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Department of Medical Microbiology and Medical Immunology, Hospital Rijnstate, Arnhem
Department of Microbiology and Infection Control, Amphia Hospital, Breda
Department of Medical Microbiology and Infectious Diseases, Erasmus MC, Rotterdam
Department of Clinical Microbiology and Infection Control, VU Medical Center and Department of Medical Microbiology, Academic Medical Center, Amsterdam
National Institute of Public Health and the Environment, Bilthoven, The Netherlands
ABSTRACT
The mecA gene was lost in 36 (14.4%) of 250 methicillin-resistant Staphylococcus aureus isolates after 2 years of storage at –80°C with the Microbank system (Pro-lab Diagnostics, Austin, Tex.). Further analysis of 35 of these isolates confirmed loss of the mecA gene in 32 isolates. This finding has important implications for the management of strain collections.
TEXT
The evaluation of new diagnostic or in vitro antimicrobial susceptibility tests for methicillin-resistant Staphylococcus aureus (MRSA) requires well-defined strain collections. In The Netherlands all MRSA strains that have been isolated since 1989 are stored at the National Institute of Public Health and the Environment (RIVM; Bilthoven, The Netherlands). In 1999 we evaluated the MRSA Screen latex agglutination test (Denka Seiken, Ltd.) by the use of a part of the RIVM collection (14). Thereafter, part of this collection was stored at the microbiology laboratory of the Amphia Hospital in Breda, The Netherlands. In 2001 we retested 250 of the MRSA isolates stored at the Amphia Hospital for methicillin resistance. Surprisingly, 36 (14.4%) of the 250 isolates no longer harbored the mecA gene.
In the present report we describe a further analysis of 35 of the 36 strains that apparently lost the mecA gene. We compared the isolates stored at the Amphia Hospital with renewed subcultures from the original isolates stored at the RIVM.
The 35 MRSA isolates are a subset of a collection of 250 MRSA isolates, comprising 247 different phage types and three isolates that were not typeable, collected in The Netherlands between 1989 and 1998. The original isolates were stored at the RIVM (site B) at room temperature in Moeller agar medium in the year they were isolated. In 1999, subcultures from the original isolates were made at site B and S. aureus identification and methicillin resistance determination were performed by multiplex PCR for the coagulase and mecA genes as described previously (4, 14). Thereafter, the isolates were transported to and stored at the laboratory of the Amphia Hospital in Breda, The Netherlands (site A), in the commercial Microbank Bacterial Preservation system (Pro-lab Diagnostics, Austin, Tex.). Each Microbank vial contains cryopreservative and approximately 25 porous beads that serve as carriers to support microorganisms. Vials were inoculated according to the manufacturer's instructions. Vials were kept in a –80°C freezer. This freezer is equipped with a sound alarm, which goes off when the temperature drops, to ensure storage conditions. In the summer of 2002, renewed subcultures from the 35 original isolates were requested from site B for further analysis.
The 35 isolates from site A and site B were compared by pulsed-field gel electrophoresis (PFGE). PFGE was performed as described by Murchan et al. (11).
On the isolates from both sites, oxacillin disk diffusion testing according to NCCLS standards using 1-μg oxacillin disks (12), mecA gene detection by Southern blotting and hybridization, and multiplex PCR for staphylococcal cassette chromosome mec (SCCmec) typing were performed.
mecA gene detection by Southern blotting. PFGE gels were blotted onto Hybond N+ membranes (Amersham, Roosendaal, The Netherlands). Membranes were probed with a PCR-amplified mecA gene DNA fragment with the chemiluminescence system provided by Boehringer (Boehringer, Mannheim, Germany). Hybridization was visualized by exposure of the blots to photographic films for various time periods.
Multiplex PCR for mec element type assignment was performed as described previously by Oliveira and de Lencastre (13). PCR-amplified material was identified by agarose gel electrophoresis.
