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MacConkey Agar



This classic agar formula is used for the detection, selection, and identification of various gram negative organisms from individual cow and environmental samples.



One of the most common causes of environmental mastitis is coliforms which are gram-negative lactose fermenters.  This pink agar is a selective media that uses the characteristic of growth and lactose fermentation to identify gram-negative organisms.  Lactose fermentation is expressed on this agar through the production of a pink colony.  Escherichia coli (E. coli) and Klebsiella spp. are the most common lactose fermenting mastitis organisms.  Both organisms produce a pink colony. E. coli colonies are pink and generally smaller compared to Klebsiella spp., which are larger and produce a pink colony with a mucoidal appearance.  Non-lactose fermenting organisms (i.e. Pseudomonas spp.) will also grow on MacConkey agar, however, the colonies will be colorless or opaque.  The antimicrobial components of this agar inhibit the growth of gram-positive and some gram-negative bacteria, such as Pasteurella spp.  When used as a secondary test, the inability of Pasteurella to grow on MacConkey agar is the primary characteristic of this organism used in its identification.

Refer to table on back for expected culture responses.


Organism Growth Reaction
Strep. agalactiae Inhibited
Staph. aureus Inhibited
Mycoplasma Not Applicable
Strep. dysgalactiae Inhibited
Strep. uberis Inhibited
E- strep Inhibited
Staph. species Inhibited
E. Coli Excellent Pink colonies
Klebsiella pneumoniae Excellent Pink mucoid colonies
Klebsiella species Excellent Pink mucoid colonies
Pseudomonas species Excellent Colorless colonies
Pseudomonas aeruginosa Excellent Green/colorless colonies
Pasteurella No Growth
Proteus No Growth
Serratia Excellent Pinkish/red colonies*
Bacillus Inhibited
Yeast No Growth
Mold Not Applicable
Nocardia Not Applicable
Prototheca Not Applicable
Trueperella pyogenes Inhibited
C. bovis Not Applicable

Additional information

Weight .06 lbs
Dimensions 3.5 × 3.5 × .5 in


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