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Defined, spores are typically Bacillus (genus Bacillus), any of a group of rod-shaped, gram-positive, aerobic or (under some conditions) anaerobic bacteria widely found in soil and water. The term bacillus has been applied in a general sense to all cylindrical or rod-like bacteria. The largest known Bacillus species, B. Megaterium, is about 1.5 μm (micrometres; 1 μm = 10−6 m) across by 4 μm long. Bacillus frequently occur in chains.

In 1877 German botanist Ferdinand Cohn provided an authoritative description of two different forms of hay bacillus (now known as Bacillus subtilis): one that could be killed upon exposure to heat and one that was resistant to heat. He called the heat-resistant forms “spores” (endospores) and discovered that these dormant forms could be converted to a vegetative, or actively growing, state. Today it is known that all Bacillus species can form dormant spores under adverse environmental conditions. These endospores may remain viable for long periods of time. Endospores are resistant to heat, chemicals, and sunlight and are widely distributed in nature, primarily in soil, from which they invade dust particles.

Some types of Bacillus bacteria are harmful to humans, plants, or other organisms. For example, B. cereus sometimes causes spoilage in canned foods and food poisoning of short duration. B. subtilis is a common contaminant of laboratory cultures (it plagued Louis Pasteur in many of his experiments) and is often found on human skin. Most strains of Bacillus are not pathogenic for humans but may, as soil organisms, infect humans incidentally. A notable exception is B. anthracis, which causes anthrax in humans and domestic animals. B. thuringiensis produces a toxin (Bt toxin) that causes disease in insects.

Medically useful antibiotics are produced by B. subtilis (bacitracin) and B. polymyxa (polymyxin B). In addition, strains of B. amyloliquefaciens bacteria, which occur in association with certain plants, are known to synthesize several different antibiotic substances, including bacillaene, macrolactin, and difficidin. These substances serve to protect the host plant from infection by fungi or other bacteria and are being studied for their usefulness as biological pest-control agents.

A gene encoding an enzyme known as barnase in B. amyloliquefaciens is of interest in the development of genetically modified (GM) plants. Barnase acts to kill plant cells that have become infected by fungal pathogens; this activity limits the spread of disease. The gene controlling production of the Bt toxin in B. thuringiensis has been used in the development of GM crops such as Bt cotton. 

Bacillus Stearothermophilus 

Geobacillus stearothermophilus is a gram positive thermophilic (heat loving) bacteria characterized by a inner cell membrane and a thick cell wall. G. stearothermophilus is a rod shaped anaerob found in thermophilic habitats like thermal vents. Many heat stable enzymes like xylanase for pulp treatment and thermolysin-like protease for production of artificial aspartame have been isolated from this thermophilic bacteria. Geobacillus stearothermophilus strain 10, is an isolated strain that was found in a hot spring in Yellowstone National Park and has been used in comparative analysis of thermophiles and mesophiles. Geobacillus stearothermophilus is constantly used in the biotech industry to test the success of sterilization cycles of equipment. Due to the bacteria’s high resistance to heat, it is a suitable Biological Indicator of microbe life after a sterilisation cycle. 

Strain Geobacillus stearothermophilus JT2 when grown on blood agar plates are observed to have an ellipsoidal shape and adhere to each other to form longitudinal chains containing two or more cells. This strain is observed to be highly motile and produces a highly temperature stable enzyme α-amylase. 

Effect of Temperature on Bacillus Stearothermophilus 

One of the obligate thermophilic bacteria, Bacillus stearothermophilus, was unable to grow at temperatures below 35 degrees C. About 80% of the population in the bacterial culture died at the temperatures, and the same extent of loss in either of the activities of oxygen consumption or synthesis of protein or nucleic acid of the organisms was observed. With the progress of death of the organisms, reduced nicotinamide-adenine dinucleotide came to be oxidized by the organisms, enzymes such as fructose-1,6-diphosphate aldolase, when the organisms were washed with phosphate buffer, were leaked out of the organisms, and an increasing amount of ribonucleoprotein was released into the culture medium. The change of the membrane state was then suggested to be one of the possible causes for the death of the organisms at the temperatures.

When tested in high temperatures of a Resistometer in Germany and the United States, it was found when increased increments of temperature were actioned, the Bacillus population of Stearothermophilus gradually declined up until 134 Degrees Celsius when all kinetics were killed within 10 seconds at this holding temperature.