Existence of a Colonizing Staphylococcus aureus Strain Isolated in Diabetic Foot Ulcers

Abstract

Staphylococcus aureus is an opportunistic bacterium capable of causing a wide range of severe diseases when it gains access to underlying tissues. Paradoxically, S. aureus is a common inhabitant of the skin microflora and colonizes the nares and other human mucosa. The purpose of this study was to determine the genetic basis for the differences in the pathogenic versus colonizing potential of S. aureus isolated from diabetic foot ulcers (DFUs). By performing optical map comparisons of a collection of S. aureus strains isolated from DFUs, we brought to light a prophage present in noninfecting bacteria. The phage, namely ROSA-like, was localized in a hotspot region ΦNM2 near the locus isd, the iron surface determinant system. The integrated phage significantly reduces the virulence of the strain and increases the biofilm formation. DFUs seem to be a specific niche of this colonizing strain. The ROSA-like phage represents the first description of a mobile element present mainly in S. aureus isolated from DFUs, which modulates the relationship of the bacteria with its human host. This phage appears to attenuate bacterial virulence and promote colonization.

Figure 1

Virulence of the different S. aureus strains in zebrafish embryos. A: Survival curves of zebrafish embryos after infection with two infecting (NSA739 and NSA18026) and two colonizing (NSA1385 and NSA1322) strains. B: Survival curves of zebrafish embryos after infection with NSA739, NSA1385, and NSA1385(P−) strains. hpi, hours post-infection.

Figure 2

Kinetics of biofilm formation for the different S. aureus strains. Lines represent the SDs from three independent experiments with triplicate for each one. BFI, BioFilm Formation Index.