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Journal of Clinical Microbiology, July 2008, p. 2438-2441, Vol. 46, No. 7
0095-1137/08/$08.00+0 doi:10.1128/JCM.00808-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
Molecular Characterization of Staphylococcus aureus Bloodstream Isolates Collected in a Dutch University Hospital between 1999 and 2006
Eric Nulens,1
Ellen E. Stobberingh,2
Helke van Dessel,2
Silvie Sebastian,2
Frank H. van Tiel,2
Patrick S. Beisser,2 and
Ruud H. Deurenberg2*
Department of Medical Microbiology, General Hospital St. Jan, Brugge, Belgium,1
Department of Medical Microbiology, University Hospital Maastricht, Maastricht, The Netherlands2
Received 29 April 2008/
Accepted 29 April 2008

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.

TEXT
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 (SCC
mec), of which six main types, I to VI, have
been distinguished. It has been suggested that MRSA originated
through the transfer of SCC
mec 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, 1x iQ Sybr green Supermix (Bio-Rad Laboratories, The Netherlands), and 5 µl of a suspension of 1.5 x 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 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.
The main
spa-CC was
spa-CC012, which harbored 43% of the MSSA
isolates (Table
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.
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
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
2).
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 SCC
mec, 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. 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. 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
* Corresponding author. Mailing address: Department of Medical Microbiology, Maastricht Infection Center University Hospital Maastricht, P.O. Box 5800, 6202 AZ Maastricht, The Netherlands. Phone: 31-43-3874644. Fax: 31-43-3876643. E-mail:
rde{at}lmib.azm.nl 
Published ahead of print on 7 May 2008. 

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Journal of Clinical Microbiology, July 2008, p. 2438-2441, Vol. 46, No. 7
0095-1137/08/$08.00+0 doi:10.1128/JCM.00808-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.
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