MeReSa Agar Base

Intended use

Recommended for the selection, isolation and identification of Methicillin Resistant Staphylococcus aureus from clinical specimens.

Composition**

Final pH ( at 25°C) 7.1±0.2

**Formula adjusted, standardized to suit performance parameters

# Equivalent to beef extract

Directions

Suspend 40.06 grams in 500 ml purified/ distilled water. Heat to boiling to dissolve the medium completely. Sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes. Cool to 45-50°C and aseptically add sterile rehydrated contents of 1 vial of MeRS Selective Supplement (FD229) and CF Selective Supplement II (FD259) both in combination for more selectivity as desired. Mix well and pour into sterile Petri plates.

Principle And Interpretation

Staphylococcus aureus sometimes referred to as “Staph” is a common bacterium found on the skin of healthy people. It is responsible for infections ranging from superficial to systemic (1,2). Staphylococcus aureus resistant to the antibiotic methicillin are referred to as Methicillin Resistant Staphylococcus aureus (MRSA) (3). Initially staphylococcal infections were treated using penicillin. But over the years, resistance to penicillin developed, so methicillin was the next drug of choice. Unfortunately certain strains (MRSA) have now developed resistance to methicillin also. Patients with breaks in their skin due to wounds, indwelling catheters or burns are those with certain risk of developing MRSA infection (4). Symptoms in serious cases may include fever, lethargy and headache. MRSA can cause UTI, pneumonia, toxic shock syndrome and even death, which can be controlled to a great extent by maintaining personal hygiene after interaction with an MRSA infected person (3).

MRSA were recognized in 1980s as a major clinical and epidemiological problem. MRSA strains were heterogeneous in their expression of resistance to β-lactam agents, in that large differences in the degree of resistance were seen among the individual cells in a population. The basis of methicillin-resistance is the production of an additional penicillin-binding protein mediated by the mec A gene, an additional gene found in methicillin-resistant Staphylococci. MeReSa Agar Base was developed to detect the presence of the mec A gene in S.aureus i.e. methicillin-resistant S.aureus.

Tryptone and HM peptone B provide nitrogenous and carbonaceous compounds, long chain amino acids, vitamins and other essential nutrients. Lithium chloride and methicillin inhibit most of the contaminating microflora except methicillin-resistant S.aureus (MRSA). Glycine and sodium pyruvate enhance the growth of Staphylococcus species. Colour of the colonies is due to the indicator mixture and mannitol used in the medium. Sodium chloride maintains the osmotic equilibrium of the medium as well as supports the growth of Staphylococcus species.

Type of specimen

Clinical samples

Specimen Collection and Handling

For clinical samples follow appropriate techniques for handling specimens as per established guidelines (5,6). After use, contaminated materials must be sterilized by autoclaving before discarding.

Warning and Precautions

In Vitro diagnostic use only. For professional use only. Read the label before opening the container. Wear protective gloves/protective clothing/eye protection/face protection. Follow good microbiological lab practices while handling specimens and culture. Standard precautions as per established guidelines should be followed while handling clinical specimens. Safety guidelines may be referred in individual safety data sheets.

Limitations

1. MIC must be carried out for confirmation.
2. Further biochemical tests must be carried out for further identification

Performance and Evaluation

Performance of the medium is expected when used as per the direction on the label within the expiry period when stored at recommended temperature.

Quality Control

Appearance Cream to yellow homogeneous free flowing powder

Gelling Firm, comparable with 2.0% Agar gel.

Colour and Clarity of prepared medium Pale pink coloured clear to slightly opalescent gel forms in Petri plates

Reaction Reaction of 8.01% w/v aqueous solution at 25°C. pH : 7.1±0.2

pH 6.90-7.30

Cultural Response Cultural characteristics observed with added MeRS Selective Supplement (FD229) and CF Selective Supplement II (FD259) both in combination after an incubation at 35-37°C for 18-24 hours.

Key : *Corresponding WDCM numbers.

Storage and Shelf life

Store dehydrated powder and prepared medium at 2-8°C. Use before expiry period on the label. On opening, product should be properly stored dry, after tightly capping the bottle in order to prevent lump formation due to the hygroscopic nature of the product. Improper storage of the product may lead to lump formation. Store in dry ventilated area protected from extremes of temperature and sources of ignition. Seal the container tightly after use. Product performance is best if used within stated expiry period.

Disposal

User must ensure safe disposal by autoclaving and/or incineration of used or unusable preparations of this product. Follow established laboratory procedures in disposing of infectious materials and material that comes into contact with clinical sample must be decontaminated and disposed of in accordance with current laboratory techniques (5,6).

Reference

1. Doyle, Beuchat and Montville, (Eds.), 1997, Food Microbiology Fundamentals and Frontiers. American Society for Microbiology, Washington, D.C.
2. Murray P. R., Baron J. H., Pfaller M. A., Jorgensen J. H. and Yolken R. H., (Ed.), 2003, Manual of Clinical Microbiology, 8th Ed., American Society for Microbiology, Washington, D.C.
3. Methicillin Resistant Staphylococcus aureus, Copyright © 1997-2005, Canadian Centre for Occupational Health and Safety, Sept 19th, 2005.
4. Dr. Alan Johnson, Methicillin resistant Staphylococcus aureus (MRSA) infection, The support group for MRSA sufferers and Dependents, AUG 1st , 2005.
5. Isenberg, H.D. Clinical Microbiology Procedures Handbook 2nd Edition.
6. Jorgensen, J.H., Pfaller, M.A., Carroll, K.C., Funke, G., Landry, M.L., Richter, S.S and Warnock., D.W. (2015) Manual of Clinical Microbiology, 11th Edition. Vol. 1.