Magazine article Journal of Continuing Education Topics & Issues

One Laboratory's Experience with the Transition from Culture to Molecular Methods for the Detection of Methicillin-Susceptible Staphylococcus Aureus and Methicillin-Resistant Staphylococcus Aureus

Magazine article Journal of Continuing Education Topics & Issues

One Laboratory's Experience with the Transition from Culture to Molecular Methods for the Detection of Methicillin-Susceptible Staphylococcus Aureus and Methicillin-Resistant Staphylococcus Aureus

Article excerpt

The introduction of molecular testing into clinical microbiology laboratories can be a major paradigm shift. Clinical microbiology laboratories have an opportunity to replace traditional methods with molecular methods that can have numerous advantages. However, educating and fulfilling the clinical needs of physicians while complying with administrative, operationa (l), staffing, and financial constraints can be daunting both during and after the introduction of molecular methods. In hospital settings, rapid molecular methods can play major roles in identifying patients with healthcare-associated infections (HAI) such as Clostridium difficile infections (CDI), colonization by vancomycin-resistant enterococci and/or detection of methicillin-susceptible Staphylococcus aureus (MSSA) and methicillin-resistant S. aureus (MRSA). Healthcare-associated infections cause significant morbidity and mortality in both hospital and community settings (1) and have been reported to result in over 6.5 billion dollars in treatment costs and excess hospital stays (2).

Staphylococcus aureus causes a significant number of infections and deaths worldwide and is a major source of HAIs. S. aureus is found primarily as normal flora in the nose; 2030% of the world population is long-term carriers of S. aureus (3). Prolonged hospital stays, proximity to patients with S. aureus carriage, endogenous S. aureus flora, and long-term exposure to antimicrobial agents can lead to carriage of MSSA and MRSA. Screening of certain populations for MSSA and MRSA has become important for therapeutic management, infection control, and epidemiological purposes. S. aureus accounts for the majority of surgical site infections at a rate of 25%. Nasal colonization with either MSSA or MRSA is the most important and significant risk factor for developing surgical site infections (4). Colonized patients can be decolonized with mupirocin ointment in the nares twice daily and given chlorhexidine baths daily for 5 days prior to surgery (5-7). The logistics of MRSA screening and prophylaxis can become complicated with patients who need to travel to a (sometimes distant) center days ahead of surgery, especially if culture-based methods are used to detect MRSA.

Traditionally, bacterial culture has been the gold standard for isolating clinically important bacteria, such as S. aureus. Commonly used, multipurpose, commercially available media that grows most facultative anaerobic bacteria and selective media designed to enhance the growth of specific bacteria and inhibit others has been used almost universally to detect S. aureus at 24 to 48 hours. Prior to 2003, most clinical microbiology laboratories cultured/ screened for MRSA by inoculating non-selective Trypticase[TM] Soy Agar with 5% Sheep Blood and a selective medium such as Mannitol Salt Agar or Phenylethyl Alcohol Agar with 5% Sheep Blood (PEA).

Then in early 2000's chromogenic medium from various manufacturers became available. These were selective and differential media that contained a chromogenic substrate that forms a colored compound when degraded by bacterial enzymes contained within S. aureus. If the media also contained a high concentration of methicillin, any growth would be considered methicillin-resistant, as well. Chromogenic medium reduced the need for subculture and additional biochemical and antimicrobial susceptibility testing. Reluctance to implement molecular testing for the detection of MSSA and MRSA include (a) clinical microbiologists are comfortable with solid media, (b) the time needed for a conventional culture seemed clinically acceptable, (c) the additional cost of a molecular method was too high, and (d) isolates may be needed for further testing.

The next major change in the detection of MRSA and MSSA was the introduction of molecular methods. In 2010, three molecular tests for S. aureus and/or MRSA were commercially available: Cepheid Xpert[R], BD GeneOhm[TM] StaphSR, and Roche Lightcycler[R]. …

Search by... Author
Show... All Results Primary Sources Peer-reviewed

Oops!

An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.