Ask A Scientist© Molecular Biology Archive

name       kiri
status     student

Question -  what type of milk bacteria grow in
organic and goats milk  and, also we did an experiment and we kept two
milk samples (organic and goats) both pasturised, for two days we placed
them on a sterilised agar  trays and they were left in contained petri dishes
for 4 days, after which they were counted i had more bacteria in the
goats milk (dilution  factor 1000) please could you tell me why bacteria
grow quicker in this milk type?
 
Your question is not an easy one to answer. 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 sugar
content) 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!

Trudy Wassenaar 
=========================================================