Inducible AmpC beta-lactamase of a new member Enterobacteriaceae

Class C β-Lactamases: Molecular Characteristics

Class C β-lactamases or cephalosporinases can be classified into two functional groups (1, 1e) with considerable molecular variability (≤20% sequence identity). These enzymes are mostly encoded by chromosomal and inducible genes and are widespread among bacteria, including Proteobacteria in particular. Molecular identification is based principally on three catalytic motifs (⁶⁴SXSK, ¹⁵⁰YXN, ³¹⁵KTG), but more than 70 conserved amino-acid residues (≥90%) have been identified, many close to these catalytic motifs. Nevertheless, the identification of a tiny, phylogenetically distant cluster (including enzymes from the genera Legionella, Bradyrhizobium, and Parachlamydia) has raised questions about the possible existence of a C2 subclass of β-lactamases, previously identified as serine hydrolases. In a context of the clinical emergence of extended-spectrum AmpC β-lactamases (ESACs), the genetic modifications observed in vivo and in vitro (point mutations, insertions, or deletions) during the evolution of these enzymes have mostly involved the Ω- and H-10/R2-loops, which vary considerably between genera, and, in some cases, the conserved triplet ¹⁵⁰YXN. Furthermore, the conserved deletion of several amino-acid residues in opportunistic pathogenic species of Acinetobacter, such as A. baumannii, A. calcoaceticus, A. pittii and A. nosocomialis (deletion of residues 304-306), and in Hafnia alvei and H. paralvei (deletion of residues 289-290), provides support for the notion of natural ESACs. The emergence of higher levels of resistance to β-lactams, including carbapenems, and to inhibitors such as avibactam is a reality, as the enzymes responsible are subject to complex regulation encompassing several other genes (ampR, ampD, ampG, etc.). Combinations of resistance mechanisms may therefore be at work, including overproduction or change in permeability, with the loss of porins and/or activation of efflux systems.

Combination Therapy of Sophoraflavanone B against MRSA: In Vitro Synergy Testing

Sophoraflavanone B (SPF-B), a known prenylated flavonoid, was isolated from the roots of Desmodium caudatum. The aim of this study was to determine the antimicrobial synergism of SPF-B combined with antibiotics against methicillin-resistant Staphylococcus aureus (MRSA). MRSA, a multidrug-resistant pathogen, causes both hospital- and community-acquired infections worldwide. The antimicrobial activity of SPF-B was assessed by the broth microdilution method, checkerboard dilution test, and time-kill curve assay. The MIC of SPF-B for 7 strains of S. aureus ranges from 15.6 to 31.25 μg/mL determined. In the checkerboard method, the combinations of SPF-B with antibiotics had a synergistic effect; SPF-B markedly reduced the MICs of the β-lactam antibiotics: ampicillin (AMP) and oxacillin (OXI); aminoglycosides gentamicin (GET); quinolones ciprofloxacin (CIP) and norfloxacin (NOR) against MRSA. The time-kill curves assay showed that a combined SPF-B and selected antibiotics treatment reduced the bacterial counts below the lowest detectable limit after 24 h. These data suggest that the antibacterial activity of SPF-B against MRSA can be effectively increased through its combination with three groups of antibiotics (β-lactams, aminoglycosides, and quinolones). Our research can be a valuable and significant source for the development of a new antibacterial drug with low MRSA resistance.

The role of AmpR in regulation of L1 and L2 beta-lactamases in Stenotrophomonas maltophilia

Stenotrophomonas maltophilia is known to produce at least two chromosomal-mediated inducible beta-lactamases, L1 and L2. Gene L2, which encodes a class A beta-lactamase, and the adjacent ampR gene form an ampR-class A beta-lactamase module. L1 belongs to the class B beta-lactamase and has no neighbor ampR-like regulatory gene. In this study, the ampR-L2 module from S. maltophilia KH was compared with ampR-beta-lactamase modules from several microorganisms with respect to the AmpR and beta-lactamase proteins and the intergenic (IG) region. S. maltophilia and Xanthomonas campestris showed the most closely phylogenetic relationship among the microorganisms considered. The regulatory role of AmpR towards L1 and L2 was further analyzed. In the absence of an inducer, AmpR acted as an activator for L1 expression and as a repressor for L2 expression, whereas AmpR was an activator for both genes in an induced state. In addition, inducibility of L1 and L2 genes depended on the presence of AmpR. The ampR transcript was weakly and constitutively expressed, but was not autoregulated.

