Cat. no. G97 Mannitol Salt Agar (MSA) with Oxacillin, 15x100mm Plate,
10 plates/bag


Hardy Diagnostics Mannitol Salt Agar (MSA) with Oxacillin is used as a primary screening medium for the simultaneous detection and differentiation of methicillin resistant Staphylococcus spp. (MRS). It can be used as a cost effective screening method for MRSA.


Mannitol Salt Agar (MSA) is used as a selective, differential media for pathogenic staphylococci. Oxacillin has been added for selective isolation of methicillin-resistant strains. Oxacillin is used instead of methicillin due to its greater stability. This plate may be used to screen environmental and clinical specimens.


Ingredients per liter of deionized water:*

Sodium Chloride 60.0gm
Phenol Red Broth Base 16.0gm
Mannitol 10.0gm
Polymyxin B 10.0mg
Oxacillin 4.0mg
Agar 15.0gm

Final pH 7.4 +/- 0.2 at 25ºC.

* Adjusted and/or supplemented as required to meet performance criteria.


Storage: Upon receipt store at 2-8ºC. away from direct light. Media should not be used if there are any signs of deterioration (shrinking, cracking, or discoloration), contamination, or if the expiration date has passed. Product is light and temperature sensitive; protect from light, excessive heat, moisture, and freezing.



Specimen Collection: Infectious material should be submitted directly to the laboratory without delay and protected from excessive heat and cold. If there is to be a delay in processing, specimens should be inoculated into the appropriate transport media and refrigerated until inoculation in a sterile container, or other appropriate means of transport. Consult appropriate references for specimen collection and transport. (1)

Method of Use: Prior to inoculation, the medium should be brought to room temperature. Inoculate the media with the specimen and streak for isolation. Incubate aerobically at 35ºC. for 24-48 hours.


Most methicillin-resistant Staphylococcus aureus (MRSA) are capable of fermenting mannitol within 24 hours. Fermentation of mannitol is indicated by a color change from red to yellow. However, delayed fermentation of mannitol may occur with a few strains of MRSA, so negative plates should be incubated for an additional 24 hours. It is recommended that a coagulase or latex agglutination test be performed on mannitol-fermenting isolates for a presumptive identification of MRSA.

MRS organisms other than Staphylococcus aureus may grow on the media in 24-48 hours but appear as mannitol non-fermentors and are coagulase-negative.


While the medium is differential and selective for MRSA, a coagulase or latex agglutination test as well as other antimicrobial and biochemical tests must be performed for complete identification.

Methicillin-resistant, coagulase-negative staphylococci will also grow on this medium but can be differentiated by their inability to ferment mannitol and a negative coagulase reaction.


Standard microbiological supplies and equipment such as loops, other culture media, swabs, applicator sticks, incinerators, and incubators, etc., as well as serological and biochemical reagents, are not provided.


Test Organisms Inoculation Method* Incubation Results
Time Temperature Atmosphere
Staphylococcus aureus
ATCC ® 43300
A 24-48hr 35°C Aerobic Growth; small colonies with yellow color change in media
Proteus mirabilis
ATCC ® 12453
A 24hr 35°C Aerobic Partial to complete inhibition
Staphylococcus aureus
ATCC ® 29213
B 24hr 35°C Aerobic Inhibited



Mannitol Salt Agar (MSA) with Oxacillin should appear clear, and light orange in color.

MRSA growing on Mannitol Salt Agar

Staphylococcus aureus (ATCC ® 43300) colonies growing on Mannitol Salt Agar with Oxacillin (Cat. no. G97). Incubated aerobically for 48 hours at 35ºC.

P. mirabilis inhibited on Mannitol Salt Agar

Proteus mirabilis (ATCC ® 12453) growth inhibited on Mannitol Salt Agar with Oxacillin (Cat. no. G97). Incubated aerobically for 48 hours at 35ºC.


1. Jorgensen., et al. Manual of Clinical Microbiology, American Society for Microbiology, Washington, D.C.

2. Tille, P., et al. Bailey and Scott's Diagnostic Microbiology, C.V. Mosby Company, St. Louis, MO.

3. Lally, R.T., T.M. Ederer and B. Woolfrey. 1985. Evaluation of Mannitol Salt Agar and Oxacillin as a screening medium for methicillin-resistant Staphylococcus aureus. J. Clin. Microbiol.; 22: 501-504.

4. Quality Assurance for Commercially Prepared Microbiological Culture Media, M22. Clinical and Laboratory Standards Institute (CLSI - formerly NCCLS), Wayne, PA.

5. Van Enk, R.A. and K. Thompson. 1992. Primary isolation medium for the recovery of methicillin-resistant Staphylococcus aureus. J. Clin. Microbiol.; 30: 504-505.

6. Isenberg, H.D. Clinical Microbiology Procedures Handbook, Vol. I, II & III. American Society for Microbiology, Washington, D.C.

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