications of Mudge and Thorwaldsen (1930) and Christian (1931), both of which dealt with obscure milk defects in which heat affected the development of sporeforming organisms. Seeking to explain observed qualitative and quantitative changes in the flora of pasteurized milk, Mudge and Thorwaldsen formulated an interesting hypothesis which assumes for certain thermophiles a complex life cycle involving both visible and invisible spore forms. The latter, normally dormant, might be induced to germinate by physical and chemical agencies, including heat. Christian believed that heating promoted the development of a "coconut" or "carbolic" taint in commercial sterilized milk by destroying a product of vegetative activity inhibitory to the germination of the spores. Some light is shed upon the heat activation reaction by experiments reported in this paper. The relationship between the amount of pregermination heat and the degree of activation, and certain factors exclusive of pregermination heat which affect the heat response are given especial consideration. METHODS AND MATERIALS Observations were made on the following organisms: 15u, 4149, 6 (American Can Company); CON ("A," Continental Can Company); 9499 (National Canners Association); G1 and H2 (Whitehouse Evaporated Milk Company); LB (Bureau of Dairy Industry); Bacillus coagulans (Iowa State College), a thermotolerant aerobe isolated from spoilage in commercially processed evaporated milk; Bacillus calidolactis, 2 strains, one from Iowa State College and 1518 from the National Canners Association, both obligate thermophilic aerobes recovered from spoiled evaporated milk and corn, respectively; Bacillus subtilis 6051 and 6634 (American Type Culture Collection); and 3679 (American Can Company),
In their early studies on disinfection, Koch, Gaffky and Loeffler (1881) reported that anthrax spores which had been heated to 900 and 95°C. required longer: to produce visible plate colonies than unheated spores. Since that time the growth-delaying action of sublethal heat has been recorded by many investigators, both for spores and vegetative cells: Bredemann (1909), for Bacillus
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