Cloning and characterization of femA and femB from Staphylococcus epidermidis

Cloning, Purification, and Biophysical Characterization of FemB Protein from Methicillin-Resistant Staphylococcus aureus and Inhibitors Screening

Methicillin-resistant Staphylococcus aureus has emerged as a leading cause of nosocomial, community acquired infections worldwide. Earlier investigations revealed that mecA-encoded FEM proteins play a role in antimicrobial resistance by developing unique peptidoglycan cross-linking which helps in the formation of protective cell membrane. In view to this, present study focused on expression, purification FEM proteins, and FemB biophysical characterization with the aid of in silico and in vitro approaches. Furthermore, we carried out biological screening assays and identified the novel potent 1,2,3-triazole conjugated 1,3,4-oxadiazole hybrid molecule which could inhibit the MRSA than the proven oxacillin.

Roles of tRNA in cell wall biosynthesis

Recent research into various aspects of bacterial metabolism such as cell wall and antibiotic synthesis, degradation pathways, cellular stress, and amino acid biosynthesis has elucidated roles of aminoacyl-transfer ribonucleic acid (aa-tRNA) outside of translation. Although the two enzyme families responsible for cell wall modifications, aminoacyl-phosphatidylglycerol synthases (aaPGSs) and Fem, were discovered some time ago, they have recently become of intense interest for their roles in the antimicrobial resistance of pathogenic microorganisms. The addition of positively charged amino acids to phosphatidylglycerol (PG) by aaPGSs neutralizes the lipid bilayer making the bacteria less susceptible to positively charged antimicrobial agents. Fem transferases utilize aa-tRNA to form peptide bridges that link strands of peptidoglycan. These bridges vary among the bacterial species in which they are present and play a role in resistance to antibiotics that target the cell wall. Additionally, the formation of truncated peptides results in shorter peptide bridges and loss of branched linkages which makes bacteria more susceptible to antimicrobials. A greater understanding of the structure and substrate specificity of this diverse enzymatic family is necessary to aid current efforts in designing potential bactericidal agents. These two enzyme families are linked only by the substrate with which they modify the cell wall, aa-tRNA; their structure, cell wall modification processes and the physiological changes they impart on the bacterium differ greatly.

On the role of an unusual tRNAGly isoacceptor in Staphylococcus aureus

In the available Staphylococcus aureus genomes, four different genes have been annotated to encode tRNA(Gly) isoacceptors. Besides their prominent role in protein synthesis, some of them also participate in the formation of pentaglycine bridges during cell wall synthesis. However, until today, it is not known how many and which of them are actually involved in this essential procedure. In the present study we identified, apart from the four annotated tRNA(Gly) genes, a putative pseudogene which encodes and expresses an unusual fifth tRNA(Gly) isoacceptor in S. aureus (as detected via RT-PCR and subsequent direct sequencing analysis). All the in vitro transcribed tRNA(Gly) molecules (including the "pseudogene-encoded" tRNA(Gly)) can be efficiently aminoacylated by the recombinant S. aureus glycyl-tRNA synthetase. Furthermore, bioinformatic analysis suggests that the "pseudo"-tRNA(Gly(UCC)) identified in the present study and two of the annotated isoacceptors bearing the same anticodon carry specific sequence elements that do not favour the strong interaction with EF-Tu that proteinogenic tRNAs would promote. This observation was verified by the differential capacity of Gly-tRNA(Gly) molecules to form ternary complexes with activated S. aureus EF-Tu.GTP. These tRNA(Gly) molecules display high sequence similarities with their S. epidermidis orthologs which also actively participate in cell wall synthesis. Both bioinformatic and biochemical data suggest that in S. aureus these three glycylated tRNA(Gly) isoacceptors that are weak EF-Tu binders, possibly escape protein synthesis and serve as glycine donors for the formation of pentaglycine bridges that are essential for stabilization of the staphylococcal cell wall.

A Study of the Regulating Gene of femA from Methicillin-resistant Staphylococcus aureus Clinical Isolates

The regulating gene of femA was studied in methicillin-resistant Staphylococcus aureus (MRSA). High-level MRSA, low-level MRSA and methicillin-sensitive S. aureus (MSSA) were identified by agar diffusion. β-Lactamases were detected by nitrocephin and the presence of the mecA gene was determined by polymerase chain reaction (PCR). Only isolates that were both β-lactamase-negative and mecA-positive were used. The femA gene and its 250 base pair (bp) upstream sequence were amplified by PCR and expression was determined by real-time fluorescent quantitative PCR. The 250 bp upstream sequence was labelled by BrightStar€ Psoralen–Biotin and detected by electrophoretic mobility shift assay (EMSA). Expression levels of femA in MSSA, low-level MRSA and high-level MRSA were 3.53 × 10−3%–29.91%, 5.54 × 10−3%–3.1 × 102% and 13.88–5.50 × 104%, respectively. EMSA detected a signal shift in 57 high-level MRSA isolates but not in four low-level MRSA and four MSSA strains. Expression of femA in high-level MRSA (non-β-lactamase-producing) was higher than in low-level MRSA and MSSA. The femA regulating gene probably lies in the 250 bp upstream sequence in MRSA and high-level expression is essential for high-level methicillin resistance.