ISCR elements: novel gene-capturing systems of the 21st century?

"Common regions" (CRs), such as Orf513, are being increasingly linked to mega-antibiotic-resistant regions. While their overall nucleotide sequences show little identity to other mobile elements, amino acid alignments indicate that they possess the key motifs of IS91-like elements, which have been linked to the mobility ent plasmids in pathogenic Escherichia coli. Further inspection reveals that they possess an IS91-like origin of replication and termination sites (terIS), and therefore CRs probably transpose via a rolling-circle replication mechanism. Accordingly, in this review we have renamed CRs as ISCRs to give a more accurate reflection of their functional properties. The genetic context surrounding ISCRs indicates that they can procure 5' sequences via misreading of the cognate terIS, i.e., "unchecked transposition." Clinically, the most worrying aspect of ISCRs is that they are increasingly being linked with more potent examples of resistance, i.e., metallo-beta-lactamases in Pseudomonas aeruginosa and co-trimoxazole resistance in Stenotrophomonas maltophilia. Furthermore, if ISCR elements do move via "unchecked RC transposition," as has been speculated for ISCR1, then this mechanism provides antibiotic resistance genes with a highly mobile genetic vehicle that could greatly exceed the effects of previously reported mobile genetic mechanisms. It has been hypothesized that bacteria will surprise us by extending their "genetic construction kit" to procure and evince additional DNA and, therefore, antibiotic resistance genes. It appears that ISCR elements have now firmly established themselves within that regimen.

Correlation of quinolone resistance levels and differences in basal and quinolone-induced expression from three qnrA-containing plasmids

This study investigated the effect of copy number and transcriptional level of the qnrA gene on plasmid-mediated quinolone resistance, and the effect of quinolones on qnrA expression, in three clinical isolates of Klebsiella pneumoniae and the corresponding Escherichia coli transconjugants. The copy number of plasmids containing qnrA was analysed and transcriptional studies were performed on transconjugants grown in the presence or absence of ciprofloxacin or moxifloxacin. None of the three clinical isolates was porin-deficient. One isolate contained a mutation in the quinolone resistance-determining region of the gyrA gene (Ser83Phe), but no gyrA mutations were present in the other two isolates, and no mutations were found in parC. Differences in qnrA copy number were observed in K. pneumoniae, but not in the corresponding E. coli transconjugants, although the qnrA gene was located in plasmids with similar mobility and Southern blot RFLP pattern. Differences in qnrA transcription, both at the basal level and following induction by quinolones, were observed among transconjugants. Expression of the qnrA gene correlated well with the level of quinolone (ciprofloxacin and moxifloxacin) resistance in E. coli transconjugants. These data suggest that the main factor determining the resistance level in the transconjugants analysed was the different levels of qnrA expression.

First case of septicemia due to a strain belonging to enteric group 58 (Enterobacteriaceae) and its designation as Averyella dalhousiensis gen. nov., sp. nov., based on analysis of strains from 20 additional cases

When enteric group 58 was first described as a distinct new group of Enterobacteriaceae in 1985, there were only five known human isolates: four from wounds and one from feces. In 1996, we investigated the first blood isolate of enteric group 58, a case of sepsis in a 33-year-old woman receiving total parenteral nutrition. Fifteen additional clinical isolates have since been identified at CDC, including several recognized from a collection of "unidentified" strains dating back to 1973. All strains were characterized with a standard set of 49 biochemical tests used for Enterobacteriaceae, and the results were analyzed to determine phenotypic relatedness and best taxonomic fit. Antibiograms were determined as a taxonomic tool. Original identifications provided by submitting laboratories encompassed a wide variety of Enterobacteriaceae, including 14 species in eight genera, the most common being Enterobacter spp., Salmonella spp., Serratia spp., Kluyvera spp., or Escherichia spp. Enteric group 58 strains have been most frequently isolated from traumatic injuries, fractures, and wounds and rarely from feces. Defining its clinical significance and distinguishing infection from colonization requires further study, but our case report indicates that serious systemic infection can occur. The vernacular name enteric group 58 was used from 1985 to 2004. In this paper, we formally name it Averyella dalhousiensis gen. nov., sp. nov., on the basis of its unique phenotype and its unique 16S rRNA gene sequence. These data indicate that enteric group 58 is not closely related to any of the existing genera or species of Enterobacteriaceae. The type strain is designated CDC 9501--97, and a phenotypic definition is given based on all 21 strains.