Characterisation of biofilm-forming Staphylococcus epidermidis clinical isolates

Objective: Staphylococcus epidermidis is one of the bacterial species mainly implicated in foreign body associated infections. We have characterized several S.epidermidis clinical isolates for their ability to form biofilms and their resistance to antibiotics. Methods: 76 S.epidermidis strains isolated from implantable medical devices have been collected from hospitals of Central Italy. Susceptibility to penicillin, methicillin, erythromycin and tetracycline has been determined in vitro by E-test according to NCCLS guidelines. The specific antibiotic resistance determinants have been checked by PCR (blaZ, mecA, ermA, ermB, ermC, msrA, tetK and tetM). The ability to form biofilms has been determined: (i) by PCR, detecting genes specific for attachment and biofilm development (icaADBC operon, aap, and atlE); (ii) by Congo Red Agar (CRA) plate test to assay the production of polisaccaridic intercellular adhesin (PIA); (iii) by crystal violet (CV) stain to determine the biofilm biomass development on polystyrene microtiter plates; (iv) and by CSLM microscopy observations to investigate biofilm structure. Results: 94% of the strains under study was resistant to penicillin, 87% to methicillin, 72% to erythromycin and 25% to tetracycline. On the side of biofilm-specific genes detection, 66% of strains was positive to ica operon genes, 82% possessed atlE gene, and 42% aap determinant. In 89% of the population, the CRA test confirmed the correlation between the presence of ica genes and slime expression. The CV assay classified the quasitotality of our strains (97%) as biofilm producers on plastic surface. In addition, the distribution of optical density values (OD540) obtained after CV stain, showed a significant statistical difference in biofilm biomass development between the ica- ADBC-positive strains and the icaADBC-negative ones. Finally, a correlation, although not always present, has been observed between ability of the strains to develop in a high-structureted biofilm and specific biofilm-formation determinants. Conclusions: The investigated bacterial population shows a very high and alarming level of resistance to all tested drugs. Although the specific determinants for biofilms formation are not always present, nevertheless all the strains are able to develop in sessile form showing that different and not still identified factors could work together in the formation and organization of staphylococcal complex microbial communities.