Studies on some morphological variants of Bacillus stearothermophilus strain NCA 1518 and on two of its thermophilic bacteriophages
Authors:   , Humbert Robert Dale 
Published: June 2024

"The remarkable heat resistance of the spores of Bacillus stearothermophilus has attracted a great deal of interest and investigation. This microorganism is of great theoretical interest because its spores represent one of the extreme parameters for the survival of living systems. The heat resistance of the spores is of practical significance because of the spoilage potential which this microorganism represents in canned foods. The spoilage produced by B. stearothermophilus is known as "flat sour" spoilage because the microorganism produces acid but no gas when it grows in canned foods. The acid and other products of the organism cause a sour flavor and an unplesant odor. The absence of gas production leaves the can flat, as opposed to the bulged or swelled condition brought about by gas production. A number of investigations showed that the heat resistance of the spores of B. strearothermophilus varies widely. Heat resistance of spores varied with the ccmposition of the medium on which the spores were produced, the temperature at which they were produced, the medium in which they were heated, the particular strain involved, and the variant type within the strain. The National Canners Association (NCA) strain 1518 of B. stearothermophilus is widely used for determinations of heat resistance. A wide variety of F values was reported in the literature. Some of these differences may be attributed to the existence of variant types within the stocks of B. stearothermophilus NCA 1518. Some of these variants, identified as Rough (R) and Smooth (s), according to their colonial morphology, were found by Fields (1963), to differ in the thermal resistance of their spores. The differences in heat resistance and the composition and reaction of these variants to a number of different selective pressures were investigated by Fields and his co-workers. Since there are spontaneous morphological mutations in microbial populations, and since there are observed differences in the heat resistance and biochemistry of the known morphological variants of B. stearothermophilus RCA. 1518, an investigation of the specific interrelationships involved with morphological mutations was desirable. The study also could help in understanding the variations in heat resistance as well as perhaps providing a little more insight into the extreme thermophilic and thermoduric nature of B. stearothermophilus. Some phases of its genetics were also explored. The approach used in this investigation was to isolate a series of rough and smooth (morphological) mutants from the rough and smooth stocks of B. stearothermophilus NCA 1518. The mutant stocks were evaluated for heat resistance of the spores and biochemical capabilities of the vegetative cells as compared to the "standard" rough and smooth strains. Studies of the relationship between hosts and their parasites often have provided valuable information regarding the host itself as well as the host-parasite system. For that reason, this research effort included some investigations into bacteriophages which parasitize B. stearotheimophilus NGA 1518. It was hoped that this might possibly lead to the development of transduction as an investigational tool available for the study of the genetics of this species."--Introduction.