Complete MRSA Nasal Screening Using a Single, New and Novel Swab Transport System
A new patented Flocked Swab with Liquid Amies, Copan, Ca, USA (ESwab™) was designed to improve the sensitivity of the traditional transport swab systems. In this new swab the organism inoculum theoretically is released into 1mL of Amies Liquid and after that it should be possible to perform different tests on the collected sample. The purpose of this study was to evaluate the Eswab™ for MRSA nasal screening.
A total of 30 patients from a Nephrology Unit of Hospital Sao Paulo (Sao Paulo, Brazil) were screened for MRSA colonization. The samples were collected using one Eswab™ per patient and they were separated in two different groups (15 samples in each group). After swabbing the anterior nares, all the swabs were vortexed in the Liquid Amies medium and from the first group of samples three aliquots of 200μl were collected from the medium and tested. For direct culture, the 200μl aliquot was inoculated directly onto MRSA screening medium agar plate (Probac, Brazil) and incubated at 35°C for 24-48h. The other 200μl aliquot was first cultured in a pre-enrichment broth medium for 24h at 35°C and then cultured onto MRSA screening medium agar for another 24-48h at 35°C. The third aliquot was used to screen MRSA by PCR. The DNA extraction was performed using QIAamp DNA Mini Kit (Qiagen, Germany), followed by an in house mecA PCR protocol. S. aureus ATCC 43300 and two MRSA clinical strains previously identified were used as positive controls. On the second group of samples, after vortexing, the swab was removed to the enrichment broth. An aliquot of 200μl of the medium was also removed for the PCR and another aliquot of 700μl was centrifuged. The pellets were reconstituted in 100μl of distilled water and the whole volume was cultured onto a selective plate for direct culture.
Among the 15 patients screened on the first group, 12 were MRSA-colonized. Both, pre-enrichment culture and PCR were concordant in 100% of the cases. In contrast, only four out of 12 patients were positive by direct culture. On the second group, 13 were MRSA colonized with 100% concordance between pre-enrichment culture and PCR results. Eleven out of these 13 were also positive by direct culture. In both groups, results from PCR were obtained after 6h, while from cultures were obtained after 48-72h.
The new Eswab™ proved to be excellent for a complete MRSA nasal screening. It was possible to perform from a single swab three different tests and have the first results faster than by conventional tests. Although direct culture were less sensitive than the others two tests, we were able to improve the sensitivity of this test by increasing the initial volume inoculated.