Mobilisation Mechanism of Pathogenicity Islands by Endogenous Phages in Staphylococcus aureus clinical strains

Mercedes Cervera-Alamar,Katerina Guzmán-Markevitch,Patricia Bernabé-Quispe,María Ángeles Tormo-Mas,Javier Pemán,Antonio Pineda-Lucena,Miglė Žiemytė,Leticia Ortí

doi:10.1038/s41598-018-34918-2

Abstract

Staphylococcus aureus pathogenicity islands (SaPIs) are a type of mobile genetic element that play a significant role in the pathogenesis and virulence of this microorganism. SaPIs are integrated in the chromosome under the control of the master repressor Stl, but they can be horizontally transferred at a high frequency due to certain bacteriophages. Thus, a phage protein can bind to the SaPI Stl and induce the SaPI cycle, spreading the SaPI virulence factors to other bacterial populations. We report the dissemination mechanism of SaPIs mediated by endogenous prophages in S. aureus clinical strains. We reveal the induction of SaPIs by a co-resident prophage in seven clinically relevant strains, and we further study this mechanism in MW2, a community-acquired methicillin-resistant S. aureus strain that contains two bacteriophages (ɸSa2mw and ɸSa3mw) and one SaPI (SaPImw2) encoding for three enterotoxins (sec, sel and ear). ɸSa2mw was identified as responsible for SaPImw2 induction, and the specific phage derepressor protein DUF3113 was determined. The Stl-DUF3113 protein interaction was demonstrated, along with the existence of variants of this protein in S. aureus phages with different abilities to induce SaPI. Both Stl and DUF3113 are present in other Staphylococcus species, which indicates that this is a generalised mechanism.

Highlights

Staphylococcus aureus is a widespread pathogen that colonises the skin and mucosa of healthy adults
To confirm whether this mechanism occurs naturally, a total of 14 S. aureus clinical strains were treated with mitomycin C to activate the SOS response and induce endogenous phages to check whether these phages are able to activate the resident SaPIs (Fig. 1)
In MW2, C and USA300 strains the hybridization signal of the specific probe for each SaPI was observed in the bulk DNA as well as in a band that corresponds to SaPI linear monomers (L) released from the small phage capsids, as it was observed in previous works, in presence of a phage able to induce and encapsidate the SaPI25,34

Summary

Introduction

Staphylococcus aureus is a widespread pathogen that colonises the skin and mucosa of healthy adults. Homologues have been identified, they are cos SaPIs and they can use the packaging mechanisms of pac and cos phages[17]. The SaPI life cycle starts by the infection of a helper phage or by activation of an endogenous helper prophage by the SOS response. This response can be caused by oxidative stress, exposure to UV-irradiation or antibiotic treatment[22,23,24]. Once the prophage is activated or the cell is infected by the phage, a bacteriophage protein binds to Stl, triggering the release of the Stl repressor from the SaPI DNA. Disruption of the Stl-SaPI DNA complex allows the expression of SaPI proteins and the initiation of the excision-replication-packaging (ERP) cycle of the SaPI25 (Fig. 1)

Methods

Results

Conclusion