Identification of the Extracytoplasmic Function sigma Factor sigma(P) Regulon in Bacillus thuringiensis

Antimicrobial resistance is major concern for public health. beta-Lactams remain an important treatment option for many diseases. However, the spread of beta-lactam resistance continues to rise. Bacillus thuringiensis and other members of the Bacillus cereus family are resistant to many beta-lactams. Resistance is dependent upon the extracytoplasmic function sigma factor sigma(P). We used label-free quantitative proteomics to identify proteins whose expression was dependent upon sigma(P). We compared the protein profiles of strains which either lacked sigma(P) or overexpressed sigma(P). We identified 8 members of the sigma(P) regulon which included four beta-lactamases as well as three penicillin-binding proteins (PBPs). Using transcriptional reporters, we confirmed that these genes are induced by beta-lactams in a sigma(P)-dependent manner. These genes were deleted individually or in various combinations to determine their role in resistance to a subset of beta-lactams, including ampicillin, methicillin, cephalexin, and cephalothin. We found that different combinations of beta-lactamases and PBPs are involved in resistance to different beta-lactams. Our data show that B. thuringiensis utilizes a suite of enzymes to protect itself from beta-lactam antibiotics. IMPORTANCE Antimicrobial resistance is major concern for public health. beta-Lactams remain an important treatment option for many diseases. However, the spread of beta-lactam resistance continues to rise. Many pathogens acquire antibiotic resistance from environmental bacteria. Thus, understanding beta-lactam resistance in environmental strains may provide insights into additional mechanisms of antibiotic resistance. Here, we describe how a single regulatory system, sigma(P), in B. thuringiensis controls expression of multiple genes involved in resistance to beta-lactams. Our findings indicate that some of these genes are partially redundant. Our data also suggest that the large number of genes controlled by sigma(P) results in increased resistance to a wider range of beta-lactam classes than any single gene could provide.