Evaluation of the ability of Streptococcus agalactiae strains isolated from genital and neonatal specimens to bind to human fibrinogen and correlation with characteristics of the fbsA and fbsB genes

Abstract

The ability of 111 Streptococcus agalactiae strains to bind to human fibrinogen was quantified. We correlated the percentages of bacteria that bound to immobilized fibrinogen with fibrinogen-binding (fbs) gene characteristics of strains and with clinical origin, serotypes, and phylogenetic positions of strains. Percentages varied from 0.4 to 29.9%. Fifty-five strains (49.5%) had the fbsB gene sensu stricto described by Gutekunst et al. (Infect. Immun., 72:3495-3504, 2004), allowing adhesion to human fibrinogen, and all of the other strains had an fgag variant gene. Ninety strains (81.1%) had a fbsA gene and 55 of them also had the fbsB gene. The other 21 strains (18.9%) had a truncated form of fbsA without the fbsB gene sensu stricto. The numbers of 48-nucleotide repeat sequences (rs) in the fbsA gene varied from 2 to 26. The population of strains with the highest ability to bind to human fibrinogen significantly more frequently had the fbsB gene sensu stricto and 4 to 7 rs in the fbsA gene (P < 0.05). However, the single strain that carried the highest number of rs (26 rs) in the fbsA gene showed high fibrinogen-binding activity (24.3%). Strains exhibiting significantly higher levels of binding to human fibrinogen belonged to a phylogenetic group of strains associated with neonatal meningitis, currently known as the ST-17 clone, that is mostly composed of serotype III strains. These findings indicate that S. agalactiae strains possess a wide variety of fbs gene content that markedly influences the ability of strains to bind to human fibrinogen. Variations in the configuration and the expression of the Fbs proteins may therefore partly explain the variability of virulence in S. agalactiae species.

FIG. 1.

PCR products obtained with the fbsA86/fbsA555rc primer set and analyzed by agarose gel electrophoresis with chromosomal DNA from different clinical S. agalactiae isolates belonging to the MLEE I (lanes 1 to 8) and to the MLEE II (lanes 10 to 14) phylogenetic divisions. Lane 9, 100-bp ladder.

FIG. 2.

Distribution of the 48-nucleotide repeat sequences in the fbsA gene. MLEE I phylogenetic division, ET 11, and ET 12 are three high-risk groups for neonatal meningitis.

FIG. 3.

Binding to fibrinogen of 111 strains of S. agalactiae in relation to the presence of the fbsA and the fbsB genes and to the numbers of 48-nucleotide repeat sequences in the fbsA gene. Boxes are means and bars are standard deviations of the means of the fibrinogen-binding percentages for the different strains in each category.