Regardless of their targets and modes of action, subinhibitory concentrations of antibiotics can have an impact on cell physiology and trigger a large variety of cellular responses in different bacterial species. Subinhibitory concentrations of β-lactam antibiotics cause reactive oxygen species production and induce PolIV-dependent mutagenesis in Escherichia coli. Here we show that subinhibitory concentrations of β-lactam antibiotics induce the RpoS regulon. RpoS-regulon induction is required for PolIV-dependent mutagenesis because it diminishes the control of DNA-replication fidelity by depleting MutS in E. coli, Vibrio cholerae and Pseudomonas aeruginosa. We also show that in E. coli, the reduction in mismatch-repair activity is mediated by SdsR, the RpoS-controlled small RNA. In summary, we show that mutagenesis induced by subinhibitory concentrations of antibiotics is a genetically controlled process. Because this mutagenesis can generate mutations conferring antibiotic resistance, it should be taken into consideration for the development of more efficient antimicrobial therapeutic strategies.

Bacterial infections caused by antibiotic-resistant isolates have become a major health problem in recent years, since they are very difficult to treat, leading to an increase in morbidity and mortality. Fosfomycin is a broad-spectrum bactericidal antibiotic that inhibits cell wall biosynthesis in both Gram-negative and Gram-positive bacteria. This antibiotic has a unique mechanism of action and inhibits the initial step in peptidoglycan biosynthesis by blocking the enzyme, MurA. Fosfomycin has been used successfully for the treatment of urinary tract infections for a long time, but the increased emergence of antibiotic resistance has made fosfomycin a suitable candidate for the treatment of infections caused by multidrug-resistant pathogens, especially in combination with other therapeutic partners. The acquisition of fosfomycin resistance could threaten the reintroduction of this antibiotic for the treatment of bacterial infection. Here, we analyse the mechanism of action and molecular mechanisms for the development of fosfomycin resistance, including the modification of the antibiotic target, reduced antibiotic uptake and antibiotic inactivation. In addition, we describe the role of each pathway in clinical isolates.

Triggering receptors expressed on myeloid cell (TREM) proteins are a family of cell surface receptors that participate in diverse cellular processes such as inflammation, coagulation, and bone homeostasis. TREM-1, in particular, is expressed on neutrophils and monocytes and is a potent amplifier of inflammatory responses. LPS and other microbial products induce up-regulation of cell surface-localized TREM-1 and the release of its soluble form, sTREM-1. Two hypotheses have been advanced to explain the origin of sTREM-1: alternative splicing of TREM-1 mRNA and proteolytic cleavage(s) of mature, membrane-anchored TREM-1. In this report, we present conclusive evidence in favor of the proteolytic mechanism of sTREM-1 generation. No alternative splicing forms of TREM-1 were detected in monocytes/macrophages. Besides, metalloproteinase inhibitors increased the stability of TREM-1 at the cell surface while significantly reducing sTREM-1 release in cultures of LPS-challenged human monocytes and neutrophils. We conclude that metalloproteinases are responsible for shedding of the TREM-1 ectodomain through proteolytic cleavage of its long juxtamembrane linker.

Low concentrations of some antibiotics have been reported to stimulate mutagenesis and recombination, which may facilitate bacterial adaptation to different types of stress, including antibiotic pressure. However, the mutagenic effect of most of the currently used antibiotics remains untested. Furthermore, it is known that in many bacteria, including Escherichia coli, stimulation of mutagenesis is mediated by the SOS response. Thus, blockage or attenuation of this response through the inhibition of RecA has been proposed as a possible therapeutic adjuvant in combined therapy to reduce the ability to generate antibiotic-resistant mutants. The aim of this work was to study the capacity of sublethal concentrations of antimicrobials of different families with different molecular targets to increase the mutant frequency of E. coli, and the effect that inactivation of recA would have on antibiotic-mediated mutagenesis. We tested the mutagenicity of the following antimicrobials: ampicillin; ceftazidime; imipenem; fosfomycin; ciprofloxacin; trimethoprim; sulfamethoxazole; trimethoprim/sulfamethoxazole; colistin; tetracycline; gentamicin; rifampicin; and chloramphenicol. Eight out of the 13 antimicrobials tested stimulate E. coli mutagenesis (slightly in most cases), with trimethoprim, alone or in combination with sulfamethoxazole, producing the highest effect. Inactivation of recA abolishes the mutagenic effect and also produces increased susceptibility to some of the tested antimicrobials. The fact that inactivation of recA reduces mutagenicity and/or increases the activity of a large number of antimicrobials supports the hypothesis that RecA inhibition might have favourable effects on antibiotic therapy.