Subject: How can the Lactobacillus bulgaricus, Streptococcus thermophilus,

How can the Lactobacillus bulgaricus, Streptococcus thermophilus, 
and Lactobacillus acidophilus bacteria be grown from yogurt to be 
studied?.  Specifically, I would like to determine the number of bacteria
in a cup of yogurt as it approaches its expiration date (it has been 
proven that the number diminishes.)  How can these bacteria be isolated
from the milk in the yogurt to be studied?  Can yogurt be simply placed
on an agar plate?  What do these bacteria require in a culture?  

Re:How can the Lactobacillus bulgaricus, Streptococcus thermophilus,