An Overview of the Factors Involved in Biofilm Production by the Enterococcus Genus

Abstract

Enterococcus species are known for their ability to form biofilms, which contributes to their survival in extreme environments and involvement in persistent bacterial infections, especially in the case of multi-drug-resistant strains. This review aims to provide a comprehensive understanding of the mechanisms underlying biofilm formation in clinically important species such as Enterococcus faecalis and the less studied but increasingly multi-drug-resistant Enterococcus faecium, and explores potential strategies for their eradication. Biofilm formation in Enterococcus involves a complex interplay of genes and virulence factors, including gelatinase, cytolysin, Secreted antigen A, pili, microbial surface components that recognize adhesive matrix molecules (MSCRAMMs), and DNA release. Quorum sensing, a process of intercellular communication, mediated by peptide pheromones such as Cob, Ccf, and Cpd, plays a crucial role in coordinating biofilm development by targeting gene expression and regulation. Additionally, the regulation of extracellular DNA (eDNA) release has emerged as a fundamental component in biofilm formation. In E. faecalis, the autolysin N-acetylglucosaminidase and proteases such as gelatinase and serin protease are key players in this process, influencing biofilm development and virulence. Targeting eDNA may offer a promising avenue for intervention in biofilm-producing E. faecalis infections. Overall, gaining insights into the intricate mechanisms of biofilm formation in Enterococcus may provide directions for anti-biofilm therapeutic research, with the purpose of reducing the burden of Enterococcus-associated infections.

Keywords: MSCRAMMs; antibiotic resistance; biofilm formation; enterococcus; extracellular DNA (eDNA); pheromone signaling; pili-mediated adhesion; quorum sensing; virulence factors.

Figure 1

PilB production and translocation via murein sacculi (MS) during cell division, in different stages of colony growth. In the early exponential (EE) phase, the pilin subunits are stored near the bacterial cross wall (CW). Later on, in the exponential (E) phase, some subunits migrate near the extremities and form pili, but others remain around the cross wall. In the stationary (S) phase, all the pilin subunits are converted into mature PilB-type pili (MP).

Figure 2

AS (aggregation substance) production and the role of the cAD1 pheromone. F- bacteria emit cAD1, a sex pheromone, in order to induce AS synthesis in F+ bacteria. Once the AS is expressed on the cell surface, it will interact with the EBS (enterococcal binding substance), and thus the two cells can now exchange the pAD1 plasmid (with the asa1 gene that encodes for AS).

Figure 3

Autocrine inhibition of pheromone-mediated transfer mechanisms.

Figure 4

Pheromone-mediated (pCF10) horizontal plasmid transfer and Asc10 expression.

Figure 5

Degradation of iCF10 and activation of conjugation system resulting in host colonization.

Figure 6

Pheromone synthesis and target gene activation produced by COB1 pheromone (the amino acids sequence is presented below).

Figure 7

cPD1 pheromone and pPD1 plasmid interaction via TraA receptor. Pheromone-mediated transfer mechanisms between a donor that contains the pPD1 plasmid and the recipient’s TraC and TraA proteins. cPD1 and cCF10 pheromones can both bind with TraC.