Abstract
The understanding of mechanisms of interactions between various bacterial cell surface proteins and host receptors has become imperative for the study of the health promoting features of probiotic enterococci. This study, for the first time, describes a novel enterococcal aggregation protein, AggE, from Enterococcus faecium BGGO9-28, selected from a laboratory collection of enterococcal isolates with auto-aggregation phenotypes. Among them, En. faecium BGGO9-28 showed the strongest auto-aggregation, adhesion to components of ECM and biofilm formation. Novel aggregation promoting factor AggE, a protein of 178.1 kDa, belongs to the collagen-binding superfamily of proteins and shares similar architecture with previously discovered aggregation factors from lactic acid bacteria (LAB). Its expression in heterologous enterococcal and lactococcal hosts demonstrates that the aggE gene is sufficient for cell aggregation. The derivatives carrying aggE exhibited the ten times higher adhesion ability to collagen and fibronectin, possess about two times higher adhesion to mucin and contribute to the increase of biofilm formation, comparing to the control strains. Analysis for the presence of virulence factors (cytolysin and gelatinase production), antibiotic resistance (antibiotic susceptibility) and genes (cylA, agg, gelE, esp, hylN, ace, efaAfs, and efaAfm) showed that BGGO9-28 was sensitive to all tested antibiotics, without hemolytic or gelatinase activity. This strain does not carry any of the tested genes encoding for known virulence factors. Results showed that BGGO9-28 was resistant to low pH and high concentrations of bile salts. Also, it adhered strongly to the Caco-2 human epithelial cell line. In conclusion, the results of this study indicate that the presence of AggE protein on the cell surface in enterococci is a desirable probiotic feature.
Highlights
Enterococci are an important group of lactic acid bacteria (LAB) which have the ability to survive various environmental conditions, allowing them to inhabit different ecological niches, such as water, soil (Giraffa, 2003), human, and animal gastrointestinal (GI) tracts (Bhardwaj et al, 2011) and food products, especially fermented dairy products (Gomes et al, 2008; Martín-Platero et al, 2009)
Considering the importance of aggregation phenomena for human health, the laboratory collection of enterococci was screened for strains exhibiting strong aggregation ability for further analysis
It was found that aggregation ability is a rare phenotype among enterococci (2.2% in our laboratory collection) as in other LAB (Miljkovic et al, 2015)
Summary
Enterococci are an important group of lactic acid bacteria (LAB) which have the ability to survive various environmental conditions, allowing them to inhabit different ecological niches, such as water, soil (Giraffa, 2003), human, and animal gastrointestinal (GI) tracts (Bhardwaj et al, 2011) and food products, especially fermented dairy products (Gomes et al, 2008; Martín-Platero et al, 2009). Many enterococci isolated from fermented dairy products have proven to be great natural probiotics and are generally considered to be beneficial and safe to the host (Eaton and Gasson, 2001; Pieniz et al, 2014). To evaluate the microorganism as novel food constituent with a health claim, several essential characteristics should be considered, including the survival throughout the GI passage, adhesion, metabolic activities and its effect on intestinal homeostasis (Miquel et al, 2015). It is believed that adherence ability of probiotic bacteria to intestinal cells is important for successful colonization and may lead to exclusion of pathogens and/or immunomodulation; expected to provide long time lasting beneficial effects for health (McNaught and MacFie, 2001; Kravtsov et al, 2008). The mechanism by which the consortium of strains from fermented milk product elicit this response is still unclear, but it seems that the effect is rapid (occurring within the first 24 h after application) and it lasts regardless of whether the consortium was introduced during a 1 day period, or with subsequent repeated applications over a several weeks (McNulty et al, 2011)
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