Screening Tests For Oxacillin Resistance and mecA- Mediated Oxacillin Resistance in Staphylococcus aureus group


Oxacillin Resistance Using Agar Dilution
Organism group S. aureus
Medium Mueller Hinton Agar with NaCl (4% w/v; 0.68mol/L)
Antimicrobial
Concentration
6ug/ml oxacillin
Inoculum Direct colony suspension to obtain 0.5 McFarland turbidity.

Using a 1ul loop that was dipped in the suspension, spot an area 10 to 15mm in diameter.

Alternatively, using a swab dipped in the suspension and expressed, spot a similar area or streak an entire quadrant.



Incubation Conditions 33-35°C; ambient air

(Testing at temperatures above 35°C may not detect MRSA.)

Incubation Length 24 hours
Results >1 colony = oxacillin resistant.

Examine carefully with transmitted light for >1 colony or light film of growth.

QC Recommendations Staphylococcus aureus ATCC ® 29213 - Susceptible

Staphylococcus aureus ATCC ® 43300 - Resistant

Further testing and
reporting
Oxacillin-resistant staphyloccci are resistant to all beta-lactam agents; other beta-lactam agents should be reported as resistant or should not be reported.



mecA -Mediated Oxacillin Resistance Using Cefoxitin Disk Diffusion
Organism group S. aureus and S. lugdunensis
Medium Mueller Hinton Agar
Antimicrobial
Concentration
30ug cefoxitin disk
Inoculum Standard disk diffusion recommendations
Incubation Conditions 33-35°C; ambient air

(Testing at temperatures above 35°C may not detect MRSA.)

Incubation Length 16-18 hours
Results ≤ 21mm = mecA positive

≥ 22mm = mecA negative

QC Recommendations Staphylococcus aureus ATCC ® 25923 - mecA negative
zone 23-29mm)

Staphylococcus aureus ATCC ® 43300 - mecA positive
(zone ≤ 21mm)



Further testing and
reporting
Cefoxitin is used as a surrogate for mecA -mediated oxacillin resistance.

Isolates that test as mecA positive should be reported as oxacillin (not cefoxitin) resistant; other beta-lactam agents should be reported as resistant or should not be reported.

REFERENCES

1. Clinical and Laboratory Standards Institute (CLSI - formerly NCCLS). 2009. Performance Standards for Antimicrobial Susceptibility Testing, Informational Supplement. M100. CLSI, Wayne, PA.

ATCC is a registered trademark of the American Type Culture Collection.

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