Molecular Characterization of a Prevalent Ribocluster of Methicillin-Sensitive Staphylococcus aureus from Orthopedic Implant Infections. Correspondence with MLST CC30

Abstract

Staphylococcus aureus is the leading etiologic agent of orthopedic implant infections. Here a ribocluster of 27 S. aureus strains underwent further molecular characterization and subtyping by multilocus sequence typing (MLST) and spa-typing. This cluster had been detected by automated ribotyping (with the EcoRI restriction enzyme) of 200 S. aureus isolates from periprosthetic infections of patients who underwent revision at the Rizzoli Orthopaedic Institute. The ribocluster, consisting of agr type III strains, with a 74% co-occurrence of bone sialoprotein-binding (bbp) and collagen-binding (cna) genes, lacked mecA and IS256, and exhibited a high prevalence of the toxic shock syndrome toxin gene (tst, 85%). Strains' relatedness was analyzed by BURP and eBURST. Two predominant spa types, t012 (32%) and t021 (36%), and one predominant sequence type, ST30 (18/27, 67%) were identified: a S. aureus lineage spread worldwide belonging to MLST CC30. Two new sequence types (ST2954, ST2960) and one new spa type (t13129) were detected for the first time. Interestingly, the 27-strain cluster detected by ribotyping corresponded exactly to MLST CC30, the sole CC identified by eBURST.

Keywords: methicillin-sensitive Staphylococcus aureus; multilocus sequence typing; orthopedic implant infections; spa-typing; virulence factors.

Figure 1

Analysis of ST allelic profiles of the MLST CC30 by eBURST v3 software. The sequence type ST30, genotypic founder of the MLST CC30, is represented by a blue circle. All other STs are SLV (single locus variant). The new STs (ST2960, ST2954) are represented by green circles. ST34, a sub-group founder, is represented by a red circle. The other ST243 is represented by a sky-blue circle. In parentheses the number of strains.

Figure 2

Analysis of the spa types clustered by BURP algorithm of the Ridom StaphType Software. On the left, within the frame, the population distribution is illustrated as per BURP population snapshot. The predict founder of a cluster is shown in blue, while the others in black. On the right, a customized representation of the CCs takes into consideration the type of mutations. Near the lines of connection, the mutations involved in the transition from a spa type to the next one are reported in detail. All DNA changes are meant to occur from the founder to the periphery. Legend: numbers along the lines refer to the repeat sequence involved in the mutation; + indicates the acquisition of a repeat sequence; -indicates the loss of a repeat sequence; within circles the numbers of the strains of each CC appear between brackets; extensive losses including more than a repeat sequence are reported along the lines between brackets; s.n. mut.: single nucleotide mutation of a sequence repeat.