The molecular basis for beta-lactamase gene expression in B. anthracis, B. cereus and B. thuringiensis

The susceptibility of most Bacillus anthracis strains to β-lactam antibiotics is intriguing considering that the B. anthracis genome harbors two β-lactamase genes, bla1 and bla2, and closely-related species, Bacillus cereus and Bacillus thuringiensis, typically produce β-lactamases. This work demonstrates that B. anthracis bla expression is affected by two genes, sigP and rsp, predicted to encode an extracytoplasmic function sigma factor and an antisigma factor, respectively. Deletion of the sigP/rsp locus abolished bla expression in a penicillin-resistant clinical isolate and had no effect on bla expression in a prototypical penicillin-susceptible strain. Complementation with sigP/rsp from the penicillin-resistant strain, but not the penicillin-susceptible strain, conferred β-lactamase activity upon both mutants. These results are attributed to a nucleotide deletion near the 5' end of rsp in the penicillin-resistant strain that is predicted to result in a nonfunctional protein. B. cereus and B. thuringiensis sigP and rsp homologues are required for inducible penicillin resistance in those species. Expression of the B. cereus or B. thuringiensis sigP and rsp genes in a B. anthracis sigP/rsp-null mutant confers resistance to β-lactam antibiotics, suggesting that while B. anthracis contains the genes necessary for sensing β-lactam antibiotics, the B. anthracis sigP/rsp gene products are insufficient for bla induction. ^ Because alternative sigma factors recognize unique promoter sequence, direct targets can be elucidated by comparing transcriptional profiling results with an in silico search using the sigma factor binding sequence. Potential σP -10 and -35 promoter elements were identified upstream from bla1 bla2 and sigP. Results obtained from searching the B. anthracis genome with the conserved sequences were evaluated against transcriptional profiling results comparing B. anthracis 32 and an isogenic sigP/rsp -null strain. Results from these analyses indicate that while the absence of the sigP gene significantly affects the transcript levels of 16 genes, only bla1, bla2 and sigP are directly regulated by σP. The genomes of B. cereus and B. thuringiensis strains were also analyzed for the potential σP binding elements. The sequence was located upstream from the sigP and bla genes, and previously unidentified genes predicted to encode a penicillin-binding protein (PBP) and a D-alanyl-D-alanine carboxypeptidase, indicating that the σ P regulon in these species responds to cell-wall stress caused by β-lactam antibiotics. ^ β-lactam antibiotics prevent attachment of new peptidoglycan to the cell wall by blocking the active site of PBPs. A B. cereus and B. thuringiensis pbp-encoding gene located near bla1 contains a potential σP recognition sequence upstream from the annotated translational start. Deletion of this gene abolished β-lactam resistance in both strains. Mutations in the active site of the PBP were detrimental to β-lactam resistance in B. cereus, but not B. thuringiensis, indicating that the transpeptidase activity is only important in B. cereus. I also found that transcript levels of the PBP-encoding gene are not significantly affected by the presence of β-lactam antibiotic. Based on these data I hypothesize that the gene product acts a sensor of β-lactam antibiotic. ^

Importance of antibiotic class to the development of Clostridium difficile infection (CDI) in adults: A systematic review

C. difficile causes gastrointestinal infections in humans, including severe diarrhea. It is implicated in 20%-30% of cases of antibiotic-associated diarrhea, in 50%-70% of cases of antibiotic-associated colitis, and in >90% of cases of antibiotic-associated pseudomembranous colitis. Exposure to antimicrobial agent, hospitalization and age are some of the risk factors that predispose to CDI. Virtually all hospitalized patients with nosocomially-acquired CDI have a history of treatment with antimicrobials or neoplastic agent within the previous 2 months. The development of CDI usually occurs during treatment with antibiotics or some weeks after completing the course of the antibiotics. ^ After exposure to the organism (often in a hospital), the median incubation period is less than 1 week, with a median time of onset of 2days. The difference in the time between the use of antibiotic and the development of the disease relate to the timing of exogenous acquisition of C. difficile. ^ This paper reviewed the literature for studies on different classes of antibiotics in association with the rates of primary CDI and RCDI from the year 1984 to 2012. The databases searched in this systematic review were: PubMed (National Library of Medicine) and Medline (R) (Ovid). RefWorks was used to store bibliographic data. ^ The search strategy yielded 733 studies, 692 articles from Ovid Medline (R) and 41 articles from PubMed after removing all duplicates. Only 11 studies were included as high quality studies. Out of the 11 studies reviewed, 6 studies described the development of CDI in non-CDI patients taking antibiotics for other purposes and 5 studies identified the risk factors associated with the development of recurrent CDI after exposure to antibiotics. ^ The risk of developing CDI in non-CDI patients receiving beta lactam antibiotics was 2.35%, while fluoroquinolones, clindamycin/macrolides and other antibiotics were associated with 2.64%, 2.54% and 2.35% respectively. Of those who received beta lactam antibiotic, 26.7% developed RCDI, while 36.8% of those who received any fluoroquinolone developed RCDI, 26.5% of those who received either clindamycin or macrolides developed RCDI and 29.1% of those who received other antibiotics developed RCDI. Continued use of non-C. difficile antibiotics especially fluoroquinolones was identified as an important risk factor for primary CDI and recurrent CDI. ^