Cloning and characterization of a gene, pbpF, encoding a new penicillin-binding protein, PBP2B, in Staphylococcus aureus

Antistaphylococcal activity of the novel cephalosporin CB-181963 (CAB-175)

OBJECTIVES

We examined the antistaphylococcal activity of the novel cephalosporin CB-181963 (formerly known as CAB-175), with emphasis on its microbiological activity and penicillin-binding protein specificities.

METHODS

Using established procedures, we examined the activity of CB-181963 against methicillin-susceptible (MSSA) and methicillin-resistant (MRSA) strains of Staphylococcus aureus in both planktonic and biofilm culture. We also determined whether CB-181963 exhibited a post-antibiotic effect (PAE). A radioactive competition assay with (3)H-labelled benzylpenicillin was used to determine penicillin-binding protein (PBP) affinities of CB-181963, including binding to PBP2a from MRSA. The potential for emergence of CB-181963-resistant mutants in MSSA and MRSA strains was examined using plating procedures.

RESULTS

CB-181963 showed excellent activity against MRSA strains resistant to other cephalosporins in both planktonic and biofilm cultures. However, in common with other cephalosporins it was unable to eradicate biofilms. CB-181963 had a short PAE compared with other beta-lactam antibiotics. CB-181963 retained activity against a strain expressing type A beta-lactamase and demonstrated affinity for PBP2a of MRSA. Mutants resistant to CB-181963 were not recovered in either MSSA or MRSA.

CONCLUSIONS

CB-181963 is a potent antistaphylococcal agent with better activity against MRSA than other cephalosporins. The anti-MRSA activity is correlated with elevated binding to PBP2a. CB-181963 may have a role in the treatment of staphylococcal infections, including those caused by MRSA and in the prophylaxis of biofilm-associated MSSA and MRSA infections. However, because of its short PAE, CB-181963 may have to be administered more frequently than other beta-lactam antibiotics, or given via prolonged infusion.

The gate controlling cell wall synthesis in Staphylococcus aureus

Glucosamine-6-P occupies a central position between cell wall synthesis and glycolysis. In the initial steps leading to peptidoglycan precursor formation glucosamine-6-P is processed sequentially to UDP-N-acetylglucosamine, while to enter the glycolysis pathway, glucosamine-6-P is isomerized by NagB to fructose-6-P. Although we could not demonstrate NagB activity, nagB inactivation significantly reduced growth. Mutational analysis showed that NagA was involved in glucosamine-6-P formation from N-acetylglucosamine-6-P, and GlmS in that from fructose-6-P. Inactivation of glmS prevented growth on glucose as sole carbon source, which resumed after complementation with N-acetylglucosamine. Transcription of glmS as well as the amount of GlmS was reduced in the presence of N-acetylglucosamine. This and the preferential incorporation of N-acetylglucosamine over glucose into cell wall material showed that N-acetylglucosamine was used exclusively for cell wall synthesis, while glucose served both cell wall synthesis and glycolysis. These observations suggest furthermore GlmS to be the key and only enzyme leading from glucose to cell wall synthesis in Staphylococcus aureus, and show that there exists a tight regulation and hierarchy in sugar utilization. Inactivation of nagA, nagB or glmS affected the susceptibility of S. aureus to cell wall synthesis inhibitors, suggesting an interdependence between efficiency of cell wall precursor formation and resistance levels.

Transcriptional induction of the penicillin-binding protein 2 gene in Staphylococcus aureus by cell wall-active antibiotics oxacillin and vancomycin

We found an increased abundance of pbpB-specific transcripts in vancomycin intermediate-resistant Staphylococcus aureus (VISA) isolates compared with that found in paired, genetically identical, susceptible isolates. This difference in expression cannot be explained by differences in the pbpB promoter sequence. Since the factors controlling pbpB gene expression have remained largely unexplored, various conditions that might affect pbpB transcript abundance were examined. In both vancomycin-susceptible and VISA strains, pbpB expression varied with the growth phase, with the highest abundance of pbpB-specific transcripts detected during mid-log phase. Interestingly, both vancomycin and oxacillin were able to induce pbpB transcription above a constitutive level. When vancomycin was absent, one of the three pbpB-specific transcripts that were usually faintly detected in non-VISA strains was more readily detected in VISA strains during mid-log but not stationary phase. This transcript was enhanced in non-VISA strains by vancomycin induction. Gel shift assays indicated that an increased amount of the putative transcription factor that binds to both P1 and P1' promoter regions is present in the cytosol of vancomycin-induced cells. Neither the SigB sigma factor nor the quorum-sensing agr locus was required for growth phase-variable pbpB expression or transcriptional induction of pbpB by vancomycin or oxacillin. Also, MecI, MecR1, BlaI, and BlaR1, regulatory proteins that mediate beta-lactam-inducible expression of mecA and beta-lactamase, were not required for antibiotic induction of pbpB transcription. These data support the idea that pbpB expression is modulated by a trans-acting factor in response to the presence of the cell wall-active antibiotics vancomycin and oxacillin.

