High throughput screening for spores and vegetative forms of pathogenic B. anthracis by an internally controlled real-time PCR assay with automated DNA preparation

Human infections with Bacillus anthracis have become rare but in cases of intentional release, masses of samples would have to be expected. Current PCR assays for anthrax are appropriate for use in single cases, but they have not been formulated for high throughput screening. This article describes a high throughput real-time PCR for anthrax, including automated sample preparation without the need for pre-culturing of samples. The assay detects single copies of target gene. An internal control monitors the whole assay including sample preparation. The limit of detection in blood was 1,066 (95%CI, 741–1,739) copies/ml, corresponding to 4.4–32.3 organisms/ml. Using spore preparations, 20 colony-forming units (CFU) per sample could be detected reliably (0.8 CFU per PCR). The extraction procedures depleted viable spores from solution by factors of 10,000 (automated procedure) and >100,000 (conventional column procedure). One hundred and ten clinical and environmental specimens were retested, 50 of them sampled during a period of heightened anthrax awareness in 2001. A widely used assay yielded two false positive results (cross-reaction with B. cereus), while the new assay tested all samples negative. The internal control operated stable in all clinical samples. The assay is capable of testing for anthrax in the high throughput mode.