Reduced susceptibility to vancomycin and biofilm formation in methicillin-resistant Staphylococcus epidermidis isolated from blood cultures

This study aimed to correlate the presence of ica genes, biofilm formation and antimicrobial resistance in 107 strains of Staphylococcus epidermidis isolated from blood cultures. The isolates were analysed to determine their methicillin resistance, staphylococcal cassette chromosome mec (SCCmec) type, ica genes and biofilm formation and the vancomycin minimum inhibitory concentration (MIC) was measured for isolates and subpopulations growing on vancomycin screen agar. The mecA gene was detected in 81.3% of the S. epidermidis isolated and 48.2% carried SCCmec type III. The complete icaADBC operon was observed in 38.3% of the isolates; of these, 58.5% produced a biofilm. Furthermore, 47.7% of the isolates grew on vancomycin screen agar, with an increase in the MIC in 75.9% of the isolates. Determination of the MIC of subpopulations revealed that 64.7% had an MIC ≥ 4 μg mL^-1, including 15.7% with an MIC of 8 μg mL^-1 and 2% with an MIC of 16 μg mL^-1. The presence of the icaADBC operon, biofilm production and reduced susceptibility to vancomycin were associated with methicillin resistance. This study reveals a high level of methicillin resistance, biofilm formation and reduced susceptibility to vancomycin in subpopulations of S. epidermidis. These findings may explain the selection of multidrug-resistant isolates in hospital settings and the consequent failure of antimicrobial treatment.     

Biofilm formation by ica-positive and ica-negative strains of Staphylococcus epidermidis in vitro

Staphylococcus epidermidis is a clinically important opportunistic pathogen that forms biofilm infections on nearly all types of indwelling medical devices. The biofilm forming capability of S. epidermidis has been linked to the presence of the ica operon in the genome, and the amount of biofilm formation measured by the crystal violet (CV) adherence assay. Six S. epidermidis strains were characterized for their ica status using PCR, and their biofilm forming ability over 6 days, using the CV assay and a flow cell system. Ica-negative strains characterized as ‘negative for biofilm formation’ based on the CV assay were demonstrated to form strongly attached biofilms after 6 days. However, the biofilms were not as extensive as the ica-positive strains. It was concluded that ica is not required for biofilm formation, nor is the 24-h CV assay generalizable for predicting the 6-day biofilm-forming ability for all S. epidermidis strains.     

In vitro effect of pH and ethanol on biofilm formation by clinicalica-positiveStaphylococcus epidermidis strains

Biofilm production is an important step in the pathogenesis ofStaphylococcus epidermidis associated biomaterial infections.Staphylococcus epidermidis strains isolated from dialysis fluid (n=9) and needle cultures (n=14) were phenotyped and genotyped for extracellular polysaccharide production and were examined for their ability to produce slime in a medium at various pH levels (3, 5, 7, 9 and 12) and with ethanol supplementation (0, 2, 5, 10 and 15%) using a semi-quantitative adherence assay. A total of 23 clinicalicaADBC positiveS. epidermidis, one reference strain (S. epidermidis CIP 106510) used as positive control, and oneicaADBC negative strain (E21) were investigated. Qualitative biofilm production analysis revealed that 15 of the 23icaADBC positive strains (65.21%) produced slime on Congo Red agar plates. Quantitative biofilm was determined by measuring the optical density at 570 nm (OD₅₇₀). The results show that the slime production depended on the pH value of the medium and the ethanol concentration. At highly acidic (pH 3) and alkaline (pH 12) levels, the OD₅₇₀ was lower, while at pH 7 the adhesion was moderate. In addition the cells adhered strongly with 2% ethanol than with the other concentrations. Our results suggest that pH and ethanol were stress factors that led toS. epidermidis biofilm formation and also play a possible role in the pathogenesis of biomaterial-related infections.     

