Prevalence of Staphylococcus aureus protein A (spa) mutants in the community and hospitals in Oxfordshire

Rowena Fung,Ruth R Miller,Kyle Knox,A Sarah Walker,Derrick W Crook,David H Wyllie,Heather Godwin,Antonina A Votintseva,Rory Bowden

doi:10.1186/1471-2180-14-63

Abstract

BackgroundStaphylococcal protein A (spa) is an important virulence factor which enables Staphylococcus aureus to evade host immune responses. Genotypes known as “spa-types”, based on highly variable Xr region sequences of the spa-gene, are frequently used to classify strains. A weakness of current spa-typing primers is that rearrangements in the IgG-binding region of the gene cause 1-2% of strains to be designated as “non-typeable”.ResultsWe developed an improved primer which enabled sequencing of all strains, containing any type of genetic rearrangement, in a large study among community carriers and hospital inpatients in Oxfordshire, UK (6110 isolates). We identified eight novel spa-gene variants, plus one previously described. Three of these rearrangements would be designated “non-typeable” using current spa-typing methods; they occurred in 1.8% (72/3905) asymptomatically carried and 0.6% (14/2205) inpatient S. aureus strains. Some individuals were simultaneously colonized by both formerly non-typeable and typeable strains; previously such patients would have been identified as carrying only currently typeable strains, underestimating mixed carriage prevalence and diversity. Formerly non-typeable strains were found in more spa-types associated with multilocus sequence type ST398 (35%), common among livestock, compared to other groups with any non-typeable strains (1-4%), suggesting particular spa-types may have been under-represented in previous human studies.ConclusionsThis improved method allows us to spa-type previously non-typeable strains with rearrangements in the spa-gene and to resolve cases of mixed colonization with deletions in one or more strains, thus accounting for hidden diversity of S. aureus in both community and hospital environments.

Highlights

Staphylococcal protein A is an important virulence factor which enables Staphylococcus aureus to evade host immune responses
Deletions appear to be over-represented in clonal lineages related to livestock In total, we found 20 spa-types from 33 individuals associated with nine types of rearrangements in the spa-gene (Additional file 2: Table S2)
Spa-typing of 6110 S. aureus isolates showed that 1.8% of samples from 1.8% community carriers and 0.6% of samples from 0.7% inpatients were formerly non-typeable

Summary

Introduction

Staphylococcal protein A (spa) is an important virulence factor which enables Staphylococcus aureus to evade host immune responses. A weakness of current spa-typing primers is that rearrangements in the IgG-binding region of the gene cause 1-2% of strains to be designated as “non-typeable”. S. aureus encodes many virulence factors including the surface Ig-binding protein A (spa) whose function is to capture IgG molecules in the inverted orientation and Typing the highly variable Xr region of the spa-gene is one of the most common methods for genotyping S. aureus. One weakness of current spa-typing methods is that rearrangements in the in the IgG-binding region of the gene, where the forward spa-primer is located, lead to. Five nonspa-typeable S. aureus clinical strains with rearrangements in the IgG-binding domain of the spa-gene were first described by Baum et al in 2009 [14]. Whilst the prevalence of such rearrangements can be directly estimated from the proportion of non-typeable strains, detecting rearrangements that do not affect spa-typing would require sequencing the whole spa-gene; such rearrangements may still be informative with respect to population structure

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Results

Conclusion