MacConkey Agar

Description

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.