Evaluation of two selective media for the isolation of Bacillus anthracis

Aims:  To evaluate two selective media, polymyxin, lysozyme, ethylenediaminetetraacetic acid, thallium acetate (PLET) agar and R&F Anthracis chromogenic agar (ChrA), for the isolation and selection of Bacillus anthracis .
Methods and Results:  Sixteen genotypically diverse B. anthracis strains were sub-cultured onto PLET and ChrA to test the sensitivity (ability of B. anthracis to grow and produce expected colony morphology) of both media. Fourteen of the 16 B. anthracis strains produced the expected morphology on PLET (88% sensitive) while 13/16 produced the expected morphology on the ChrA medium (81% sensitive). Seventeen other Bacillus strains and 18 non Bacillus spp. strains were used to evaluate the media's selectivity (ability to inhibit non- B. anthracis growth). PLET inhibited growth of 14/35 strains (40% selective), including six Bacillus strains, while ChrA inhibited 3/35 (9% selective). In addition, we did not observe any differences between the recovered CFU on PLET or ChrA when plating extractions of spiked soil.
Conclusions:  Polymyxin, lysozyme, ethylenediaminetetraacetic acid, thallium acetate agar was more selective and sensitive than ChrA.
Significance and Impact of the Study:  Although both media are more expensive than sheep blood agar, for samples with high numbers of bacteria, they can be used to isolate B. anthracis with proper training and experience and with the knowledge that there are limitations to each media.