Methicillin-susceptible ST398 Staphylococcus aureus responsible for bloodstream infections: an emerging human-adapted subclone?

Abstract

In the course of an annual 3-month bloodstream infections (BSI) survey conducted during a four-year period in 31 healthcare institutions located in three noncontiguous French regions, we report 18 ST398 Staphylococcus aureus BSI. ST398 BSI incidence showed a seven-fold increase during the study period (0.002 per 1,000 patient days in 2007 vs. 0.014 in 2010). ST398 BSI isolates differed from the pig-borne multiresistant clone: 17/18 BSI isolates were methicillin susceptible and none was of t011, t034 or t108 pig-borne spa-types. ST398 BSI isolates had homogenous resistance patterns (15/18 with only Ery(r)) and prophagic content (all harboured the hlb-converting Sau3int phage). The clustering of BSI and pig-borne isolates by spa-typing and MLVA, the occurrence of Sau3int phage in BSI isolates and the lack of this phage in pig-borne isolates suggest that the emergence of BSI isolates could have arisen from horizontal transfer, at least of the Sau3int phage, in genetically diverse MSSA ST398 isolates. The acquisition of the phage likely plays a role in the increasing ability of the lysogenic ST398 isolates to colonize human. The mode of acquisition of the non pig-borne ST398 isolates by our 18 patients remains unclear. ST398 BSI were diagnosed in patients lacking livestock exposure and were significantly associated with digestive portals of entry (3/18 [16.7%] for ST398 vs. 19/767 [2.5%] for non ST398 BSI; p = .012). This raises the question of possible foodborne human infections. We suggest the need for active surveillance to study and control the spread of this human-adapted subclone increasingly isolated in the hospital setting.

Figure 1. Phage content and MLVA dendrogram obtained with the 18 strains of ST398 BSI isolates and 12 strains of ST398 representing the diversity of the pig-borne clone.

The dendrogram was calculated using the MLVA data obtained for18 ST398 BSI isolates, 12 pig-borne ST398 MSSA and MRSA isolates and 3 reference strains (COL, MSSA476 and MW2). On the basis of their MLVA fingerprints, the isolates segregated into different clusters. Results of antibiotic susceptibility tests [resistance to Oxa (methicillin), Te (tetracycline), E (erythromycin), K (kanamycin), To (tobramycin)], spa-typing, as well as determination of prophage content of Sa1int to Sa7int genes assessed by Phage integrase multiplex PCRs, are also indicated. The BSI isolates appear underlined with grey colour in the figure.