| Description |
A retrospective and prospective study was performed on clinical human blood cultures that grew coagulase-negative staphylococcus species (CNS). The purpose of the retrospective study was to determine whether there were multiple phenotypes of CNS present, when it was initially thought that one strain of CNS was present. For the retrospective study, 183 archived and frozen CNS isolates recovered from blood cultures were retrieved from the Associated Regional and University Pathologists (ARUP) microbiology laboratory. After incubating the isolates on sheep blood agar (SBA) for five days, this retrospective study showed that 29.5% of the CNS had multiple phenotypes and that 7.1% of the CNS were different strains and/or species. The prospective study had three objectives: first, to determine prospectively whether multiple phenotypes of CNS from blood cultures could be detected after five days of incubation; second, to evaluate the accuracy and correlation of current biochemical and commercial methodologies available for the identification of CNS; and finally, to determine the clinical significance of CNS recovered from blood cultures and whether multiple phenotypes, strains, and/or species were present in patients with true CNS sepsis. For the prospective study, 178 blood cultures that grew CNS were evaluated. After incubating the CNS on the initial SBA subculture medium for five days, it was determined that 20% of the total cultures had multiple phenotypes present. It was shown that 18% of the total cultures truly had multiple strains and/or species of CNS present in each based on the isolates exhibiting different biochemical patterns. These isolates were identified with a conventional biochemical scheme as well as a new Gram Positive ID2 panel (MicroScan, Dade Inc. West Sacramento, CA) and a new API Staph strip (bioMerieux Vitek, Hazelwood, MO). These commercial identification methods were compared to a conventional biochemical scheme for their accuracy in the identification of the CNS isolated. The MicroScan accurately identified 85% of the CNS and the API Staph Strip accurately identified 78% of the CNS. However, it was difficult to determine conclusively which commercial method is better because each method showed strengths and weaknesses in its ability to identify certain species of CNS. Finally, a clinical chart review was performed on 75% of the patients with the positive blood cultures for CNS in the prospective study. The clinical chart review showed that 74.5% of the CNS were contaminants. Almost all the (25.5%) CNS that were not contaminants were associated with nosocomial or immunocompromised patients. Interestingly, 28.6% of the CNS that were considered clinically significant, showed multiple strains and/or species of CNS present. This was based on biochemical differences in each phenotype compared with others in the same culture. Furthermore, 25.6% of the patients with CNS that were considered contaminants were still treated with antibiotics, usually vancomycin. This indicates a probable overuse of vancomycin which could lead to increasing populations of CNS developing vancomycin resistance. |
