Detection and Fate of Bacillus anthracis (Sterne) Vegetative Cells and Spores Added to Bulk Tank Milk†

Abstract

A preparation of Bacillus anthracis (Sterne strain) spores was used to evaluate commercially available reagents and portable equipment for detecting anthrax contamination by using real-time PCR and was used to assess the fate of spores added directly to bulk tank milk. The Ruggedized Advanced Pathogen Identification Device (RAPID) was employed to detect spores in raw milk down to a concentration of 2,500 spores per ml. Commercially available primers and probes developed to detect either the protective antigen gene or the lethal factor gene both provided easily read positive signals with the RAPID following extraction from milk with a commercially available DNA extraction kit. Nucleotide sequence analysis of the vrrA gene with the use of DNA extracted from spiked milk provided molecular data that readily identified the spores as B. anthracis with a 100% BLAST match to the Sterne and Ames strains and easily distinguished them from B. cereus. Physical-fate and thermal-stability studies demonstrated that spores and vegetative cells have a strong affinity for the cream fraction of whole milk. A single treatment at standard pasteurization temperatures, while 100% lethal to vegetative cells, had no effect on spore viability even 14 days after the treatment. Twenty-four hours after the first treatment, a second treatment at 72°C for 15 s reduced the viability of the population by ca. 99% but still did not kill all of the spores. From these studies, we conclude that standard pasteurization techniques for milk would have little effect on the viability of B. anthracis spores and that raw or pasteurized milk poses no obstacles to the rapid detection of the spores by molecular techniques.