Identification of Coagulase-Negative Staphylococci by SodA Gene Sequence Analysis

A total number of 50 internal sodA gene sequences with 416 nucleotides was analyzed to determine the discriminative power of the gene for the identification of the Coagulase-Negative Staphylococci (CoNS). The mean similarity between these CoNS species was found to be 80.4% indicating a good discriminative power although low sequence divergence was observed between some of the species. The relatedness between these staphylococcal species was further explored by constructing phylogenetic analysis. Six clusters was revealed, with four of these supported by significant bootstrap values of >95% while the other two clusters were less robustly supported at bootstrap values of about 92%. Following that, the identification of 200 clinical isolates of CoNS isolates were determined phenotypically by a commercial diagnostic kit and also genotypically using the sodA gene sequencing method. The Microgen Staph ID was found to be quite reliable for commonly isolated clinical CoNS species e.g., S. capitis subsp. capitis, S. haemolyticus and a majority of the S. epidermidis isolates. However, for the less commonly encountered species, the kit was unreliable. Identification by sodA sequencing method was found to be more reliable with homology values of at least 98%. However, like other nucleotide sequence based identification, the discriminative power of sodA at subspecies level was poor whereby the final identification had to be supplemented with biochemical tests.