Characteristics of the Species
On the basis of 16S rRNA gene sequence analysis, the group 5 of the Bacillus genus has been defined as a phenotypically and phylogenetically coherent group of thermophilic bacilli displaying a high degree of similarity among their 16S rRNA gene sequences (98.5–99.2%). Based on phenotypic and genotypic characteristics, some existing Bacillus species within group 5 were reclassified into the new genus Geobacillus, and the former Bacillus stearothermophilus is now Geobacillus stearothermophilus. For simplicity, all studies previously referring to as B. stearothermophilus are referred to in this article as G. stearothermophilus.
Geobacillus stearothermophilus is a thermophilic, aerobic, spore-forming bacterium with ellipsoidal spores that distend the sporangium. It is a heterogeneous species in which the distinguishing features are a maximum growth temperature of 65–75 C, a minimum growth temperature of 40 C, and a limited tolerance to acid. The bacterium does not grow at 37 C; its optimum growth is at 55 C with a fast growth rate (a generation time of w 15–20 min). Starch hydrolysis is typical, although some strains do not hydrolyze starch. Hydrolysis of casein and reduction of nitrate to nitrite are variable. Growth in 5% NaCl is scant. The heterogeneity of the species is indicated by the wide range of DNA base composition as well as the diversity of the phenotypic characters (Table 1). Minimum pH for the growth of G. stearothermophilus is 5.2; the minimum water activity (aw) for growth at optimum temperature is 0.93.
Geobacillus stearothermophilus was first isolated from cream-style corn by P.J. Donk in 1917. The bacterium is a common inhabitant of soil, hot springs, desert sand, Arctic waters, ocean sediments, food, and compost. The incidence of G. stearothermophilus in foods is related to the distribution of the microorganism in soil, water, and plants. Foods that have been heated or desiccated generally possess an enriched and varied flora of bacterial spores. Especially, milk contains minerals, such as calcium, magnesium, and so on, which stimulate spore formation of Geobacillus spp. during dairy processes. Some Geobacillus strains are able to sporulate in a laboratory medium (tryptone soya broth supplemented with CaCl2, MnSO4, FeSO4, or MgCl2) with a maximum yield (105–107 spores ml1) in 12–18 h. Geobacillus stearothermophilus is included in the usual microflora of cocoa bean fermentation as well as of cocoa powder. It is the dominant microorganism of beet sugar and is isolated from pasteurized milk, ultrahigh-heat-treated milk, and milk powders.
The incidence of G. stearothermophilus spores in canned foods is of particular interest. The spores enter canneries in soil, on raw foods, and in ingredients (e.g., spices, sugar, starch, and flour). The presence of G. stearothermophilus spores in some containers of any given lot of commercially sterile low-acid canned foods may be considered normal. If the food is to be distributed in nontropical regions where temperatures do not exceed about 40 C for significant periods of time, complete eradication of the microorganism is not necessary because it cannot grow at such low ambient temperatures. For tropical conditions, the thermal process must be sufficient to inactivate
spores of G. stearothermophilus that might otherwise germinate and multiply under these conditions. Geobacillus stear- othermophilus is the typical species responsible for thermophilic flat sour spoilage of low-acid canned foods or coffee during storage in automatic vending machines.
Spores or vegetative cells of G. stearothermophilus from dairy manufacturing plants attach to stainless steel surfaces and form biofilms. A doubling time of 25 min has been calculated for this organism grown as a biofilm. The formation of biofilms within the plant is the cause of contamination of manufactured dairy products.
The importance of thermophilic spoilage organisms in the food industry has generated considerable interest in the factors affecting heat resistance, germination, and survival of their spores. Because it grows at high temperatures, G. stearothermophilus tends to produce heat-resistant spores. The genetic variation, however, in moist as well as dry heat resistance between different strains of G. stearothermophilus is of considerable magnitude (Table 2). The main factors affecting these discrepancies are the composition of the sporulation medium, the sporulation temperature, and the chemical state of the bacterial spore, as well as the heating conditions in terms of the water activity, the pH, and the ionic environment of the heating medium, the presence of organic substances, the composition of the atmosphere, and so on. Under dry conditions G. stearothermophilus spores show the greatest increase in heat resistance (Table 2). At high water activity, the decimal reduction at 100 C (D100-value) of G. stearothermophilus spores is no less than 800 min, and under dry conditions, the D100 is about 1000 min. There is a need for technologies that require short thermal processing times to eliminate bacterial spores in foods. The superheat steam processing and drying system, which has been applied in Asian noodles, potatoes, and potato chips, is effective for the reduction of G. stearothermophilus ATCC 10149 spores. The thermal resistance constant (z-value, i.e., the tem- perature increase needed for a 10-fold decrease in the D value) calculated for superheated steam-processing temperatures between 130 and 175 C is 25.4 C, which is similar to those reported for conventional steam treatment.
In low-acid canned foods, D120 values of 4–5min and z-values of 14–22 C have been reported. Values of D decrease when the pH is reduced from 7.0 to 4.0. Values of z appear to be higher when the medium is acidified, although the difference is not statistically significant. Organic acids and glucono-delta- lactone have the same effect as acidulants in reducing the heat resistance of G. stearothermophilus spores. Sodium chloride reduces heat resistance of G. stearothermophilus when present at relatively low levels (i.e., less than 0.5 mol l1). The increased heat resistance of the spores of a strain of G. stearothermophilus during incomplete rehydration of dried pasta indicates possible implications in regard to food safety, as the reported D121 values range from 4.6 to 6.5 min and the z-values range from 10.7 to 15.6 C, may not be applied for products that are rehydrated during heat treatment.
When a dormant heat-resistant spore is activated and germinates to form a vegetative cell, its heat resistance is lost.