AST- Staphylococcus aureus and MRSA
HOME      CONTACT US      EQA PROGRAMS      RESULTS     CRITIQUES      NEWSLETTER      SITE MAP  
Click on the drop-down button for links to critiques from other sources:

Contact us to obtain critiques older than 5 years.
Scroll down to find Critique number of older challenges and a brief description.
Some older archived critiques are included in this list, but CMPT advises visitors to also consult recent literature.

Disclaimer: Please consult most recent CLSI guidelines and SOPs for your laboratory. See CLSI information page.
YEAR CRITIQUE SUMMARY
2007 M072-2 Finger (patient allergic to penicillin): S. aureus Cloxacillin or a first-generation cephalosporin is the drug of choice for most S. aureus infections.  However, where MRSA is a possibility, or the patient has a history of allergy to penicillin other agents such as trimethoprim–sulfamethoxazole or clindamycin may be used.  Clindamycin has good bone and tissue penetration and is therefore often the drug used in these situations.  A concern with the use of clindamycin is the induction of resistance that may occur during treatment.

Resistance to macrolide, lincosamide and streptogramin B (MLSB) antibiotics most commonly occurs as a result of acquisition of erythromycin resistance methylase (erm) genes.  These genes encode enzymes that alter the ribosomal binding sites of these antibiotics.  MLSB resistance may be constitutive or inducible.  Strains with MLSB constitutive resistance will demonstrate resistance to MLS antibiotics on in vitro testing.  Strains with inducible resistance will be resistant to the macrolides (erythromycin), but appear susceptible to the lincosamides (clindamycin).  Erythromycin is an effective inducer of clindamycin resistance and this factor is used in the “D zone test” to determine the presence of inducible clindamycin resistance in both staphylococci and beta-hemolytic streptococci.

The “D zone test” is a disk approximation test performed by the Kirby Bauer Method where a 2 µg clindamycin disk is placed 15 mm away from the edge of a 15 µg erythromycin disk.  Following overnight incubation strains that have inducible resistance will show flattening of the clindamycin zone in the area next to the erythromycin disc “D zone”.  Clindamycin is reported as resistant on these isolates 3.   S. aureus isolates showing susceptibility to clindamycin and no “D zone” [negative] are reported as susceptible to clindamycin. Isolates may be resistant to macrolides via an efflux mechanism encoded by the msr A gene; these strains show resistance to erythromycin only 3.  As noted under AST, CLSI also suggests that a comment  be added “This isolate is presumed to be resistant based on detection of inducible clindamycin resistance.  Clindamycin may still be effective in some patients 4.
2006 M061-2 Nares: MRSA, AST not graded
2004 M043-2 Wound (impetigo): MRSA and group A streptococci
MRSA: In light of the recent literature it may be prudent for laboratories to either begin screening or refer their MRSA isolates for vancomycin resistance.
GAS: With increased beta-lactam allergies in the general population, and in the case of a multipathogen infection, importance has been placed on erythromycin susceptibility testing. Increased rates of erythromycin resistance among group A streptococci has been reported world-wide. In Canada, studies have uncovered rates of resistance of 14%, with some laboratories quoting rates even higher. An inducible or constitutive cross resistance to clindamycin may or may not be present, depending on the mechanism of resistance. Inducible clindamycin resistance can be detected using a disk approximation test using clindamycin and erythromycin, and looking for flattening of the clindamycin zone. Organisms that show this flattening are demonstrating inducible clindamycin resistance and should be re reported as ‘clindamycin resistant'. Macrolide resistance has been shown to be more frequently associated with strains of S. pyogenes isolated from invasive infections as opposed to pharyngitis.
  M042-4 Deep wound-post C-section: Antibiotic susceptibility testing should be routinely performed on all S. aureus isolates from deep wound specimens in order to not miss methicillin-resistant Staphylococcus aureus (MRSA). This strain of B. fragilis was penicillin resistant because it produced a ß-lactamase. Although most laboratories did not perform extensive antibiotic susceptibility testing of this isolate, it would be important to provide a penicillin result and/or an interpretive comment on the report to guide the choice of antibiotics in this mixed infection.
2002 M022-3 Leg MRSA- vancomycin susceptibility testing should be included as part of the susceptibility report for MRSA strains.
2001 M013-2 Boil - nursing home patient MRSA - vancomycin susceptibility testing should be included as part of the susceptibility report for MRSA strains.
top M011-3 Abdominal wound S. aureus and Enterococcus gallinarum. S. aureus that are clearly sensitive to penicillin should have a beta-lactamase test performed and reported. With respect to the ambiguity of the significance of enterococci in superficial swab specimens, susceptibility testing was not graded. The isolate was susceptible to penicillin/ampicillin and had low level resistance.