Rapid detection of methicillin-resistant Staphylococcus aureus directly from sterile or nonsterile clinical samples by a new molecular assay

A rapid procedure was developed for detection and identification of methicillin-resistant Staphylococcus aureus (MRSA) directly from sterile sites or mixed flora samples (e.g., nose or inguinal swabs). After a rapid conditioning of samples, the method consists of two main steps: (i) immunomagnetic enrichment in S. aureus and (ii) amplification-detection profile on DNA extracts using multiplex quantitative PCR (5'-exonuclease qPCR, TaqMan). The triplex qPCR assay measures simultaneously the following targets: (i) mecA gene, conferring methicillin resistance, common to both S. aureus and Staphylococcus epidermidis; (ii) femA gene from S. aureus; and (iii) femA gene from S. epidermidis. This quantitative approach allows discrimination of the origin of the measured mecA signal. qPCR data were calibrated using two reference strains (MRSA and methicillin-resistant S. epidermidis) processed in parallel to clinical samples. This 96-well format assay allowed analysis of 30 swab samples per run and detection of the presence of MRSA with exquisite sensitivity compared to optimal culture-based techniques. The complete protocol may provide results in less than 6 h (while standard procedure needs 2 to 3 days), thus allowing prompt and cost-effective implementation of contact precautions.

Cell wall targets in methicillin-resistant staphylococci

Multiresistant staphylococci pose an alarmingly growing problem, especially in serious hospital infections. The recent emergence of strains with reduced susceptibility against vancomycin, the last remaining drug effective against methicillin (multi) resistant Staphylococcus aureus, highlights the urgent need for new antimicrobial agents and new therapeutic regimen. Previously, new drugs were discovered exclusively in bacterial whole cell growth assays. Today's more rational approach depends on the identification of suitable target genes and proteins. These should be bacteria-specific and essential for growth either in vitro or in vivo. Targets within cell wall synthesis and remodeling pathways might be particularly attractive because the bacterial cell wall is a unique structure occurring only in prokaryots; many of the antibiotics in use today have confirmed its 'drugability'. However, several potential targets within this field have not yet been exploited successfully for anti-staphylococcal therapy and some were discovered only recently. After a short summary of known potential targets a set of genes involved in the pentaglycine interpeptide bridge formation of the staphylococcal cell wall will be introduced as interesting targets to combat multiresistant staphylococcal infections. Copyright 1999 Harcourt Publishers LtdCopyright DUMMY.

Molecular characterization of femA from Staphylococcus hominis and Staphylococcus saprophyticus, and femA-based discrimination of staphylococcal species

The femA gene encodes a protein precursor which plays a role in peptidoglycan biosynthesis in Staphylococcus aureus and is also considered as a factor influencing the level of methicillin resistance. A femA homologous gene was recently characterized in S. epidermidis, entailing the possibility of femA phylogenetic conservation in staphylococcal species. Accordingly, we assessed the presence of femA homologous genes in S. hominis and S. saprophyticus. Strategy for identification relied upon alignment of S. aureus and D. epidermidis femA sequences and upon identification of potentially conserved regions. Amplifications of portions of the femA genes were performed under permissive annealing conditions, by using several sets of primers designed to match the consensus regions. DNA sequencing of overlapping PCR fragments led to the characterization of the entire femA genes of S. hominis and S. saprophyticus, and provided more precise information on the femA start codon for all five species. The genomic organization of all these femA genes appeared highly conserved, with alternance of homologous and variable regions. On this basis, a consensus sequence of the femA gene was defined and interspecies variations were exploited to design strategies for staphylococci species-specific identification, including multiplex PCR amplification and a reverse hybridization assay.

Increased overall antibiotic susceptibility in Staphylococcus aureus femAB null mutants

The staphylococcal pentaglycine side chain of the peptidoglycan is reduced to one glycine in femAB null mutants. This is associated with increased susceptibility to methicillin and to a whole range of unrelated antibiotics as well. Genetic evidence suggests that femAB null mutants are only viable because of a compensatory mutation in an unlinked site.

Specificities of FemA and FemB for different glycine residues: FemB cannot substitute for FemA in staphylococcal peptidoglycan pentaglycine side chain formation

The femAB operon codes for two nearly identical approximately 50-kDa proteins involved in the formation of the staphylococcal pentaglycine interpeptide bridge. Sequencing and analysis of the femA region of mutants isolated by chemical mutagenesis and selection for lysostaphin resistance revealed point mutations leading to the expression of truncated FemA proteins. These femA mutants, although still producing an intact FemB, exhibited a phenotype identical as that described for femAB double mutants. Thus, FemA seems to be essential for the addition of glycine residues 2 and 3 only, whereas FemB is involved in the attachment of exclusively glycine residues 4 and 5. Although FemB has 39% identity with FemA, it cannot substitute for FemA. The FemA and FemB proteins seem to be highly specific in regard to the position of the glycine residues that they attach.

Effects of various types of Triton X on the susceptibilities of methicillin-resistant staphylococci to oxacillin

We examined the effect of six types of the nonionic detergent Triton X on the susceptibilities of methicillin-resistant staphylococci to oxacillin. We used five methicillin-resistant Staphylococcus aureus isolates and 17 methicillin-resistant coagulase-negative staphylococci isolates. All strains of S. aureus, S. epidermidis and S. sciuri had enhanced susceptibility to oxacillin following exposure to the types of Triton X having 7-13 polymerized ethylene oxides. These strains were altered from homogeneously resistant to heterogeneously resistant by Triton X-100. Those types of Triton X that affected the resistance level also promoted the release of lipoteichoic acid. These results and those of previous studies suggest that Triton X might act on factors other than the mecA or femA products.