The Mouse in the Bag Scenario
The Mouse inthe Bag Scenario
Dear Bob[In the last SproutNet, regarding seed sampling forpathogen testing], “You implied that uneven distribution doesn’t cause aproblem. Is this what you meant to say? Suppose all the pathogensare in one bag.”
The only argument I hear about the accuracy of seed samplingand testing is in regard to even distribution of the pathogen throughout thelot. A typical argument is, “What if a mouse crawls into a bag when the seedis being bagged?”
To this I have to say, in all likelihood, nothing. First, not all mice are infected with a human pathogen. And if the onethat happens to crawl into a bag is infected, one must ask an equallycompelling question: “What kind of a sprout grower would soak theirseed when they find a dead mouse in the bag?” So in order for thisoft reiterated scenario to come to fruition several things have to happen:
- A mouse sneaks into the bag undetected (as if a mouse is attracted to alfalfa, radish or mung seed anyhow. I can’t get the squirrels in our neighborhood to eat them.);
- The mouse happens to be a carrier;
- The seed sampling and testing procedure misses the contaminated seed;
- The sprout grower never notices the dead mouse and uses the seed;
- The growers seed sanitizing procedures do not kill the pathogens;
- The growers post testing does not detect the pathogens;
But to answer your question as it relates to evendistribution, there are about 11,300,000 seeds in a bag of alfalfa seed. Let’suppose you have a full truckload (880 bags) of alfalfa seed that iscontaminated at 4cfu/kg, but it is all in one bag. That bag would have80,000 pathogen cells in it. It is possible that a contaminated seed maycontain more than one pathogen cell, so for the sake of discussion let’s cutthat number in half and say there are 40,000 contaminated seeds in the bag. In this scenario, about 1 in every 282 seeds is contaminated, for a cfu/kg countof 3520. When you sample the seed according to our seed-sampling planyou take out a 20-gram sample from each bag. A 20-gram samplehas 10,000 seeds in it. In this scenario, on average there will be 35.5contaminated seeds per sample. Again we are only trying to capture onepathogen cell, not get an number of how many cells are present in the bag orlot.
The probability of capture if all the pathogens arein one bag is over 99.999%. The probability of capture if thepathogens are spread over half the bags is over 99.999%. Theprobability, if it were evenly distributed throughout the lot, is also over99.999%.