Conversion of Staphylococcus epidermidis strains from commensal to invasive by expression of the ica locus encoding production of biofilm exopolysaccharide

Abstract

To test if biofilm formation in Staphylococcus epidermidis is dependent on the polysaccharide intercellular adhesin, whose biosynthesis is driven by the ica locus, a plasmid containing the ica locus was transferred to three ica-negative strains. Using in vitro biofilm assays and a rat central venous catheter infection model, we confirmed the importance of the ica locus for biofilm production and pathogenesis of S. epidermidis.

FIG. 1.

Semiquantitative biofilm assay of ica-negative S. epidermidis strains and corresponding ica locus-expressed isogenic strains. Plasmid pYM12 (containing icaRADBC) or the control plasmid pYJ90 was transformed into three ica-negative S. epidermidis strains, ATCC 12228, HB, and Tü3298 respectively. Each experiment was repeated eight times. When A₄₉₂ exceeded 0.12, the strain was defined as biofilm positive. The mean of eight experiments ± standard error is shown.

FIG. 2.

Scanning electron microscopy of S. epidermidis biofilm. Bacteria adhered to the cover slides were fixed, treated, and observed by SEM. The bar represents a 1.5-μm scale.

FIG. 3.

PIA production in ica-negative S. epidermidis wild-type and corresponding ica locus-expressed strains. PIA was extracted from 24-h cultures by boiling with 0.5% EDTA for 5 min. Three-μl samples of the extract were spotted on nitrocellulose membrane, and PIA production was assayed by immunodot blot and evaluated by densitometry as described (15). The experiment was performed twice. (A) Representative immunodot blot. (B) Bars show the mean ± standard error: 1, Tü3298-ica; 2, Tü3298; 3, ATCC 12228-ica; 4, ATCC 12228; 5, HB-ica; 6, HB.