Thermal Inactivation of Bacillus anthracis Spores in Cow's Milk

Abstract

Decimal reduction time (time to inactivate 90% of the population) (D) values of Bacillus anthracis spores in milk ranged from 3.4 to 16.7 h at 72°C and from 1.6 to 3.3 s at 112°C. The calculated increase of temperature needed to reduce the D value by 90% varied from 8.7 to 11.0°C, and the Arrhenius activation energies ranged from 227.4 to 291.3 kJ/mol. Six-log-unit viability reductions were achieved at 120°C for 16 s. These results suggest that a thermal process similar to commercial ultrahigh-temperature pasteurization could inactivate B. anthracis spores in milk.In 2001, the intentional release of anthrax spores through the U.S. Postal Service prompted increasing concern for a potential bioterrorist attack, including food terrorism. Due to the vulnerability of the food supply system, food and water could be deliberately contaminated and are considered two of the most viable targets for bioterrorism (7). Milk, as a product consumed every day especially by children, poses a high probability/high severity risk potential if Bacillus anthracis spores were purposely added to it at the farm, truck delivery, or processor level (23). In order to enhance milk safety against purposeful contamination with a biological agent, it is necessary to develop effective heat treatments capable of inactivating biological agents, such as large amounts of anthrax spores, that could be deployed immediately by milk processors in the event of a bioterrorist attack (23).Bacillus anthracis is a spore-forming, gram-positive bacillus that causes anthrax. There is very limited information about heat inactivation of B. anthracis spores in the literature, as most studies have focused on the heat resistance of other Bacillus species. The effects of different heating methods (boiling, moist heat, and dry heat) on heat inactivation of B. anthracis spores have also been reported (6,12,17,19,20). In general, it was reported that in different buffer solutions B. anthracis spore counts could be reduced by at least 10 6 CFU/ml if the spores were heated at 90°C for 20 min, at 100°C for 10 min, and 105°C for 5 to 10 min (12,19). At 120°C, the spores were destroyed after 5 to 15 min (20). The thermal kinetics of B. anthracis inactivation in buffer solutions, however, cannot be used to predict its destruction in foods, because the heat resistance can be increased by food components, such as protein and fat (3, 18).Because milk is not considered a vehicle of natural B. anthracis infections, very few studies have investigated the thermal resistance of B. anthracis spores in milk. In one recent study, standard pasteurization conditions (63°C, 30 min; 72°C, 15 s) killed 4 log CFU/ml vegetative B. anthracis cells but had no effect on spores (14). The authors concluded that standard pasteurization processes had little effect on the viability of B.anthracis spores (14). In another study, Novak et al. (13) reported the inactivation of B. anthracis strain Sterne in skim milk by 0.45 log CFU/ml after 90 min at 72°C, 8.1 log CFU/ml after 60 min at 100°C, and 7.7 log...

Behavioural pattern of vegetative cells and spores of Bacillus cereus as affected by time-temperature combinations used in processing of Indian traditional foods

“…As B. anthracis was in the same cluster as that of B. cereus, a similarity was observed in an earlier study with spores of B. anthracis Sterne (Novak et al 2005), wherein the D-values recorded at 100°C in skim milk was almost near to those recorded in the present study. In contrast to the above, studies with different cultures like B. anthracis in whole milk at 100°C (Xu et al 2006) and a strain of B. cereus ATCC 9818 at 100°C in distilled water, BHI broth and skim milk (Novak et al 2005) revealed higher D-values (14.4 to 35.7 min) in comparison to those recorded in this study with three native food isolates of B. cereus.…”

Inactivation of Bacillus anthracis Spores by a Combination of Biocides and Heating under High-Temperature Short-Time Pasteurization Conditions

“…The results of that study confirmed that standard HTST pasteurization processes (e.g., 72°C for 15 s) had little effect on inactivation of B. anthracis spores (11,12,24). Six-log reductions in viability were achieved if the spores were heated at 120°C for 16 s. This observation suggests that a thermal process similar to commercial ultra-high-temperature (135 to 140°C for 1 to 2 s) pasteurization could inactivate B. anthracis spores in the event of a deliberate attack by terrorists.…”

“…Since pasteurization is a standard process in milk plants, it could be adapted to develop a thermal process capable of inactivating B. anthracis spores. Such a process could be easily adopted by high-temperature short-time (HTST) milk pasteurizers in the event of a bioterrorist attack for decontamination and disposal.A series of thermal inactivation kinetic parameters at 72 to 103°C were determined in our previous study (24). The results of that study confirmed that standard HTST pasteurization processes (e.g., 72°C for 15 s) had little effect on inactivation of B. anthracis spores (11,12,24).…”

“…A series of thermal inactivation kinetic parameters at 72 to 103°C were determined in our previous study (24). The results of that study confirmed that standard HTST pasteurization processes (e.g., 72°C for 15 s) had little effect on inactivation of B. anthracis spores (11,12,24).…”

“…B. anthracis Sterne strain 7702, avirulent Ames strain ANR-1, and Sterne derivative 9131 were provided by Theresa Kohler, University of Texas-Houston Medical School. Sterne strain 7702 harbors pXO1, which encodes the edema factor toxin, but lacks pXO2 (4,14,24). Strain ANR-1 is a pXO2-cured nonencapsulated variant of the Ames strain that harbors pXO1 and pXO2 (21).…”