recA mediated spontaneous deletions of the icaADBC operon of clinical Staphylococcus epidermidis isolates: a new mechanism of phenotypic variations

Phenotypic variation of Staphylococcus epidermidis involving the slime related ica operon results in heterogeneity in surface characteristics of individual bacteria in axenic cultures. Five clinical S. epidermidis isolates demonstrated phenotypic variation, i.e. both black and red colonies on Congo Red agar. Black colonies displayed bi-modal electrophoretic mobility distributions at pH 2, but such phenotypic variation was absent in red colonies of the same strain as well as in control strains without phenotypic variation. All red colonies had lost ica and the ability to form biofilms, in contrast to black colonies of the same strain. Real time PCR targeting icaA indicated a reduction in gene copy number within cultures exhibiting phenotypic variation, which correlated with phenotypic variations in biofilm formation and electrophoretic mobility distribution of cells within a culture. Loss of ica was irreversible and independent of the mobile element IS256. Instead, in high frequency switching strains, spontaneous mutations in lexA were found which resulted in deregulation of recA expression, as shown by real time PCR. RecA is involved in genetic deletions and rearrangements and we postulate a model representing a new mechanism of phenotypic variation in clinical isolates of S. epidermidis. This is the first report of S. epidermidis strains irreversibly switching from biofilm-positive to biofilm-negative phenotype by spontaneous deletion of icaADBC

The Bacterial Insertion Sequence Element IS256 Occurs Preferentially in Nosocomial Staphylococcus epidermidis Isolates: Association with Biofilm formation and Resistance to Aminoglycosides

Staphylococcus epidermidis is a normal constituent of the healthy human microflora, but it is also the most common cause of nosocomial infections associated with the use of indwelling medical devices. Isolates from device-associated infections are known for their pronounced phenotypic and genetic variability, and in this study we searched for factors that might contribute to this flexibility. We show that mutator phenotypes, which exhibit elevated spontaneous mutation rates, are rare among both pathogenic and commensal S. epidermidis strains. However, the study revealed that, in contrast to those of commensal strains, the genomes of clinical S. epidermidis strains carry multiple copies of the insertion sequence IS256, while other typical staphylococcal insertion sequences, such as IS257 and IS1272, are distributed equally among saprophytic and clinical isolates. Moreover, detection of IS256 was found to be associated with biofilm formation and the presence of the icaADBC operon as well as with gentamicin and oxacillin resistance in the clinical strains. The data suggest that IS256 is a characteristic element in the genome of multiresistant nosocomial S. epidermidis isolates that might be involved in the flexibility and adaptation of the genome in clinical isolates

Mechanism of chromosomal transfer of Enterococcus faecalis pathogenicity island, capsule, antimicrobial resistance, and other traits

The Enterococcus faecalis pathogenicity island (PAI) encodes known virulence traits and >100 additional genes with unknown roles in enterococcal biology. Phage-related integration and excision genes, and direct repeats flanking the island, suggest it moves as an integrative conjugative element (ICE). However, transfer was observed not to require these genes. Transfer only occurred from donors possessing a pheromone responsive-type of conjugative plasmid, and was invariably accompanied by transfer of flanking donor chromosome sequences. Deletion of plasmid transfer functions, including the cis-acting origin of transfer (oriT), abolished movement. In addition to demonstrating PAI movement by a mechanism involving plasmid integration, we observed transfer of a selectable marker placed virtually anywhere on the chromosome. Transfer of this selectable marker was observed to be accompanied by chromosome-chromosome transfer of vancomycin resistance, MLST markers, and capsule genes as well. Plasmid mobilization therefore appears to be a major mechanism for horizontal gene transfer in the evolution of antibiotic resistant E. faecalis strains capable of causing human infection