A Dangerous Alliance in the Microverse: Synergistic Interactions Between Fungi and Bacteria

Researchers within the Cluster of Excellence identify how Candida albicans and Enterococcus faecalis cooperate to amplify host damage.

Scanning electron microscope image: Bacteria of the species Enterococcus faecalis (purple) attach to hyphae of the fungus Candida albicans (turquoise). Source: Leibniz-HKI
Scanning electron microscope image: Bacteria of the species Enterococcus faecalis (purple) attach to hyphae of the fungus Candida albicans (turquoise). Source: Leibniz-HKI

Rivals or allies? Microorganisms in the human body are in constant interaction. An international research team led by the Leibniz-HKI and supported by the "Balance of the Microverse" Cluster of Excellence has revealed a dangerous alliance between the yeast Candida albicans and the bacterium Enterococcus faecalis. The study, published in PNAS, identifies the bacterial toxin cytolysin as the key driver of this synergistic damage.

Bacteria on our mucous membranes are often considered antagonists to fungi, keeping their growth in check. However, when the microbial balance is disturbed—such as after antibiotic therapy or due to a weakened immune system—this dynamic can shift dramatically. The new study led by Prof. Dr. Ilse Jacobsen demonstrates that under specific conditions, these microbes cease to be rivals and instead amplify their pathogenic impact.

Cytolysin as the Key Factor

Using cell culture and mouse models, the team discovered that not all E. faecalis strains act as accomplices. Significant cell damage occurred only with strains producing cytolysin, a toxin that perforates cell membranes. Strains lacking the gene for this toxin did not cause additional damage. "The cytolysin-producing variants have proven to be the dangerous ones," emphasizes Jacobsen, explaining why disease severity can vary despite the presence of similar microorganisms.

Mechanisms of Cooperation The researchers identified two central mechanisms driving this alliance:

  • Direct Contact: Bacteria attach themselves to the fungal hyphae, effectively hitching a ride to come into close contact with host cells. This allows cytolysin to act precisely where it causes the most damage.

  • Nutrient Depletion: Candida albicans consumes glucose rapidly. The resulting energy deficiency weakens the host cells, making them significantly more susceptible to the bacterial toxin.

Implications for Microverse Research These findings underscore the complexity of microbiological interactions. The danger of an infection depends not merely on a single species, but on the specific encounter of microbes and the molecular tools they employ. This insight is crucial for developing targeted therapies for co-infections.

Original Publication:
Kapitan M, et. al., Jacobsen ID (2025) Synergistic interactions between Candida albicans and Enterococcus faecalis promote toxin-dependent host cell damage. Proc Natl Acad Sci U S A 122(46), e2505310122.
https://doi.org/10.1073/pnas.2505310122