The Possible Role of Staphylococcus epidermidis LPxTG Surface Protein SesC in Biofilm Formation

Staphylococcus epidermidis is the most common cause of device-associated infections. It has been shown that active and passive immunization in an animal model against protein SesC significantly reduces S. epidermidis biofilm-associated infections. In order to elucidate its role, knock-out of sesC or isolation of S. epidermidis sesC-negative mutants were attempted, however, without success. As an alternative strategy, sesC was introduced into Staphylococcus aureus 8325–4 and its isogenic icaADBC and srtA mutants, into the clinical methicillin-sensitive S. aureus isolate MSSA4 and the MRSA S. aureus isolate BH1CC, which all lack sesC. Transformation of these strains with sesC i) changed the biofilm phenotype of strains 8325–4 and MSSA4 from PIA-dependent to proteinaceous even though PIA synthesis was not affected, ii) converted the non-biofilm-forming strain 8325–4 ica::tet to a proteinaceous biofilm-forming strain, iii) impaired PIA-dependent biofilm formation by 8325–4 srtA::tet, iv) had no impact on protein-mediated biofilm formation of BH1CC and v) increased in vivo catheter and organ colonization by strain 8325–4. Furthermore, treatment with anti-SesC antibodies significantly reduced in vitro biofilm formation and in vivo colonization by these transformants expressing sesC. These findings strongly suggest that SesC is involved in S. epidermidis attachment to and subsequent biofilm formation on a substrate.     

Regulation of the intercellular adhesin locus regulator (icaR) by SarA, sigmaB, and IcaR in Staphylococcus aureus

The staphylococcal accessory regulator SarA and the alternative sigma factor σ^B have been previously identified as positive regulators, and IcaR as a negative regulator, of icaADBC expression. Here, we show that in Staphylococcus aureus SarA and σ^B are also required for icaR expression and that IcaR does not have a significant effect on its own expression.     

Antimicrobial activity of tigecycline alone or in combination with rifampin against Staphylococcus epidermidis in biofilm

Staphylococcus epidermidis is a commensal inhabitant of the healthy human skin, but in the recent years, it has been recognized as a nosocomial pathogen especially in immunocompromised patients. The pathogenesis of S. epidermidis is thought to be based on its capacity to form biofilms on the surface of medical devices, where bacterial cells may persist, protected from host defence and antimicrobial agents. Rifampin has been shown to be one of the most active antimicrobial agents in the eradication of the staphylococcal biofilm. However, this antibiotic should not be used in monotherapy. Therefore, one of the objectives of our research was to study the efficacy of the tigecycline/rifampin combination against methicillin-resistant S. epidermidis embedded in biofilms. Of the 80 clinically significant S. epidermidis isolates, 75 strains possess the ability to form a biofilm. These bacteria formed the biofilm via ica-dependent mechanisms. However, other biofilm-associated genes, including aap (encoding accumulation-associated protein) and bhp (coding cell wall-associated protein), were present in 85 and 29 % of isolates, respectively. The biofilm structures of S. epidermidis strains were also analyzed in confocal laser scanning microscopy (CLSM) and the obtained image demonstrated differences in their architecture. In vitro studies showed that the MIC value for tigecycline against S. epidermidis growing in the biofilm ranged from 0.125 to 2 μg/mL. Tigecycline in combination with rifampin demonstrated higher activity against bacteria embedded in biofilms than tigecycline alone.     

Biofilm density and detection of biofilm-producing genes in methicillin-resistant Staphylococcus aureus strains

Many serious diseases caused by Staphylococcus aureus appear to be associated with biofilms. Therefore, we investigated the biofilm-forming ability of the methicillin-resistant S. aureus (MRSA) isolates collected from hospitalized patients. As many as 96?% strains had the ability to form biofilm in vitro. The majority of S. aureus strains formed biofilm in ica-dependent mechanism. However, 23?% of MRSA isolates formed biofilm in ica-independent mechanism. Half of these strains carried fnbB genes encoding surface proteins fibronectin-binding protein B involved in intercellular accumulation and biofilm development in S. aureus strains. The biofilm structures were examined via confocal laser scanning microscopy (CLSM) and three-dimensional structures were reconstructed. The images obtained in CLSM revealed that the biofilm created by ica-positive strains was different from biofilm formed by ica-negative strains. The MRSA population showed a large genetic diversity and we did not find a single clone that occurred preferentially in hospital environment. Our results demonstrated the variation in genes encoding adhesins for the host matrix proteins (elastin, laminin, collagen, fibronectin, and fibrinogen) and in the gene involved in biofilm formation (icaA) within the majority of S. aureus clones.     

