Answer to Question #217704 in Microbiology for Doritos

Question #217704
Describe the steps you would take to identify Staphylococcus aureus from a patient’s blood sample :
Expert's answer

Staphylococcus aureus is the most pathogenic species and is implicated in a variety of infections. S. aureus is with some frequency found as normal human flora in the anterior nares (nostrils). It can also be found in the throat, axillae, and the inguinal and perineal areas. Approximately 30% of adults and most children are healthy periodic nasopharyngeal carriers of S. aureus. Around 15% of healthy adults are persistent nasopharyngeal carriers. The colonization rates among health care workers, patients on dialysis, and people with diabetes are higher than in the general population.

Staphylococcus aureus (SA) remains a major pathogen for human beings, causing infections of skin, soft tissue, bone, and other organs. Bacteremia due to this organism is common, and often occurs in association with medical interventions such as intravenous lines and implantable devices. With the increase in methicillin-resistant S. aureus (MRSA), there has been increasing dependence upon vancomycin, a drug that is inferior to the beta-lactams in its activity against methicillin-susceptible S. aureus (MSSA). If the microbiology laboratory had the ability to identify S. aureus (SA) and its drug susceptibility within hours rather than days, focused therapy would be possible earlier in the course of illness. Clinicians would be able to discontinue antibiotics when SA is not present, to discontinue other broad-spectrum antibiotics when SA is present, or to replace empiric vancomycin with nafcillin when MSSA is identified.

The GeneXpert system (Cepheid) uses real-time PCR to detect genes that encode Staphylococcus aureus protein A (SPA), the staphylococcal cassette chromosome (SCC) and methicillin resistance (mecA). All blood cultures with Gram stain revealing Gram positive cocci in clusters will be tested by PCR the day they became positive. Wound swabs submitted for routine bacteriologic culture will be tested within 48 hr of collection. Results will be compared with those of standard bacteriologic culture. In addition, discrepancies between the GeneXpert and wound culture results will be reviewed in the medical record to ascertain whether antibiotic use at the time of specimen collection is associated with false positive results in which the wound culture yields no S. aureus but PCR detects staphylococcal DNA components.

In the second phase of the study, PCR results for wound swabs and blood cultures will be reported to physicians immediately upon completion of the reaction. The clinical impact of early identification of S. aureus will be determined by comparing antibiotic treatment and clinical outcome of patients for whom early identification was available with those of patients for whom conventional bacteriological culture was the sole diagnostic test.


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