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Bob@sproutnet.com

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The Mouse in the Bag Scenario

SproutNet

International Specialty Supply

June 30, 2002

Dear Bob

[In the last SproutNet, regarding seed sampling for pathogen testing], "You implied that uneven distribution doesn't cause a problem.  Is this what you meant to say?  Suppose all the pathogens are in one bag."

Dear Grower

The only argument I hear about the accuracy of seed sampling and testing is in regard to even distribution of the pathogen throughout the lot.  A typical argument is, "What if a mouse crawls into a bag when the seed is 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 one that happens to crawl into a bag is infected, one must ask an equally compelling question:  "What kind of a sprout grower would soak their seed when they find a dead mouse in the bag?"  So in order for this oft reiterated scenario to come to fruition several things have to happen:

1. 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.);
2. The mouse happens to be a carrier;
3. The seed sampling and testing procedure misses the contaminated seed;
4. The sprout grower never notices the dead mouse and uses the seed;
5. The growers seed sanitizing procedures do not kill the pathogens;
6. The growers post testing does not detect the pathogens;

But to answer your question as it relates to even distribution, 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 is contaminated at 4cfu/kg, but it is all in one bag.  That bag would have 80,000 pathogen cells in it.  It is possible that a contaminated seed may contain more than one pathogen cell, so for the sake of discussion let's cut that 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 count of 3520.  When you sample the seed according to our seed-sampling plan you take out a 20-gram sample from each bag.  A 20-gram sample has 10,000 seeds in it.  In this scenario, on average there will be 35.5 contaminated seeds per sample.  Again we are only trying to capture one pathogen cell, not get an number of how many cells are present in the bag or lot. 

The probability of capture if all the pathogens are in one bag is over 99.999%.  The probability of capture if the pathogens are spread over half the bags is over 99.999%.  The probability, if it were evenly distributed throughout the lot, is also over 99.999%.