The metalloprotease SepA governs processing of accumulation‐associated protein and shapes intercellular adhesive surface properties in Staphylococcus epidermidis

The otherwise harmless skin inhabitant Staphylococcus epidermidis is a major cause of healthcare‐associated medical device infections. The species' selective pathogenic potential depends on its production of surface adherent biofilms. The Cell wall‐anchored protein Aap promotes biofilm formation in S. epidermidis, independently from the polysaccharide intercellular adhesin PIA. Aap requires proteolytic cleavage to act as an intercellular adhesin. Whether and which staphylococcal proteases account for Aap processing is yet unknown. Here, evidence is provided that in PIA‐negative S. epidermidis 1457Δica, the metalloprotease SepA is required for Aap‐dependent S. epidermidis biofilm formation in static and dynamic biofilm models. qRT‐PCR and protease activity assays demonstrated that under standard growth conditions, sepA is repressed by the global regulator SarA. Inactivation of sarA increased SepA production, and in turn augmented biofilm formation. Genetic and biochemical analyses demonstrated that SepA‐related induction of biofilm accumulation resulted from enhanced Aap processing. Studies using recombinant proteins demonstrated that SepA is able to cleave the A domain of Aap at residue 335 and between the A and B domains at residue 601. This study identifies the mechanism behind Aap‐mediated biofilm maturation, and also demonstrates a novel role for a secreted staphylococcal protease as a requirement for the development of a biofilm.     

A transposase-independent mechanism gives rise to precise excision of IS256 from insertion sites in Staphylococcus epidermidis

The mobile element IS256 causes phase variation of biofilm formation in Staphylococcus epidermidis by insertion and precise excision from the icaADBC operon. Precise excision, i.e., removal of the target site duplications (TSDs) and restoration of the original DNA sequence, occurs rarely but independently of functional transposase. Instead, the integrity of the TSDs is crucial for precise excision. Excision increased significantly when the TSDs were brought into closer spatial proximity, suggesting that excision is a host-driven process that might involve most likely illegitimate recombination.     

Antibiotic Susceptibility of Biofilm Cells and Molecular Characterisation of Staphylococcus hominis Isolates from Blood

Objectives

We aimed to characterise the staphylococcal cassette chromosome mec (SCCmec) type, genetic relatedness, biofilm formation and composition, icaADBC genes detection, icaD expression, and antibiotic susceptibility of planktonic and biofilm cells of Staphylococcus hominis isolates from blood.

Methods

The study included 67 S. hominis blood isolates. Methicillin resistance was evaluated with the cefoxitin disk test. mecA gene and SCCmec were detected by multiplex PCR. Genetic relatedness was determined by pulsed-field gel electrophoresis. Biofilm formation and composition were evaluated by staining with crystal violet and by detachment assay, respectively; and the biofilm index (BI) was determined. Detection and expression of icaADBC genes were performed by multiplex PCR and real-time PCR, respectively. Antibiotic susceptibilities of planktonic cells (minimum inhibitory concentration, MIC) and biofilm cells (minimum biofilm eradication concentration, MBEC) were determined by the broth dilution method.

Results

Eighty-five percent (57/67) of isolates were methicillin resistant and mecA positive. Of the mecA-positive isolates, 66.7% (38/57) carried a new putative SCCmec type. Four clones were detected, with two to five isolates each. Among all isolates, 91% (61/67) were categorised as strong biofilm producers. Biofilm biomass composition was heterogeneous (polysaccharides, proteins and DNA). All isolates presented the icaD gene, and 6.66% (1/15) isolates expressed icaD. This isolate presented the five genes of ica operon. Higher BI and MBEC values than the MIC values were observed for amikacin, vancomycin, linezolid, oxacillin, ciprofloxacin, and chloramphenicol.

Conclusions

S. hominis isolates were highly resistant to methicillin and other antimicrobials. Most of the detected SCCmec types were different than those described for S. aureus. Isolates indicated low clonality. The results indicate that S. hominis is a strong biofilm producer with an extracellular matrix with similar composition of proteins, DNA and N-acetylglucosamine; and presents high frequency and low expression of icaD gene. Biofilm production is associated with increased antibiotic resistance.     

