The Prevalence of Resistance to Methicillin in Staphylococcus aureus Strains Isolated from Patients by PCR Method for Detec-tion of mecA and nuc Genes

Abstract

Background: Methicillin-resistant Staphylococcus aureus (MRSA) is the main cause of hospital infection emerged over the last decades. Rapid detection of MRSA is important for patient care and proper usage of infection control. Detection of mecA genes (encoding resistance to methicillin and other similar antibiotics) and nuc genes (encoding staphylococcal thermostable nuclease) by PCR method is now considered for rapid identification of MRSA strain. The aim of this study was to determine the prevalence of MRSA isolated from patients in Tehran, Iran by PCR method for detection of mecA and nuc genes.

Method: Phenotypic method such as microscopic and colony morphology and catalase and coagulase tests were used for identification of S. aureus isolates. DNA was extracted from all isolates and the presence of nuc and mecA gene was detected by PCR method. For determination of MRSA by phenotypic methods, oxacillin disk diffusion test were used. Data were analyzed by SPSS software.

Results: Out of 126 clinical sample identified by phenotypic method, 101 isolates had nuc gene. In disk diffusion tests by oxacillin disk, 78.2% of isolates were considered to be MRSA, but in PCR method for mecA gene, 69% isolates were positive.

Conclusions: The results showed a high prevalence of methicillin-resistance among S. aureus isolates. Identifying MRSA strains, isolating MRSA-positive patients and carrier's treatment in a hospital to prevent MRSA infection is important in limiting the spread of MRSA. The PCR method for detection of nuc and mecA genes has potential for rapid and accurate diagnosis of MRSA strains.

Keywords: MRSA; Methicillin-resistant Staphylococcus aureus; mecA; nuc.

Fig. 1

Image of agarose gel electrophoresis after amplification of nuc gene (270 bp) for some isolates by PCR method. Lane M is 100 bp DNA ladder, lanes 1 to 5 and 7 to 11 is nuc gene, NC: negative control, PC: positive control (S. aureus ATCC29213)

Fig. 2

Image of agarose gel electrophoresis after amplification of mecA gene (310 bp) for some isolates by PCR method. Lane M is molecular size marker (100 bp DNA ladder), lanes 3 to 6, 8 and 11 is mecA gene, PC: Positive control (Methicillin resistant S. aureus clinical isolate containing mecA gene), NC: Negative control

Fig. 3

Frequency distribution of MRSA and MSSA isolates in hospitals. Most of MRSA strains were obtained from Imam Khomeini, Mostafa Khomeini and Motahari Hospitals, while most of MSSA strains were obtained from Shariati Hospital

Fig. 4

Frequency distribution of MRSA and MSSA isolates in wards. The majority of the strains obtained from different hospital wards were identified as MRSA strains, except the respiratory ward that the prevalence of MRSA and MSSA were almost same

Fig. 5

Frequency distribution of MRSA and MSSA isolates in clinical specimens. The prevalence of MRSA strains in all clinical specimens was significantly higher than MSSA, except for wounds specimen.