Bacterial Biofilms on Fungal Surfaces

Bacterial biofilm formation on fungi participates in the synergistic degradation of substrates, antagonism of fungal growth, bacterial utilization of fungi as nutrient sources, and the formation of more complex synergistic associations for the purposes of nutrient acquisition. While bacterial biofilm formation has been described in many systems, the molecular mechanisms that govern these interactions are not yet well understood. Analysis of physical interactions between Pseudomonas aeruginosa and the dimorphic opportunistic fungal pathogen Candida albicans has provided insights into factors involved in attachment and matrix production, and has demonstrated a role for the bacterial quorum sensing molecule, 3-oxo-C12-homoserine lactone, within bacterial-fungal biofilms. Subsequent to P. aeruginosa biofilm formation on the fungus, extracellular bacterial products contribute to the death of the fungal hyphae. Studies focused on the interactions between bacteria and fungi in the phytosphere have illustrated additional processes that contribute to bacterial biofilm formation on fungi including bacterial chemotaxis towards fungal cells, the crossfeeding of nutrients between interacting species, and the expression of specific genes upon contact with fungal cells. By understanding bacterial biofilm formation on fungi, we will gain insight into economically important interactions, such as those involved in the bacterial biocontrol of fungal plant pathogens. Furthermore, using tractable bacterial-fungal biofilm model systems, we may uncover important elements of bacterial biofilms on other living surfaces such as plant and animal tissues. Overview: bacterial biofilms on fungal surfaces From the perspective of the bacterium, there are a number of ways in which biofilm formation on fungi can be beneficial. First, bacterial colonization of a fungal surface may enable the bacteria to exploit the fungus as a source of nutrients (Hogan and Kolter, 2002). Bacteria may scavenge nutrients from the fungal cell wall, consume fungi-secreted products, or induce lysis of the fungal cells thereby liberating the intracellular contents for consumption by the local bacterial population. Second, in communities where bacteria and fungi are in competition for nutrients, biofilm formation on fungal cells could enhance bacterial antagonism of fungi by concentrating bacterially derived antifungal compounds. Third, biofilm formation on the surface of fungal hyphae would enable bacteria to “travel” with fungi as they extend into new areas in search of nutrients. Fourth, bacterial attachment to the hyphal surfaces may enhance synergistic actions of bacteria and fungi needed to breakdown complex substrates. Lastly, bacterial colonization of a fungal surface may be a first step in caister.com/biofilmsbooks