Conformational and thermodynamic changes of the repressor/DNA operator complex upon monomerization shed new light on regulation mechanisms of bacterial resistance against beta-lactam antibiotics

In absence of β-lactam antibiotics, BlaI and MecI homodimeric repressors negatively control the expression of genes involved in β-lactam resistance in Bacillus licheniformis and in Staphylococcus aureus. Subsequently to β-lactam presence, BlaI/MecI is inactivated by a single-point proteolysis that separates its N-terminal DNA-binding domain to its C-terminal domain responsible for its dimerization. Concomitantly to this proteolysis, the truncated repressor acquires a low affinity for its DNA target that explains the expression of the structural gene for resistance. To understand the loss of the high DNA affinity of the truncated repressor, we have determined the different dissociation constants of the system and solved the solution structure of the B. licheniformis monomeric repressor complexed to the semi-operating sequence OP₁ of blaP (1/2OP₁blaP) by using a de novo docking approach based on inter-molecular nuclear Overhauser effects and chemical-shift differences measured on each macromolecular partner. Although the N-terminal domain of the repressor is not subject to internal structural rearrangements upon DNA binding, the molecules adopt a tertiary conformation different from the crystallographic operator–repressor dimer complex, leading to a 30° rotation of the monomer with respect to a central axis extended across the DNA.

These results open new insights for the repression and induction mechanisms of bacterial resistance to β-lactams.

     

Differential Roles of Poly-N-Acetylglucosamine Surface Polysaccharide and Extracellular DNA in Staphylococcus aureus and Staphylococcus epidermidis Biofilms

Staphylococcus aureus and Staphylococcus epidermidis are major human pathogens of increasing importance due to the dissemination of antibiotic-resistant strains. Evidence suggests that the ability to form matrix-encased biofilms contributes to the pathogenesis of S. aureus and S. epidermidis. In this study, we investigated the functions of two staphylococcal biofilm matrix polymers: poly-N-acetylglucosamine surface polysaccharide (PNAG) and extracellular DNA (ecDNA). We measured the ability of a PNAG-degrading enzyme (dispersin B) and DNase I to inhibit biofilm formation, detach preformed biofilms, and sensitize biofilms to killing by the cationic detergent cetylpyridinium chloride (CPC) in a 96-well microtiter plate assay. When added to growth medium, both dispersin B and DNase I inhibited biofilm formation by both S. aureus and S. epidermidis. Dispersin B detached preformed S. epidermidis biofilms but not S. aureus biofilms, whereas DNase I detached S. aureus biofilms but not S. epidermidis biofilms. Similarly, dispersin B sensitized S. epidermidis biofilms to CPC killing, whereas DNase I sensitized S. aureus biofilms to CPC killing. We concluded that PNAG and ecDNA play fundamentally different structural roles in S. aureus and S. epidermidis biofilms.     

Establishment of an Arbitrary PCR for Rapid Identification of Tn917 Insertion Sites in Staphylococcus epidermidis: Characterization of Biofilm-Negative and Nonmucoid Mutants

Transposon mutagenesis with the Enterococcus faecalis transposon Tn917 is a genetic approach frequently used to identify genes related with specific phenotypes in gram-positive bacteria. We established an arbitrary PCR for the rapid and easy identification of Tn917 insertion sites in Staphylococcus epidermidis with six independent, well-characterized biofilm-negative Tn917 transposon mutants, which were clustered in the icaADBC gene locus or harbor Tn917 in the regulatory gene rsbU. For all six of these mutants, short chromosomal DNA fragments flanking both transposon ends could be amplified. All fragments were sufficient to correctly identify the Tn917 insertion sites in the published S. epidermidis genomes. By using this technique, the Tn917 insertion sites of three not-yet-characterized biofilm-negative or nonmucoid mutants were identified. In the biofilm-negative and nonmucoid mutant M12, Tn917 is inserted into a gene homologous to the regulatory gene purR of Bacillus subtilis and Staphylococcus aureus. The Tn917 insertions of the nonmucoid but biofilm-positive mutants M16 and M20 are located in genes homologous to components of the phosphoenolpyruvate-sugar phosphotransferase system (PTS) of B. subtilis, S. aureus, and Staphylococcus carnosus, indicating an influence of the PTS on the mucoid phenotype in S. epidermidis.