A type III-like restriction endonuclease functions as a major barrier to horizontal gene transfer in clinical Staphylococcus aureus strains

Abstract

Staphylococcus aureus is an versatile pathogen that can cause life-threatening infections. Depending on the clinical setting, up to 50% of S. aureus infections are caused by methicillin-resistant strains (MRSA) that in most cases are resistant to many other antibiotics, making treatment difficult. The emergence of community-acquired MRSA drastically changed the picture by increasing the risk of MRSA infections. Horizontal transfer of genes encoding for antibiotic resistance or virulence factors is a major concern of multidrug-resistant S. aureus infections and epidemiology. We identified and characterized a type III-like restriction system present in clinical S. aureus strains that prevents transformation with DNA from other bacterial species. Interestingly, our analysis revealed that some clinical MRSA strains are deficient in this restriction system, and thus are hypersusceptible to the horizontal transfer of DNA from other species, such as Escherichia coli, and could easily acquire a vancomycin-resistance gene from enterococci. Inactivation of this restriction system dramatically increases the transformation efficiency of clinical S. aureus strains, opening the field of molecular genetic manipulation of these strains using DNA of exogenous origin.

Fig. 1.

Sequence of the type III-like restriction endonuclease from strain SA564. The sequence corresponding to the SF2 helicases is indicated in purple, and some conserved residues of the typical helicase motifs I, II, III, and VI are indicated in yellow.

Fig. 2.

Summary of intrastrain transfer of plasmid DNA and its barriers. Black arrows indicate compatibility for transformation, and red arrows denote compatibilities generated by mutations, indicated by (|—). For the introduction of plasmid DNA by transformation into clinical S. aureus strains, the DNA is first transformed into the laboratory strain RN4220 (black arrow). From there, plasmid DNA may be transformed into some clinical strains, like SA564 (black arrow), but not into others (UAMS-1). Only after disruption of the hsdR gene in UAMS-1 can plasmid DNA prepared from RN4220 be transformed into UAMS-1 (red arrow). Finally, disruption of the gene encoding the type III-like restriction endonuclease improves transformation from RN4220 into UAMS-1 and, most importantly, allows direct transformation from E. coli (or E. faecalis) into the clinical strains SA564 and UAMS-1 (red arrows).