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Biofilms: A developmental niche for vancomycin-intermediate Staphylococcus aureus
Jenelle E Chapman1, Shilpa E George2, Christiane Wolz2
1Department of Medical Microbiology, Immunology, and Cell Biology, Southern Illinois University School of Medicine, USA.
Abstract:
Staphylococcus aureus are gram-positive bacteria responsible for a wide array of diseases, ranging from skin and soft tissue infections to more chronic illnesses such as toxic shock syndrome, osteomyelitis, and endocarditis. Vancomycin is currently one of the most effective antibiotics available in treating patients infected with methicillin-resistant S. aureus (MRSA), however the emergence of vancomycin-resistant S. aureus (VRSA), and more commonly vancomycin-intermediate S. aureus (VISA), threaten the future efficacy of vancomycin. Intermediate resistance to vancomycin occurs due to mutations within the loci of Staphylococcal genes involved in cell wall formation such as rpoB, graS, and yycG. We hypothesized the VISA phenotype may also arise as a result of the natural stress occurring within S. aureus biofilms, and that this phenomenon is mediated by the RecA/SOS response. Wildtype and recA null mutant/lexAG94E strains of S. aureus biofilms were established in biofilm microtiter assays or planktonic cultures with or without the addition of sub-inhibitory concentrations of vancomycin (0.063 mg/l - 0.25 mg/L ciprofloxacin, 0.5 mg/l vancomycin). Efficiency of plating techniques were used to quantify the subpopulation of biofilm-derived S. aureus cells that developed vancomycin-intermediate resistance. The results indicated that a greater subpopulation of cells from wildtype biofilms (4.16 × 102 CFUs) emerged from intermediate-resistant concentrations of vancomycin (4 μg/ml) compared with the planktonic counterpart (1.53 × 101 CFUs). Wildtype biofilms (4.16 × 102 CFUs) also exhibited greater resistance to intermediate-resistant concentrations of vancomycin compared with strains deficient in the recA null mutant (8.15 × 101 CFUs) and lexA genes (8.00 × 101 CFUs). While the VISA phenotype would be an unintended consequence of genetic diversity and potentially gene transfer in the biofilm setting, it demonstrates that mutations occurring within biofilms allow for S. aureus to adapt to new environments, including the presence of widely used antibiotics.
Insights
Staphylococcus aureus biofilms promote vancomycin intermediate resistance through the RecA/SOS response. This adaptation in biofilms, unlike planktonic cultures, enhances bacterial survival against antibiotics.
Area of Science:
- Microbiology
- Antibiotic Resistance
- Bacterial Pathogenesis
Background:
- Staphylococcus aureus causes diverse infections, with MRSA strains often treated by vancomycin.
- Emerging vancomycin-intermediate S. aureus (VISA) and vancomycin-resistant S. aureus (VRSA) threaten vancomycin's efficacy.
- VISA is linked to mutations in cell wall synthesis genes (e.g., rpoB, graS, yycG).
Purpose of the Study:
- To investigate if vancomycin-intermediate resistance in S. aureus arises from stress within biofilms.
- To determine if the RecA/SOS response mediates VISA development in S. aureus biofilms.
Main Methods:
- S. aureus biofilms and planktonic cultures were exposed to sub-inhibitory vancomycin concentrations.
- Wildtype, recA null mutant, and lexA mutant strains were utilized.
- Efficiency of plating quantified vancomycin-intermediate resistant subpopulations.
Main Results:
- Wildtype biofilms showed significantly higher vancomycin-intermediate resistance (4.16 × 10^2 CFUs) than planktonic cultures (1.53 × 10^1 CFUs).
- Biofilms exhibited greater resistance compared to recA null mutant (8.15 × 10^1 CFUs) and lexA mutant (8.00 × 10^1 CFUs) strains.
- The RecA/SOS pathway appears crucial for mediating this resistance.
Conclusions:
- Biofilm environments facilitate the emergence of vancomycin-intermediate resistance in S. aureus.
- The RecA/SOS response plays a key role in S. aureus adaptation to vancomycin stress within biofilms.
- This adaptive mechanism highlights how biofilms contribute to antibiotic resistance and bacterial survival.