PCR for the mecA gene and nuc gene was performed directly on a bead from the 35 Microbank vials. A single bead was incubated in 100 μl of Tris-EDTA glucose buffer (pH 8.0) containing 1 mg of lysostaphin/ml. After removal of the bead, DNA was extracted according to the protocol published by Boom et al. (2). DNA was dissolved in 100 μl of 10 mM Tris-HCl (pH 8.0), and 5-μl portions were subjected to mecA PCR according to the procedure of Murakami et al. (10). The PCR for the nuc gene was performed as described previously by Brakstad et al. (3).
Broth enrichment culture. A single bead from a Microbank vial was added to Mueller-Hinton (MH) broth and incubated for 48 h at 35°C; thereafter, the broth was subcultured onto an MH agar plate and an MH agar plate supplemented with 2% NaCl. To facilitate the recognition of mecA gene-positive isolates, two 1-μg oxacillin disks were placed on each agar plate. The presence of the mecA gene was confirmed by MRSA screen test (bioMerieux, Marcy l'Etoile, France) and PCR for the mecA gene as described by Murakami et al. (10).
To determine statistical significance, the Fisher's exact test was used. Statistical significance was accepted when the P value was <0.05.
Table 1 shows the numbers and percentage of isolates per year of isolation that lost the mecA gene as found in our previous evaluation in 2001. Only 2.1% of the MRSA strains isolated in 1998 lost the mecA gene, while the percentages were significantly higher, ranging between 11.1 and 23.9%, among the strains isolated earlier (P = 0.005).
By comparing the 35 isolates from site A and the original isolates stored at site B by PFGE, the possibility that there had been a mix-up between MRSA and methicillin-sensitive isolates in the freezer at site A was ruled out: all pairs showed a high degree of similarity (Fig. 1.). Nevertheless, small differences were nearly always present, possibly associated with physical loss of the mecA-containing genomic locus. In Fig. 1, band differences between the isogenic pairs are highlighted. Major genetic rearrangements seem to have occurred. When the actual chromosomal differences are assessed, 11 out of 35 pairs cannot be discriminated on the basis of PFGE. For six pairs, the chromosomal size of the isolate from site A, ranging between 10,000 and 450,000 bp, is larger than that of the isolate from site B (Table 2). For the remaining 18 strains, deletions ranging between 10,000 and 350,000 bp can be documented for the isolates from site A (Table 2).
In Fig. 2, the results from our consecutive attempts to detect the mecA gene from the isolates stored at site A and site B are presented. In Table 2, the results of the oxacillin disk diffusion, the mecA gene detection by Southern blotting, and the SCCmec multiplex PCR are presented per isolate. In summary, the mecA gene was detected, either by Southern blotting or by SCCmec multiplex PCR, in 3 (8.6%) of the 35 isolates from site A and in 14 (40.0%) of the 35 corresponding isolates from site B (P = 0.004).
The PCRs for the nuc gene and the mecA gene that were performed directly on the beads from the Microbank vials detected the nuc gene in each of the 35 vials; however, the mecA gene could only be detected from beads from two vials. From one of these two vials, one of the SCCmec-positive isolates (strain 95-900) was cultured.
Enrichment culture retrieved nine (25.7%) mecA gene-positive strains from the 35 Microbank vials. Among these nine strains were the three SCCmec PCR-positive strains and the two strains from the Microbank vials in which the mecA was detected by PCR directly on a bead. Subculturing the broth onto MH agar (with or without 2% NaCl) onto which 1-μg oxacillin disks were placed yielded similar results, recovering seven and nine strains, respectively. The subculture had to be incubated for at least 48 h. In addition to culturing a bead in a nonselective MH broth, we also cultured in two selective MH broths containing 4 mg of oxacillin/liter and 16 mg of cefoxitin/liter, respectively. This, however, did not result in a higher yield of mecA gene-positive isolates; a total of seven mecA gene-positive isolates were detected in each of the two selective broths.
Loss of the mecA gene in such a large percentage of MRSA isolates during storage at –80°C with the Microbank system has never been described before. Hürlimann-Dalel et al. described the apparent loss of the mecA gene in methicillin-resistant S. aureus isolates stored as lyophilized cultures (7). However, they did not confirm the presence of the mecA gene at the time the isolates were stored; therefore, it is not certain that all isolates carried the mecA gene to start with (7). Loss of the mecA gene has also been observed in vivo (5, 9). Katayama et al. have demonstrated that the SCCmec, which contains the mecA gene, can be integrated to and excised from the S. aureus chromosome (8). However, spontaneous excision of the SCCmec did not occur appreciably in the strain that was examined (8).
