Jpn. J. Infect.Dis., 53 (2), 82-84, 2000

Laboratory and Epidemiology Communications

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Epidemiological Analysis of Methicillin-Resistant Staphylococcus aureus Outbreaks in a Neonatal Intensive Care Unit by Genomic DNA Fingerprinting Using Pulsed-Field Gel Electrophoresis

Aki Kaneko, Hirofumi Miyazawa, Fumiko Kirikae, Kiminori Toyooka, Masahito Hashimoto, Shinji Yamasaki, Mitsuharu Hasegawa, Chiaki Takeuchi, Tadatoshi Kuratsuji, Morito Sumiya, Oichirou Kobori, Yoshio Yazaki and Teruo Kirikae*

International Medical Center of Japan,
Toyama 1-21-1, Shinjuku, Tokyo 162-8655

Communicated by Hiroshi Yoshikura

(Accepted May 26, 2000)

Nosocomial infection caused by methicillin-resistant Staphylococcus aureus (MRSA) is a critical problem in neonatal intensive care units (NICUs) (1). Genome typing using pulsed-field gel electrophoresis (PFGE) is useful for investigating the source, transmission, and spread of MRSA infection (2).

An MRSA outbreak, though with no clinical manifestations, occurred in NICU twice successively with an interval of 7 months. The hospital had total of about 900 beds and the NICU had five beds. Isolates from individual patients affected by the two outbreaks and those from other wards during the second outbreak were tested for chromosomal DNA type (contour-clamped homogeneous electric field system, CHEF Mapper^TM: Bio-Rad Laboratories, Hercules, Calif., USA), antibiotic resistance (WalkAway^TM, Dade Behring, Deerfield, Ill., USA), enterotoxin serotype (SET-RPLA: Denka Seiken Co., Tokyo), toxic shock syndrome toxin-1 (TSST-1) production (TST-RPLA: Denka Seiken), and coagulase serotype (Denka Seiken).

Eight different PFGE patterns of SmaI DNA digests were detected (Fig. 1). Band-based cluster analysis (Molecular Analyst^TM: Bio-Rad) revealed that these eight PFGE patterns shared more than 93 % similarity (Fig. 2). Sensitivity to antibiotics is shown in Table 1; there were five different patterns. All isolates except No. 27 produced enterotoxin type C (Table 2). In addition, No. 44 produced type B enterotoxin. No. 27 did not produce any type of enterotoxin. All the isolates produced TSST-1 and coagulase type II (Table 2).

PFGE pattern, antibiotic pattern and enteterotoxin serotype were identical for all the 5 isolates from the first outbreak in the NICU and for 4 in 5 isolates from the second outbreak (Table 2), while isolates from other wards showed larger diversity. These results indicated that, though there was an interval of 7 months, the two MRSA outbreaks in the NICU was due to a single strain.

REFERENCES

1. Baltimore, R.S.(1998): Neonatal nosocomial infections. Semin. Perinatol., 22, 25-32.
2. Ichiyama, S., Ohta, M., Shimokata, K., Kato, N., and Takeuchi, J. (1991): Genomic DNA fingerprintings by pulsed-field gel electrophoresis as an epidemiological marker for study of nosocomial infections caused by methicillin-resistant Staphylococcus aureus. J. Clin. Microbiol., 29, 2690-2695.

*Corresponding author: Fax: +81-3-3202-7364, E-mail: tkirikae@ri.imcj.go.jp

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