What is the usual sensitivity pattern in MRSA isolates?
Most of the MRSA isolates also demonstrate resistance to multiple classes of
antimicrobial agent, including clindamycin and erythromycin and sometimes quinolones,
chloramphenicol, tetracycline, trimethoprim-sulfamethoxazole and the aminoglycosides.
What are the quality control measures that should be adopted for sensitivity testing?
Following recommendations should be taken into consideration:
Depth of Muller- Hinton
agar should not be less or more than 20 ml in 90 mm Petri plate.
Inoculum density should be adjusted to McFarland 0.5 turbidity standards.
Incubation time and temperature should be according to the recommended protocol of each method.
These QC strains should be run parallel when the test is performed.
Why are MRSA important?
Pathogenicity: MRSA have many virulence factors that enable them to cause
disease in normal hosts. For example, MRSA are frequent causes of healthcare-associated bloodstream and
catheter-related infections. MRSA are also an emerging cause of community-associated infections, especially
skin and soft tissue infections and necrotizing pneumonia.
Limited treatment options: Vancomycin and two newer antimicrobial agents, linezolid and daptomycin, are among the drugs that are used for treatment of severe healthcare-associated MRSA infections. Although some strains remain susceptible to trimethoprim-sulfamethoxazole, gentamicin, or rifampin, these drugs are not typically used as first-line agents. Because of the rapid emergence of resistance to rifampin, this drug should never be used as a single agent to treat MRSA infections.
MRSA are transmissible: A MRSA outbreak can occur when one strain is transmitted to other patients or close contacts of the infected persons in the community. Often this occurs when a patient or health-care worker is colonized with aa MRSA strain (i.e., carries the organism but shows no clinical signs or symptoms of infection) and, through contact, spreads the strain to another person. Hand washing and screening patients for MRSA should be performed to decrease transmission and reduce the number of patients infected with MRSA.
Is it difficult to detect Oxacillin/Methicillin resistance?
Accurate detection of Oxacillin/Methicillin
resistance can be difficult due to the presence of two subpopulations (one susceptible and the other resistant)
that may coexist within a culture of staphylococci. All cells in a culture may carry the genetic information
for resistance, but only a small number may express the resistance in vitro. This phenomenon is termed
heteroresistance and occurs in staphylococci resistant to penicillinase-stable penicillin, such as oxacillin.
Cells expressing heteroresistance grow more slowly than the oxacillin-susceptible population and may be missed
at temperatures above 35oC. This is why CLSI recommends incubating isolates being tested against oxacillin,
methicillin, or nafcillin at 33-35oC (maximum of 35oC) for a full 24 hours before reading.
Why are oxacillin and cefoxitin tested instead of methicillin?
First, methicillin is no longer commercially
available in the United States. Second, oxacillin maintains its activity during storage better than methicillin
and is more likely to detect heteroresistant strains. However, cefoxitin is an even better inducer of the mec A
gene and disk diffusion tests using cefoxitin give clearer endpoints and are easier to read than tests with oxacillin.
If oxacillin is tested, why are the isolates called “MRSA” instead of “ORSA”?
When resistance was first described in
1961, methicillin was used to test and treat infections caused by S. aureus. However, oxacillin, which is in the same class
of drugs as methicillin, was chosen as the agent of choice for testing staphylococci in the early 1990s. The acronym MRSA
is still used by many to describe these isolates because of its historic role.
RESISTANCE IN STAPHYLOCOCCUS AUREUS
If intermediate results are obtained when testing Oxacillin 1ug disc for S.aureus then test for cefoxitin disk (mecA or PBP2a), or Oxacillin MIC test, or Oxacillin salt agar-screening test. Report the result of alternate test result rather than the intermediate result.
CLINICAL MICROBIOLOGY PROCEDURES HANDBOOK — ASM (American Society Manual 3rd edition)
CLSI (Clinical and laboratory standard instituted) Jan 2014 Performance standards for antimicrobial susceptibility testing. CLSI approved standard M100-S24. Clinical and Laboratory Standards Institute, Wayne, PA.
July 2018
Dr.Moiz Ahmed
Year 1 Resident
Department of pathology and laboratory medicine
The Aga Khan University Hospital, Karachi