FIG 6.

Effect of spectinomycin on signal kinetics and limits of detection. (A) Detection of B. anthracis spores in soil in the presence of spectinomycin. Spores (1.3 × 10⁷ CFU/g) were inoculated into soil (0.1 g) and maintained overnight (∼16 h) at 4°C before being mixed with Wβ::luxAB-2 (final concentration, 4 × 10⁷ PFU/ml) and incubated in beef infusion broth (containing 0.1 M l-alanine) at 35°C. Spectinomycin (100 μg/ml) was added 4 h after phage infection, and bioluminescence was measured over time following the addition of n-decanal. (B) Limits of detection. B. anthracis spores (1.3 × 10⁴ to 1.3 × 10⁷ CFU/g) were inoculated into soil and incubated with the reporter phage and spectinomycin as described above. Samples were read for bioluminescence after 3 and 6 h. Numbers represent the means ± SD (n = 3). *, P < 0.05 (two-way ANOVA) for results with spectinomycin-treated samples compared to those with untreated samples (A) or for results compared with those of phage-only controls (B).