Protein A of Staphylococcus aureus is a pathogenic factor whose encoding gene, spa, shows a variation in length in different strains. In this study the spa gene variation in S. aureus isolated from healthy carriers and patients was studied, We also compared this variation among MRSA with MSSA strains. 208 strains of Staphylococcus aureus which we were isolated from Gorgan, north of Iran were studied, 121 cases from patients and 87 cases from healthy carriers, 59 out of them were MRSA and 149 MSSA. Samples DNA were extracted and amplified by specific primer of spa gene. In 4 (3.8%) strains of them no spa gene was detected, and 10.6% had a dual band (1200 and 1400？bp). In strains with one band, the length of spa gene differed from 1150 to 1500？bp. The most prevalent length was 1350–1400？bp (37%). The frequencies of short spa bands (1150–1200？bp) in patients strains were significantly higher. In 4 (3.8%) strains of them no spa gene was detected, and 10.6% had a dual band (1200 and 1400？bp). In strains with one band, the length of spa gene differed from 1150 to 1500？bp. The most prevalent length was 1350–1400？bp (37%). The frequencies of short spa bands (1150–1200？bp) in patients strains were significantly higher. The spa gene length of 1350–1400？bp in MSSA was more than in MRSA strains ( ). The average length of spa in isolated strains from urinary tract infections was more than others. It is concluded that the length of spa gene depends either on resistance to Methicillin or the source of S. aureus isolation. 1. Introduction Staphylococcus aureus is one of the most important infectious pathogens in either hospitals or within the community. Protein A is a virulence factor with molecular weight of 42？KD [1]. It is covalently anchored to the peptidoglycan of S. aureus. 90% of protein A is found in the cell wall and the remaining 10% is free in the cytoplasm of bacteria. In some strains of S. aureus, protein A is unable to adhere to the cell wall and therefore is released into the media (secretary protein). This is mainly seen among meticillin-resistant S. aureus (MRSA) strains [2]. Protein A is an antiphagocytic protein that is based on its ability to bind the Fc portion of immunoglobulin G (IgG). Its NH2-terminal part contains five homologous IgG-binding units, A, B, C, D, and E, consisting of approximately 58 amino acids each. The COOH-terminal part of this protein which is thought to bind to the cell wall of Staphylococcus aureus consists of several repeats of an octapeptide. This protein acts as antiplatelet, anticomplement, and mytogen, [1, 3]. It is
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