text: Bacteria are everywhere.  There's many many ways bacteria grows in and on meat in particular, because it's an excellent protein source for growth.  \n\nContamination can come from dirty utensils used to cut and prepare the meat, as well as surface tops, cutting boards, ect..\nAlso, through airborne particles, dirty hands of the person preparing the meat, or even breathing over the meat while it's being prepared.  Also, the animal could have bacteria already in it's system, which is more than likely usually the case.  Animals and humans have billions of bacteria in our intestinal tract especially.  You even have bacteria known as "normal flora" on your skin right now! \nMany times, contamination occurs in meat when it is processed in slaughterhouses, because fecal matter from the intestinal tract gets mixed with the meat (sounds gross, but that's where E. Coli comes from, as well as some forms of Serratia) \nThe FDA and USDA actually run tests on the meat before it is sent out though to make sure it isn't contaminated.  They normally do something called a spread plate count or dilution count.  This is where they put a certain quantity of meat, grind it up, mix with sterile water in a sterile blender type instrument and add maybe a milliliter of the substance to test tubes of water that have a measured amount of water in it, and it then gets diluted further the same way in about 4 test tubes.  The first would be a 1:10 ratio, then 1:100, 1:1000, ect.. Then, usually about a milliliter is taken out from those tubes and put on a petri dish with a special type of nutrient agar for the bacteria to form colonies.  After the plates are incubated, they are removed and you count the colonies of bacterial growth.  Then they do a mathematical calculation where they take the colonies seen and multiply it by the dilution factor, this gives them the colony formula units.  If the CFU is over a certain amount per gram, then it is discarded or recalled.  To test for E. Coli in particular, they use something called EMB agar, which is diluted sheep red blood cells. The bacteria is spread onto the petri dish of this agar, and then incubated.  If the petri dish has a bright, metallic green colony growth, it is positive for E. Coli.