What do the numbers mean?
“CFU” stands for “colony forming unit”, and is the estimate for the number of viable (living and colonizing) bacteria or fungi in a sample. This indicates a visible colony sample, or plate sample, rather than a microscopic exam, and requires a significant amount of growth to achieve visibility. This gives an easy and reliable estimation of cells without requiring the use of a microscope. In summation, “the purpose of plate counting is to estimate the number of cells present based on their ability to give rise to colonies under specific conditions of nutrients, temperature and time.”
Each microbe has a different threshold of failure:
1000 cfu e coli (10 colonies per plate)
10000 cfu salmonella (100 colonies per plate)
5000 cfu mold/yeast (50 colonies per plate)
How does differentiation happen between the strains? For example, can e coli cross contaminate the mold/yeast plate?
Sequoia runs three separate 3M petrifilm plate tests for each microbiological sample: e coli, salmonella and molds/yeast (listeria is run for fruit/veggie samples). These plates are designed to stimulate the growth of the target microbe only. If a sample is positive for e coli, but not for salmonella, for instance, colony growth will be obvious on the e coli plate, but absent from the salmonella one.
Why do molds show up in micro or in FMI but not in the other test and vice versa?
This is a very common concern – often, a sample will fail a visual inspection for mold, yet the microbiological exam will turn up no spores. Or, conversely, the sample will look clean visually yet test high for molds on the 3M petrifilm. Quite simply, The petrifilm culture is not designed to recognize powdery mildew.
The visual mold we identify in the FMI exam is called powdery mildew, a common fungal disease of plants. This is a respiratory irritant in humans when it is inhaled, however it is not toxic to us. Molds grown from your sample in a petrifilm culture are a different story. The variety of molds and yeasts tested for on our petrifilm create toxicity in food products, and cause disease in humans.
How do I grow an uncontaminated plant?
Basic food-safe growing practices will go a long way to prevent e coli and salmonella in your plants. Make certain that any compost you use is fully ready – incompletely composted material hasn’t yet reached the temperatures required for killing off pathogens. Make sure you and any people handling the plants and flowers have washed their hands. Sanitize ALL tools. Keep animals out of your growing areas – chickens may help with insect control, but risk spreading biological contaminants. Use potable water. Contact your local Master Gardener for specific growing tips.