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J Clin Microbiol. Jul 2008; 46(7): 2438–2441.
Published online May 7, 2008. doi:  10.1128/JCM.00808-08
PMCID: PMC2446898

Molecular Characterization of Staphylococcus aureus Bloodstream Isolates Collected in a Dutch University Hospital between 1999 and 2006[down-pointing small open triangle]

Abstract

We observed that, between 1999 and 2006, up to 50% of the methicillin-susceptible Staphylococcus aureus (MSSA) bloodstream isolates in our hospital had a genetic background common to endemic methicillin-resistant S. aureus clones (clonal complex 5 [CC5], CC8, CC22, CC30, and CC45). Furthermore, several successful MSSA lineages, such as CC7 and CC15, were observed.

Methicillin-resistant Staphylococcus aureus (MRSA) can cause a wide variety of infectious diseases (15). MRSA has a clonal population structure, and five major clonal complexes (CCs), i.e., CC5, CC8, CC22, CC30, and CC45, as determined with multilocus sequence typing (MLST), have been observed (4). In MRSA, β-lactam antibiotic resistance is mediated by the staphylococcal cassette chromosome mec (SCCmec), of which six main types, I to VI, have been distinguished. It has been suggested that MRSA originated through the transfer of SCCmec from MRSA into methicillin-susceptible S. aureus (MSSA) and that the genetic background determines the stability of the new MRSA clone (9, 13).

Since no information exists about the MSSA population structure in Dutch hospitals and the relation to MRSA lineages, the genetic background of MSSA in the Maastricht University Hospital was investigated with spa typing.

Between 1999 and 2006, 501 MSSA bloodstream isolates were cultured from individual patients in the Maastricht University Hospital, a tertiary 715-bed university hospital. For each year, the first 25 MSSA isolates, corresponding to ca. 40% of the isolates, were analyzed. Several isolates were not viable when cultured from the stocks and were excluded from the study.

Real-time amplification of the spa locus was performed with the primers spa-1113F and spa-1514R (1). The reaction conditions included 0.5 μM spa-1113F, 0.25 μM spa-1514R, 1× iQ Sybr green Supermix (Bio-Rad Laboratories, The Netherlands), and 5 μl of a suspension of 1.5 × 108 CFU/ml of the isolate in a total volume of 25 μl. The amplification was performed on a Bio-Rad MyiQ single-color real-time PCR detection system, using the following program: 4 min at 95°C and 30 s at 65°C, followed by 35 cycles of 30 s at 65°C, 15 s at 95°C, and 30 s at 65°C. Sequencing of the spa locus was performed as described previously (10). The spa types were assigned through the Ridom SpaServer (http://spaserver.ridom.de) and clustered into spa-CCs using the algorithm based upon repeat pattern (BURP) with Ridom StaphType 1.4 using the default settings (5, 22). Since it has been shown that spa typing/BURP results are in agreement with results obtained by MLST (20, 22), the associated MLST CCs were allocated through the SpaServer.

Ninety-three spa types were observed among the MSSA isolates. These spa types were clustered into 13 spa-CCs, seven singletons, and four spa types that were excluded from the analysis, because the spa locus was less than five spa repeats in length (17) (Table (Table1).1). The most common spa types were t091 (12.2%), t084 (6.7%), t002 (4.4%), t008 (3.9%), t127 (3.9%), t012 (3.3%), and t015 (3.3%). Each of the remaining 86 spa types (62.3% of the isolates) accounted for between 2.2 and 0.6% each.

TABLE 1.
Distribution of spa types and spa-CCs among MSSA isolates

The main spa-CC was spa-CC012, which harbored 43% of the MSSA isolates (Table (Table1).1). This spa-CC consisted of various MLST CCs. It consisted mainly of MSSA associated with CC7 (n = 23; founder t091), CC15 (n = 14; founder t084), and CC30 (n = 26; founder t012) (Fig. (Fig.1).1). Additional spa-CCs included spa-CC008 (9%), spa-CC002 (9%), spa-CC015 (6%), and spa-CC127 (6%), associated with CC8, CC5, CC45, and CC1, respectively. The remaining spa-CCs accounted for 3% or less of the isolates (Table (Table1).1). Four of the seven isolates that were excluded from BURP analyses were associated with CC45. No spa-CC was replaced with another spa-CC during the study period (Table (Table22).

FIG. 1.
(a) Heterogeneous clonal structure of spa-CC012. At the right, spa type t012 is the founder of CC30, and at the left, spa type t084 is the founder of CC15 and spa type t091 is the founder of CC7. (b) Related spa repeat successions of the spa types of ...
TABLE 2.
Distribution of spa-CCs in time (1999 to 2006)

Up to 50% of the MSSA had a genetic background observed in epidemic hospital-associated MRSA clones, i.e., CC5, CC8, CC22, CC30, or CC45 (4), and community-associated (CA) MRSA clones, i.e., CC1, CC8, CC30, or CC59 (23). Although we took a random sample of the MSSA isolates available, our results are comparable to a study in Belgium, in which 45% of the MSSA isolates had a genetic background common to MRSA clones (8). MSSA associated with CC1, CC5, CC8, CC30, and CC45 have been described previously in Brazil, Germany, among Danish isolates from the 1960s and 1970s, and in the Dutch and English community (6, 7, 18, 19, 24, 25). CC59 is a common CA-MRSA lineage in Asian countries, such as Singapore and Taiwan, but has also recently been observed in The Netherlands (3, 11, 12, 23). The observation that MSSA with genetic backgrounds common to MRSA were found might suggest that these MSSA isolates could be a recipient for SCCmec, since this element is suggested to be mobile (9).

Several MSSA lineages were found that were not associated with MRSA, such as CC7, CC9, CC12, CC15, CC25, CC51, and CC101. The observation that more MSSA CCs were found compared to MRSA CCs suggests that MSSA is more heterogeneous. Similar results have been observed in Belgium, Brazil, England, Germany, and Portugal (2, 7, 8, 14, 25). Recently, MSSA associated with CC7, CC9, CC12, CC15, CC25, CC51, and CC101 have been observed in Belgium and in the English community, although no MRSA strains associated with these lineages were found (7, 8). Similarly, in Portugal, MSSA strains associated with CC9, CC12, CC15, CC51, and CC25 have been found in the community and in hospitals, whereas no MRSA strains from these CCs were observed (2). The CC51 genetic background has been found among MSSA in Denmark from the 1960s and in the Dutch community between 1997 and 2002 (6, 16). The observation that 50% of the MSSA in the present study had a genetic background uncommon to MRSA clones may suggest that these genetic backgrounds do not provide a genomic environment necessary for the stable integration of SCCmec, as has been shown in a previous study (13).

Previous studies have shown a good concordance between spa typing/BURP and MLST (20, 22). The observation that spa-CC012 was heterogeneous (Fig. (Fig.1a)1a) could be explained by the fact that large chromosomal replacements, including the spa locus, between different S. aureus lineages have occurred, as has been shown previously between CC8 and 30 (18, 22), or could be due to related spa repeat successions in different MSSA lineages, possibly caused by recombination in the spa locus (Fig. (Fig.1b)1b) (21). A spa-CC containing CC7 and CC15 has been described previously (20).

Acknowledgments

We thank Hajo Grundmann from the National Institute of Public Health and the Environment in The Netherlands for critical reading of the manuscript.

Footnotes

[down-pointing small open triangle]Published ahead of print on 7 May 2008.

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