New Insights into the Enterococcus faecium and Streptococcus gallolyticus subsp. gallolyticus Host Interaction Mechanisms

Abstract

Enterococcus faecium and Streptococcus gallolyticus subsp. gallolyticus (S. gallolyticus) were classically clustered into the Lancefield Group D streptococci and despite their taxonomic reclassification still share a similar genetic content and environment. Both species are considered as opportunistic pathogens. E. faecium is often associated with nosocomial bacteraemia, and S. gallolyticus is sporadically found in endocarditis of colorectal cancer patients. In both cases, the source of infection is commonly endogenous with a translocation process that launches through the intestinal barrier. To get new insights into the pathological processes preceding infection development of both organisms, we used an in vitro model with Caco-2 cells to study and compare the adhesion, invasion and translocation inherent abilities of 6 E. faecium and 4 S. gallolyticus well-characterized isolates. Additionally, biofilm formation on polystyrene, collagen I and IV was also explored. Overall results showed that E. faecium translocated more efficiently than S. gallolyticus, inducing a destabilization of the intestinal monolayer. Isolates Efm106, Efm121 and Efm113 (p < .001 compared to Ef222) exhibited the higher translocation ability and were able to adhere 2-3 times higher than S. gallolyticus isolates. Both species preferred the collagen IV coated surfaces to form biofilm but the S. gallolyticus structures were more compact (p = .01). These results may support a relationship between biofilm formation and vegetation establishment in S. gallolyticus endocarditis, whereas the high translocation ability of E. faecium high-risk clones might partially explain the increasing number of bacteraemia.

Fig 1

E. faecium (A) and S. gallolyticus (B) TER response. TER values were monitored before and after infection throughout an 8 h period and results were expressed as percentages of mean TER (±SD) at each time point in relation to basal value (before infection). E. faecium (C) and S. gallolyticus (D) translocation across an epithelial monolayer. Bacterial translocation was expressed as mean (± SD) colony forming units (CFU/ml). Translocation results for E. faecium strains were compared to Efm222, *p< .05. These figures summarize the results of three independent experiments comprising 3–6 replicates of each isolate.

Fig 2. Bacterial adherence and invasion of epithelial Caco-2 cells.

Adherence was analysed after 2 h of exposure while the viable internalized bacteria were counted after cell lysis. Results were presented as a percentage of the bacterial inocula and differences between the enterococcal and the streptococcal groups were compared using Mann Whitney U-test.

Fig 3. Production of biofilm on polystyrene, collagen I and collagen IV coated surfaces.

Isolates were classified as non-biofilm producers (OD₆₀₀<0.120), weak producers (0.120<OD₆₀₀≤0.240) and strong producers (OD₆₀₀>0.240). Results are expressed as mean values (± SD). Biofilm production among the enterococcal and the streptococcal groups was compared using Mann Whitney U-test.