Aerobic plate count Petrifilm Plate, red dots are colony forming units, black dots have been counted using a Sharpie pen

Who would have thought having your own on-farm lab would be so easy- and affordable? I am kicking myself for not trying this sooner.  Doing our own, in house, milk quality tests will help our small, licensed dairy to stay on top of cleaning regimens and milk quality. Even though our results will not be official (you have to be a certified lab to have official results) they will still assist us and even help inspectors know that our food safety program is more complete. So just what is a “plate count and how do you do it yourself?

Plate counts were traditionally preformed by taking a small sample of a substance and pouring or swabbing it onto a glass petri dish that held had a gelled growth medium. The plate was then kept warm for a certain number of hours after which a lab technician would literally count the number of “dots” on the plate. The dots were each a cluster of bacteria called a “colony forming unit” (cfu for short).  The most common plate test is the “standard or aerobic plate count” (SPC or APC).

Fortunately for us, 3M makes a wonderful, simplified product called Petrifilm Plates. These plates are ready to use, needing no added growth medium. They are also inexpensive, costing about .70 cents each (for the aerobic count) and come in a box of 100. You will also need an incubator and luckily, a compact, low tech unit (costing about 70.00) is sold by the same company from which we already buy a lot of our supplies, Nelson Jameson (www.nelsonjameson.com). They also sell the 3M Petrifilm plates and other needed

Compact Incubator from Nelson Jameson

supplies. In addition to the aerobic count plates, it is a good idea to also buy coliform plates (a box of 50 is 38.00). You will need a count plate spreader (a little plastic disc made especially for spreading the sample onto the Petrifilm plate) and, if you want to do swab tests on dry surfaces, 3M Quick Swabs work great. The Quick Swabs are a bit more expensive, about 1.50 each and come in a box of 50.

The SPC grows all kinds of bacteria from milk or swabs of surfaces- even the good bacteria. For example, if you took a sample of milk during cheesemaking, the plate count numbers would be through the roof, but that is what you would want. Milk fresh from the udder, however, should have very low counts, preferable less than 1,000 cfu per milliliter.

Step by Step Instructions for Plating a Milk Sample

1. Obtain a 1 ml sample of milk using a sterilized 1ml syringe or a pipette.
2. Lift the film on the room temperature Petrifilm plate and place the sample in the center.
3. Lower the film gently.
4. Center the plate spreader, smooth side up, over the sample, lower onto film and press firmly to spread the sample in an even circle.
5. Place the Petrifilm in the incubator at 90 F (note: the compact incubator from Nelson Jameson states that the shelf temperature is 10 degrees lower than the thermometer readout, so adjust your temperature accordingly) and incubate for 48 hours.
6. After 45-50 hours (48 is ideal) remove the plate from the incubator.
7. Using a fine tipped Sharpie pen, count each red dot, no matter how small, using the pen to mark as you count (so that you don’t double count any cfu’s).
8. If the plate has very few red dots, then count the entire plate. If there are quite a few, you can count one square and multiply the result by 20.  Do this with several squares so that you get an accurate average. (Each square represents 1 square centimeter and the plate area is 20 square centimeters, thus the multiplication by 20)

If an undiluted sample grows too many cfu’s it is impossible to get a good count, since the plate will be over crowded with overlapping colonies.  You can carefully dilute the sample with sterile water by 50% and then multiply the resulting count by two. (For example, say I diluted the ml of milk with half sterile water and then count between 200 and 250 cfu’s per square. I would then multiply that number by two for 400-500, and then multiply that by 20 for 4,000-5,000 cfu/ml.)

Another useful Petrifilm plate is called the coliform count plate. These have a growth medium that will only allow for coliforms (harmless and bad) to grow. So if you want to know how many of those cfu’s on your standard count are coliforms, this test is a great follow up. Coliforms are the most common problem bacteria in milk and in a cheese plant (and sometimes the deadliest). So low coliform counts from work surfaces and equipment, as well as in milk and brine, are a great confirmation of good processes. Coliform counts should be much lower than SPC’s, a reading of less than 10 cfu/ml is ideal.

Petrifilm plates should be stored in a cool, dry area. Be sure to tightly seal the individual film packets. They are so sensitive that they can simply be exposed (with the cover film pealed back) to the air and culture contaminants via that route. So you don’t want to expose them until ready to inoculate.

You should know that you may not run tests for anyone other than yourself. You can let people run their own using your incubator, but you may not run a test and provide a count result, that is only for certified professionals. It doesn’t mean you can’t have a “plating party” and show others how to count their own!

There are several other Petrifilm plates that I will probably try out on our farm, including staph aureus for udder health, and yeasts and molds, for cheese quality. I’ll be sure to share whatever I learn with all of you.  But for now, this is enough to work on. Oh, as a way of logging our results, I plan on take a photograph of each counted plate and keeping those on file.

Your Happy Lab Geek, Gianaclis