The present study confirms the loss of the mecA gene in 32 of the 35 strains stored at site A that were examined more closely. In three isolates the mecA gene could be detected again without any extra effort. Enrichment culture retrieved mecA gene-positive isolates from nine Microbank vials. Of the original 35 strains stored at site B, 21 (60%) had also lost the mecA gene. This suggests significant genetic instability in these strains.
Comparison of the isolates stored at site A and at site B by PFGE showed that these isolates are essentially the same isolates, although differences were detected in 24 of the 35 strains. Insertion and deletion events have occurred during storage. The availability of isogenic pairs of strains showing differently sized deletions and insertions allows for detailed examination of the region of excision.
All vials still contained viable S. aureus isolates; the result of nuc PCR performed directly on a bead from the Microbank system was positive, and cultures yielded S. aureus. Surprisingly, the result of PCR for the detection of the mecA gene directly on a bead was positive for two vials only. Not only the mecA gene-containing strain but also the genetic element itself was lost upon storage at –80°C.
A statistically significantly lower percentage (2.1%) of strains collected in 1998 lost the mecA gene compared to the results seen with strains collected in earlier years. It can be hypothesized that MRSA strains consist of a heterogeneous population of both mecA-positive and mecA-negative cells. These mecA gene-negative cells initially comprise only a small minority of the population. If the mecA gene-negative cells better resist the storage conditions, they have a selective advantage; the longer the storage period, the larger the effect of this selective advantage and, hence, the larger the effect of longer storage periods on the loss of mecA-positive cells. The fact that in 1999 all isolates, irrespective of their year of isolation, were mecA positive seems to refute our hypothesis. However, when they were stored at –80°C in 1999, the older strains could have contained a larger percentage of mecA-negative cells to begin with, due to their longer storage period, than did the more recently isolated strains.
One of the explanations for the different numbers of mecA gene-positive isolates detected in the study in 2001 and in the present study might be that each time, another single bead from the Microbank vial was used. Each Microbank vial contains approximately 25 beads. Probably, not all the beads are coated with identical amounts of mecA gene-positive cells. This agrees with our hypothesis that each MRSA strain consists of a heterogeneous population of cells containing both mecA gene-positive and -negative cells in the first place.
Preservation of strains in a microbiology laboratory is of great importance for quality control, teaching, and research (6). Freezing is a very common method of preservation and storage of microorganisms (1). Studies concentrate on the viability of the microorganisms after a certain storage period. Little attention is given to the influence of storage conditions on characteristics of the stored strain such as antimicrobial susceptibility. The Microbank Bacterial Preservation system (Pro-lab Diagnostics) is a well-known system for freezer storage of all kinds of microorganisms and is used in laboratories all over the world. At the laboratory of the RIVM (site B), MRSA isolates are stored at room temperature in Moeller agar medium. Statistically significantly more of the isolates from site B (40%) contained the mecA gene than did the isolates stored at site A (8.6%; P = 0.004). In this study we concentrated on 35 isolates of a collection that consists of 250 MRSA isolates. We do not know what happened to the mecA gene in the other isolates. Therefore, we cannot conclude that the mecA gene was lost more frequently at site A. One of the issues that remain is whether loss of the mecA gene is related to the storage method.
This study clearly demonstrates that mecA can be lost from MRSA strains stored at –80°C with the Microbank system. This has important implications for the management of strain collections. Prior to the use of MRSA isolates that have been previously stored at –80°C in any study, they have to be checked for the presence of the mecA gene at that moment in time. Maybe storage of MRSA strains can be improved by altering the storage conditions by, for example, the addition of oxacillin to the cryopreservative. This needs to be evaluated in future studies.
ACKNOWLEDGMENTS
We thank Piet Willemse, Marieke Leyten, Lisette Zwijgers, and Willem van Leeuwen for excellent technical assistance and Annemarie van 't Veen for discovering the loss of the mecA gene during her study on the isolates in 2001.
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