β-Lactamase Gene Expression in a Penicillin-Resistant Bacillus anthracis Strain

Expression of the bla1 and bla2 genes in an archetypal Bacillus anthracis strain is insufficient for penicillin resistance. In a penicillin-resistant clinical isolate, both genes are highly transcribed, but bla1 is the major contributor to high-level resistance to ampicillin. Differential expression of the bla genes is dependent upon strain background. Ciprofloxacin, doxycycline, and penicillin G are currently recommended by the Centers for Disease Control and Prevention and the Food and Drug Administration as therapeutics for inhalation and cutaneous anthrax (16). Prototypical Bacillus anthracis strains are susceptible to all three of these antibiotics. There have been no reports of naturally occurring ciprofloxacin- or doxycycline-resistant B. anthracis strains, but naturally occurring penicillin-resistant B. anthracis isolates have been reported (5, 19, 20, 32), and surveys of clinical and soil-derived strains have revealed penicillin G resistance in up to 16% of isolates tested (3, 6, 8, 10, 21, 25, 26).