Got milk?  If you answered yes, then you have psychrophiles, because psychrophiles can grow in cold environments like refrigerators.  Psychrophiles are usually nonpathogenic but they are responsible for breaking down the proteins and fats in milk causing the milk to go rancid.  Psychrophiles make lipases and proteinases that break down fats and proteins in the milk causing it to curdle.  To test for bacteria and get an estimated level of psychrophiles in milk a test, called a PI test, is commonly used.  A  PI test is a preincubated bacteria count.  This test is done by incubating bacteria for 18 hours at 12.8^oC after which a standard plate count or a plate loop count is performed to determine an approximate number of bacteria present.  High levels of psychrophiles can be present when proper sanitation is not used during collecting and processing milk.