Genetic diversity in Staphylococcus epidermidis clinical and environmental isolates biofilm forming skills

Context S. epidermidis is now recognized as an emerging nosocomial opportunistic pathogen involved in infections on indwelling medical devices. Device contamination easily occurs during insertion or use (in case of catheters), often due to a lack of hygiene. S. epidermidis cells are able to anchor themselves onto biomaterials and form dense, persistent biofilms. The factors mediating biofilm formation and their genetic determinants have been individually characterized over the past decade. Specific genes were pointed out to be necessary to produce viable biofilms whereas others were found to be more accessory or redundant. However, previous studies aiming to understand biofilm formation by S. epidermidis focused on a limited number of strains, neglecting intrinsic genotypic diversity. A lack of transversal analyses has delivered an incomplete knowledge about the proportion of strains which display a potential virulence or persistence in infections cases. Objectives The present work aims to give an account of the variety of genotypic profiles related to biofilm formation existing within the S. epidermidis species through the characterization of both clinical and environmental isolates. Methods In this study, fifty clinical and fifty environmental (random sampling from healthy carriers and surfaces) S. epidermidis isolates were tested for the presence of 14 key genes involved in biofilm formation and invasiveness. The tested genes belong to specific categories: adhesive factors, accumulation mediators, regulation, virulence and resistance to environmental threats. Results and Discussion The data were analyzed by statistical methods of clustering in which weights could be attributed to the tested genes according to reported experimental evidences. The results showed that clinical and environmental isolates coexist in only 1 of the 4 clusters. The other clusters are exclusively composed either of environmental or of clinical isolates. First, this analysis tends to confirm the selection of persistent strains in clinical settings. The harsh selection pressure due to extensive use of antimicrobial compounds might thus be favoring the occurrence of invasive strains. Second, our results helped in identifying correlations existing between the presence/absence of particular genes therefore establishing criteria to discriminate potential invasive strains from inoffensive commensals. Conclusions This study gives a significant overview of the existing genotypic diversity and proposed indicators allowing classifying strains within the S. epidermidis species according their potential to form persistent biofilms. The collection of characterized strains offers possibilities to investigate the dynamics of biofilm formation in further details