Related Experiment Video
Updated: Jun 5, 2025
Antimicrobial Synergy Testing by the Inkjet Printer-assisted Automated Checkerboard Array and the Manual Time-kill Method
Published on: April 18, 2019
Unexpected vulnerability of Enterococcus faecium to polymyxin B under anaerobic condition
Yongjun Son1,2, Bitnara Kim1, Pureun Kim1
1Laboratory of Molecular Environmental Microbiology, Department of Environmental Science and Ecological Engineering, Korea University, Seoul, Republic of Korea.
Abstract:
Gram-positive Enterococcus faecium exhibited higher susceptibility (>4-fold) to polymyxin B (PMB), the canonical antimicrobial peptide against Gram-negative bacteria, under anaerobic condition than aerobic condition. Anaerobically grown E. faecium exhibited high vulnerability to PMB, leading to alteration of cell surface and morphology, as observed based on their high dansyl-PMB affinity (>2.9-fold), a proportion (>8.5-fold) of propidium iodide-stained cells, and observation of scanning electron microscopy results. Interestingly, our transcriptomic and chemical analyses revealed that enterocin B, produced anaerobically, imposes a burden on the cellular envelope when cells are exposed to PMB. This scenario was also supported by PMB susceptibility tests and killing curves, which showed that ΔentB knockout mutant cells were more resistant to PMB (32 µg/mL) compared to wild-type cells (4 µg/mL) under anaerobic condition. Fluorescent D-amino acid and BOCILLIN™-fluorescent profiling of transpeptidase activities in ΔentB mutant cells under anaerobic condition revealed similar levels of activity to those observed in WT cells under aerobic condition. The high level of secreted bacteriocins in WT under anaerobic condition likely led to significant membrane depolarization and loosening of the peptidoglycan layer, making the cells more permeable to PMB. Overall, our findings suggest that anaerobically produced bacteriocins, in conjunction with PMB, contribute to the killing of E. faecium by destabilizing its cell envelope.
Insights
Gram-positive Enterococcus faecium is more vulnerable to polymyxin B (PMB) under anaerobic conditions. Anaerobic conditions and enterocin B production destabilize the cell envelope, increasing susceptibility to PMB.
Area of Science:
- Microbiology
- Antimicrobial Resistance
- Bacteriophage Therapy
Background:
- Polymyxin B (PMB) is a critical antibiotic for Gram-negative infections.
- Enterococcus faecium is a Gram-positive bacterium.
- Bacterial susceptibility to antibiotics can vary with oxygen availability.
Purpose of the Study:
- To investigate the susceptibility of Gram-positive Enterococcus faecium to polymyxin B (PMB) under different oxygen conditions.
- To elucidate the mechanisms underlying altered PMB susceptibility in E. faecium.
Main Methods:
- Comparative analysis of E. faecium susceptibility to PMB under aerobic and anaerobic conditions.
- Cell surface and morphological analysis using dansyl-PMB affinity and propidium iodide staining.
- Scanning electron microscopy (SEM) for morphological changes.
- Transcriptomic and chemical analyses to identify contributing factors.
- PMB susceptibility tests and killing curves for wild-type and ΔentB mutant strains.
- Bacterial transpeptidase activity profiling.
Main Results:
- Anaerobically grown E. faecium showed significantly higher susceptibility (>4-fold) to PMB compared to aerobically grown cells.
- Anaerobic growth led to cell surface alterations, increased membrane permeability, and morphological changes.
- Enterocin B, produced under anaerobic conditions, was identified as a key factor contributing to PMB susceptibility.
- ΔentB knockout mutants exhibited increased resistance to PMB under anaerobic conditions.
- Anaerobically produced bacteriocins likely destabilize the cell envelope, enhancing PMB efficacy.
Conclusions:
- Anaerobic conditions dramatically increase E. faecium susceptibility to polymyxin B.
- The production of enterocin B under anaerobic conditions plays a crucial role in this heightened susceptibility.
- Targeting anaerobic pathways or bacteriocin production could be a strategy to combat E. faecium infections.
