A preliminary evaluation of a new selective agar supplemented with desferrioxamine for detection of methicillin-resistant Staphylococcus aureus

Screening for MRSA in ICU patients. How does PCR compare with culture?

BACKGROUND

The polymerase chain reaction (PCR) for the detection of methicillin-resistant Staphylococcus aureus (MRSA) has the potential to reduce the time before the identification of nasal carriage, and therefore help in the control and prevention of spread.

OBJECTIVE

In a pilot study in a general intensive care unit (ICU) we compared culture with PCR for MRSA detection.

METHODS

Between October and November 2005, 170 swabs taken from 63 patients on admission, and then twice weekly were cultured on Columbia Blood Agar, CHROMagar MRSA (CHROMagar Microbiology, Paris, France) and by enrichment and swaps were also tested for MRSA by real-time PCR, using the IDI-MRSA assay.

RESULTS

Five patients, previously not know to be positive for MRSA, were detected, but PCR detected only three of these initially. The quickest time to detection using culture was with CHROMagar MRSA, which detected 80% of MRSA. Real-time PCR was rapid (2.25 h) and facilitated the optimization of antibiotic therapy in two of three positive PCR patients, but PCR was less specific and more expensive than CHROMagar MRSA.

CONCLUSIONS

PCR facilitates the rapid detection of MRSA and has the potential to contribute to preventing spread, but should continue to be used in conjunction with culture.

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

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.

Guidelines for the laboratory diagnosis and susceptibility testing of methicillin-resistant Staphylococcus aureus (MRSA)

These evidence-based guidelines have been produced after a literature review of the laboratory diagnosis and susceptibility testing of methicillin-resistant Staphylococcus aureus (MRSA). We have considered the detection of MRSA in screening samples and the detection of reduced susceptibility to glycopeptides in S. aureus. Recommendations are given for the identification of S. aureus and for suitable methods of susceptibility testing and screening for MRSA and for S. aureus with reduced susceptibility to glycopeptides. These guidelines indicate what tests should be used but not when the tests are applicable, as aspects of this are dealt with in guidelines on control of MRSA. There are currently several developments in screening media and molecular methods. It is likely that some of our recommendations will require modification as the new methods become available.