Identification and molecular characterization of mannitol salt positive, coagulase-negative staphylococci from nasal samples of medical personnel and students

Abstract

The identification of mannitol salt positive, coagulase-negative staphylococci (CNS) is often disregarded when Staphylococcus aureus is screened in clinical samples using mannitol salt agar. However, the emergence of CNS as important human pathogens has indicated that reliable methods for the identification of clinically significant CNS are of great importance in understanding the epidemiology of infections caused by them. The identification and molecular characterization of mannitol salt positive CNS from nasal samples of medical personnel and students is reported here. A total of 84 mannitol salt positive staphylococcal isolates were obtained from 240 nasal samples, of which 15 were CNS. The API STAPH system classified the CNS isolates into six species, and one-third of the isolates were identified with confidence levels of <80 %. 16S-23S rRNA intergenic spacer length polymorphism analysis (ITS-PCR) identified only two species (Staphylococcus haemolyticus and Staphylococcus saprophyticus). This identification was confirmed by antibiotyping, species-specific PCR and PFGE. The results from this study indicate that ITS-PCR is a potentially useful and reliable tool, enabling hospital laboratories to obtain rapid, full and accurate identification of CNS at the species level. INTRODUCTIONThe identification of bacterial pathogens in human infection plays a key role in the management of patients in health care institutions. In many clinical laboratories, Staphylococcus aureus is usually isolated on nonspecific media (e.g. blood agar) and then presumptively identified before definitive overnight characterization (Kloos & Bannerman, 1995). In an attempt to achieve presumptive isolation in a single step, mannitol salt agar (MSA) was developed in 1945 for the selective isolation of pathogenic staphylococci in the clinical microbiology laboratory (Chapman, 1945;Blair et al., 1967). The growth and production of yellow colonies, due to the high salt content of the medium and fermentation of mannitol, is regarded as a presumptive tool in the identification of S. aureus. It is also described as a characteristic for the differentiation of coagulase-positive staphylococci from coagulase-negative staphylococci (CNS) (Duguid, 1989). However, there are reports that some CNS can also produce yellow colonies on MSA (Martinez et al., 1992;Merlino et al., 1996;Mir et al., 1998;Jayaratne & Rutherford, 1999;Simor et al., 2001;Zadik et al., 2001). Mannitol salt positive CNS impede and delay the isolation and subsequent identification of S. aureus in clinical samples on the primary plate. Nevertheless, they have attracted very little attention and in most cases are not identified to species level.As a group, the CNS are among the most frequently isolated bacteria in the clinical microbiology laboratory. They are becoming increasingly important as causative agents of hospital-acquired bacteraemia, with the increasing use of prosthetic devices and other invasive technologies in medical institutions (von Eiff et al., 2002 The aim of this stud...

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Asymptomatic bacteriuria in pregnancy in osogbo with special reference to staphylococcus saprophyticus

“…It has been suggested that the mechanism of oxacillin resistance in S. saprophyticus may be other than the production of PBP2a. A study by Shittu et al (2006) in Ile-Ife, Nigeria also showed two S. saprophyticus isolated from the nose of medical personnel to exhibit resistance to oxacillin but lack mecA gene. In addition, whole genome sequence of S. saprophyticus has shown that its staphylococcus cassette chromosome (SCC) does not contain the mec antibiotic resistance determinant (Kuroda et al, 2005).…”

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