Mastitis - Plating Milk Samples

Set up on a clean area of the laboratory bench.

You will need the mastitis milk sample that was taken using good aseptic technique, a permanent marking pen, a sterile swab, and a culture tri-plate. The pen and swab are in the drawer in front of the incubator. Tri-plates are kept in the white refrigerator.

Using a permanent marker, record the cow's ID, the date, and the affected quarter on the bottom of the plate. Write on the area with the lightest media (MacConkey) to make reading the numbers easier.

Tri-plates have three media: Factor (red media used primarily for the identification of Staph. aureus and Staph. Species), Modified TKT (dark burgundy media used to identify Strep. species), and MacConkey (pinkish media used to identify gram negative bacteria). There is a book explaining more about these plates and interpreting culture findings that is kept on top of the incubator.

Using a sterile swab, dip it into the milk to saturate it (it holds at least 0.1 ml of milk) and spread the milk evenly on the surface of each sector on the tri-plate. Make sure that the entire surface of the agar has been covered. Be sure to re-dip the swab between each section. This will help insure accurate results.

Place the lid back on the plate and place the plate in the incubator upside down (agar containing part of the plate on top with the agar face down). Check that the incubator is turned on and set to the temperature marked on its thermostat control.

Place the milk sample in the rack in the top of the freezer section of the white refrigerator.

Plates can be read in as little as 12 hours, but should be checked again at 24 hours to identify any slow growing organisms.