Identification of six major outer membrane proteins from Actinobacillus actinomycetemcomitans

We have identified six major sarcosyl-insoluble outer membrane proteins (Omp) of Actinobacillus actinomycetemcomitans Y4, and designated them as Omp100, Omp64, Omp39, Omp29, Omp18 and Omp16 according to the molecular mass. A similar N-terminal sequence was found in the first 15 amino acid residues of Omp16 and Omp18. The N-terminal sequence of Omp29 matched perfectly with the sequence previously identified. We cloned and determined the DNA sequences of three complete genes encoding Omp100, Omp64 and Omp18/16, and one incomplete gene encoding Omp39. Each Omp revealed homologies with some bacterial virulence factors responsible for adhesion, invasion, serum resistance, or protein antigenicity. Serum from patients with periodontitis suspected to be related to A. actinomycetemcomintans infection strongly reacted with Omp100, Omp29 and Omp16 as did serum from mice immunized with A. actinomycetemcomitans Y4 whole bacteria. These findings suggest that Omps of A. actinomycetemcomitans can be associated with periodontal disease.

Increased glycan chain length distribution and decreased susceptibility to moenomycin in a vancomycin-resistant Staphylococcus aureus mutant

A vancomycin-resistant Staphylococcus aureus mutant, COL-VR1 (MIC, 16 microg/ml), was isolated from methicillin-resistant S. aureus COL by exposure to vancomycin. COL-VR1 also showed decreased susceptibility to teicoplanin (8-fold), methicillin (2-fold), macarbomycin (8-fold), and moenomycin (16-fold). Macarbomycin and moenomycin are thought to directly inhibit transglycosylase activity. Characterization of the mutant revealed a thickened cell wall and suppression of penicillin-induced lysis, although the amounts of the five penicillin-binding proteins (PBPs 1, 2, 3, 4, and 2') and the profiles of peptidoglycan hydrolases were not altered. Analysis of muropeptide profile and glycan chain length distribution by reversed-phase high-pressure liquid chromatography revealed slightly decreased peptide cross-linking and an increased average glycan chain length compared to those of the parent. These results together suggest that a transglycosylase activity was enhanced in the mutant. This may represent a novel mechanism of glycopeptide resistance in S. aureus.

Cloning and sequencing of the gene, fmtC, which affects oxacillin resistance in methicillin-resistant Staphylococcus aureus

Two Tn551 insertional mutants with reduced methicillin resistance were isolated from methicillin-resistant Staphylococcus aureus KSA8. These two mutants showed increased susceptibility to beta-lactam antibiotics and bacitracin, but not to fosfomycin and vancomycin. Tn551 in these mutants was inserted into the same gene, termed fmtC. The fmtC gene has an open reading frame of 840 amino acid residues with an estimated molecular mass of 96.9 kDa. The N-terminal half of the deduced FmtC protein is very hydrophobic, implying that this protein is a membrane-associated protein.

Cloning, characterization, and inactivation of the gene pbpC, encoding penicillin-binding protein 3 of Staphylococcus aureus

The gene pbpC from Staphylococcus aureus was sequenced: it encodes a 691-amino-acid protein with all of the conserved motifs of a class B high-molecular-weight penicillin-binding protein (PBP), including the transpeptidase conserved motifs SXXK, SXN, and KTG. Insertional inactivation of pbpC and introduction of the intact gene in a laboratory mutant missing PBP 3 showed that the pbpC gene encodes the staphylococcal PBP 3. Inactivation of pbpC caused no detectable change in the muropeptide composition of cell wall peptidoglycan and had only minimum, if any, effect on growth rates, but caused a small but significant decrease in rates of autolysis. Cells of abnormal size and shape and disoriented septa were produced when bacteria with inactivated pbpC were grown in the presence of a sub-MIC of methicillin.

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.