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I am not experienced in diagnostic bacteriology and am interested in understanding more about it in the context of analyzing food purity.

For example, imagine I have a supply of raw milk which might be, for example, a 20 gallon tank. I would like to ensure that listeria, e. coli and salmonella are not present in the milk.

Is it sufficient to take a single drop of the milk and examine it under a microscope or do I have to do more extensive sampling, for example, sampling every inch vertically or something like that?

What is the procedure to make sure my milk is free of the bacteria, or is that impossible?

I am not interested in procedures to kill the bacteria. I know those. I want to understand the diagnostic processes to tell if bacteria is there.

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  • $\begingroup$ Starting on page 23 are outlined procedures and methods for sampling of milk as required by FDA standards. Of course, individual manufacturers will have internal QA procedures that ensure any state-level testing does not result in penalty to the manufacturer, but since I'm not in the industry, I can't give a more accurate answer. Also, Pasteurization is a method used to make sure microbial counts in the milk are low (not good) or zero (microbes are reintroduced as the milk is opened and used). $\endgroup$
    – CKM
    Commented Nov 14, 2015 at 1:44
  • $\begingroup$ As @Kendall says, pasteurization (see here ) is what's generally used to kill pathogenic bacteria in milk. Looking at the milk under a microscope won't help. You'd have to do an ELISA-based assay or PCR... The easiest thing by far is properly carried out pasteurization procedure. $\endgroup$
    – userABC123
    Commented Nov 14, 2015 at 9:14

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A simple way to know if milk contains any bacterial contaminations is to take ~5 ml of the milk and add a couple of drops of methylene blue to it. If the milk is sterile, it will stay blue. If any bacteria are present they tend to take up the dye, leaving the sample colourless after ~1 hour. A more extensive way would be to plate 3-5 dilutions of the milk sample on LB or nutrient agar. You don't have to do much extensive sampling. Just make sure that you take a couple of well-stirred samples.

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    $\begingroup$ This would seem to only test for any bacteria, not just particular harmful bacteria. $\endgroup$
    – Imprisoned Rhesus
    Commented Nov 14, 2015 at 13:30
  • $\begingroup$ Try to put some references! $\endgroup$
    – Dexter
    Commented Nov 14, 2015 at 16:37
  • $\begingroup$ @ImprisonedRhesus, if you need to test for particular harmful bacteria, PCR would be the preferrable option as it does not depend on the species and unlike biochemical tests, where you may have to spend more time and run more experiments $\endgroup$
    – The Molecular Biologist
    Commented Nov 27, 2015 at 12:52
  • $\begingroup$ @TheMolecularBiologist Okay, if I use PCR I assume I would have to test for particular bacteria. Can I sample anywhere in a vat of milk? In other words does the bacteria spread out so it is present everywhere, or is there the possibility the bacteria would be in just one part of the milk and my sampling could miss it? $\endgroup$
    – Imprisoned Rhesus
    Commented Jan 15, 2016 at 19:28

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