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Dehydrated Culture Media

OXACILLIN RESISTANCE SCREENING AGAR BASE

CODE: CM1008

A medium for screening for methicillin resistant Staphylococcus aureus (MRSA) directly from routine swab samples.

Typical Formula*

gm/litre

Peptone

11.8

Yeast Extract

9.0

Mannitol

10.0

Sodium chloride

55.0

Lithium chloride

5.0

Aniline Blue

0.2

Agar

12.5

PH 7.2 ± 0.2 @ 25°C

 

* Adjusted as required to meet performance standards 

ORSAB SELECTIVE SUPPLEMENT

Code: SR0195

Vial contents (each vial is sufficient for 500ml of medium)

per vial
per litre

Polymyxin B

25,000IU

50,000IU

Oxacillin

1.0mg

2.0mg

Directions
Suspend 51.75g of Oxacillin Resistance Screening Agar Base in 500ml of distilled water and bring gently to the boil to dissolve. Sterilise by autoclaving at 121°C for 15 minutes. Cool to 50°C and aseptically add the contents of one vial of ORSAB Selective Supplement SR0195, reconstituted as directed below. Mix well and pour into sterile Petri dishes.

Description
ORSAB is intended as a medium for the screening for methicillin resistant Staphylococcus aureus (MRSA) directly from routine swab samples. The screening of patients and staff for the early detection of MRSA colonisation is essential if epidemics are to be prevented. ORSAB is based on traditional Mannitol Salt Agar CM0085 with a reduction in salt concentration from 75g/l (7.5%) to 55g/l (5.5%). This lower level of salt is still sufficient to inhibit most bacteria other than staphylococci whilst optimising growth of low numbers of MRSA.

Oxacillin Resistance Screening Agar Base is a nutritious and selective medium containing peptones for growth, a high salt concentration and lithium chloride to suppress non-staphylococcal growth with mannitol and aniline blue for the detection of mannitol fermentation.

The antibiotics contained in ORSAB Selective Supplement SR0195 are oxacillin at 2 mg/litre to inhibit methicillin sensitive Staphylococcus aureus (MSSA) and polymyxin B for the suppression of other bacteria able to grow at such a high salt concentration, e.g. Proteus spp.

Typical colonies of MRSA are intense blue in colour on a colourless background enabling the organism to be more easily identified in mixed culture than the pale yellow colonies seen on Mannitol Salt Agar.

Culture Medium

Colony Colour

 

Positive

Negative

ORSAB
CM1008 & SR0195

Intense blue on colourless media

Straw / No Growth

Mannitol Salt Agar CM0085

Pale yellow on red media

Pink-Red / No Growth

Technique
Take a routine swab sample from the patient or person to be screened. Rub the swab onto an ORSAB plate in one set of streaks near the plate perimeter. The sample material should then be streaked across the plate using the diminishing sweep technique. Incubate at 37°C for 24 hours.
Examine after 24 hours for blue colonies. Confirm suspected MRSA with coagulase test Staphytect Plus DR0850 or Dryspot Staphytect Plus DR0100 and PBP2’ DR0900. Re-incubate negative plates for a further 24 hours if necessary. Do not re-incubate positive plates.

Storage conditions and Shelf Life
Store the dehydrated medium at 10-30°C and use before the expiry date on the label.
ORSAB Selective Supplement should be stored at 2-8°C.
Store the prepared medium at 2-8°C.

Appearance
Dehydrated medium: Straw-grey coloured, free-flowing powder
Prepared medium: Blue-grey coloured gel

Quality control

Positive control:

Expected results

Staphylococcus aureus (MRSA strain) ATCC® 43300 *

Good growth; blue coloured colonies

Negative controls:

 
Staphylococcus aureus (MSSA strain) ATCC® 25923 *No growth
Escherichia coli ATCC® 25922 *No growth
* This organism is available as a Culti-Loop®
 
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