Milk bacteria

1 ANSWERS

1. Brett
   

   Milk is normally sterile when it leaves the body, but it is a very good source of food for all kinds of bacteria.When an animal suffers from mastitis, milk can already contain bacteria in vivo, for instance certain E. coli bacteria. Also, during the milking process, milk can get easily infected by faeces contact and by bacteria present on the udder. Since milk is such a good growth medium, it is better to sterilize or pasteurize before drinking, because such bacteria rapidly multiply. In this respect there is no difference between organic milk and normal cows' milk. In some cases milk can contain pathogenic bacteria, such as pathogenic E.coli, salmonella, and listeria. For that reason drinking raw milk is not without a risk. Goats milk has a different composition (different fat and sugarcontent) so that may explain why bacteria grew better in your experiment. It is also possible that the milk became more heavily infected during the milking process, which may be less automated than milking cows. You say you used pasteurized milk, and put it on sterile agar. So presumably you measured the growth of bacteria that were present in the milk, and growth was delayed by pasteurization. I can't say which bacteria these would have been. Lactobacilli are a likely possibility. E. coli and staphilocci, or even yeasts, are also candidates. You could do a Gram stain to see if they are positive or negative. This, together with the shape of the bacteria under the microscope (cocci,rods, etc.) could give you more clues. The experiment clearly shows why we keep milk refrigerated!

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