The Function And Regulation Of Intercellular Adhesin (ica) Genes In Staphylococcus Epidermidis

The function and regulation of intercellular adhesin (ica) genes in Staphylococcus epidermidisStaphylococcus epidermidis is an important opportunistic pathogen which causes nosocomial infections predominantly in immunocompromised patients, especially in patients with indwelling medical devices. The pathogenesis of S. epidermidis is closely associated with its capability to adhere to the surfaces of polymeric devices and accumulate into multi-layered biofilm. It is crucial to study the biofilm forming related genes in S. epidermidis, thereby revealing the mechanisms of its pathogenesis and for the development of effective preventive and therapeutic measures against intravascular catheter associated infections caused by S. epidermidis.Sequencing and comparative study of microbial genomes provided a completely new platform for the studies of pathogenic bacteria. In the present study, the whole genome sequence of biofilm negative S. epidermidis strain ATCC 12228 was analyzed and compared with that of biofilm positive S. epidermidis strain RP62A and that of Staphylococcus aureus strain N315 to identify biofilm related genes. Among them, the function of the intercellular adhesin (ica) locus and its' expression regulation were studied in details. I. By genome wide sequence comparison between S. epidermidis ATCC 12228 and S. aureus N315, a number of virulence genes encoding toxins and exoenzymes were found absent in S. epidermidis ATCC 12228. In contrast, the genome sequences of biofilm negative S. epidermidis strain ATCC 12228 and that of biofilm positive S. epidermidis strain RP62A (the genome sequence of S. epidermidis RP62A was licensed by The Institute for Genomic Research to be used only for comparative study) were found highly homologous, while the ica locus, which correlates with the synthesis of polysaccharide intercellular adhesin (PIA), was absent in S. epidermidis ATCC 12228. This finding suggested that most likely, the ica genes determined the differences in the biofilm formation and the pathogenesis among S. epidermidis strains. Data analysis on clinical S. epidermidis isolates also showed that 77% of the ica locus positive isolates formed biofilm whereas none of the ica negative isolates formed biofilm.II. To characterize the function of ica genes, the entire ica locus was cloned and transformed into ica negative S. epidermidis strains ATCC 12228, HB and TU3298. Semi-quantitative biofilm assay showed that the knock-in of ica locus converted all the three biofilm negative S. epidermidis strains to biofilm positive. This morphological conversion was also confirmed by scanning electron microscopy. Interestingly, the biofilm forming ability of the ica locus transformed clinical isolate, HB, was significantly higher than that of commensal S. epidermidis strains ATCC 12228 and TU3298. This observation implies that other factors were involved in the ica product-PIA-biofilm pathway. To demonstrate the influence of ica genes on the pathogenicity of commensal S. epidemidis, strain ATCC 12228 and its ica locus transformed isogenic counterpart were studied in a rat central venous catheter (CVC)-associated infection model. The number of bacteria recovered from the liver, kidney and heart of infected animals was remarkably higher for the ica positive ATCC 12228, compared to its parental strain. It indicated that the transformed S. epidermidis ATCC 12228-ica was with increased virulence. In summary, all the experimental results suggested that ica genes played an important role in the conversion of a S. epidemidis avirulent strain to a relatively virulent strain. However, the extend of biofilm formation of ica locus transformed ATCC 12228 was much lower than that of ica locus transformed HB and wild type ica positive strain RP62A. Therefore, though the ica locus per se was important in determining the biofilm formation and virulence of S. epidermidis, regulatory genes, possibly other genes, seemed to be involved in the complex process of biofilm formation at the whole bacterial cell level. III. Biof...