Bacteria Identification

There are 3 general groups of bacteria that can be counted:

-non-coliform bacteria. These are mostly gram negative bacteria but are not considered coliforms in this test because they do not ferment lactose to produce gas. They are mostly environmental bacteria and not associated with fecal contamination.

-total coliform bacteria. These are bacteria that ferment lactose and produce gas, the classical definition of coliform bacteria. They include E. coli. They can be found naturally occurring in the environment but are often the result of fecal contamination.

-E. coli. These are bacteria that are usually associated with fecal contamination as they are usually only found in the human intestinal tract. They produce the Beta-glucuronidase enzyme, which allows them to be differentiated from other total coliform bacteria (via the blue color change in Petrifilm)

Here is a hierarchical way to look at it:

                           gram negative bacteria	                         => Red color change
                           /                    \
                        non coliform             \                                        => No bubbles
                                                  \
                                              total coliforms                        => bubbles around colony
                                           /               \
                                          non-E. coli       \                          => red colonies with bubbles
                                                         \
                                                          E. coli                     => blue colonies with bubbles

For example, if a Petrifilm result is 10 red colonies, 5 red colonies with bubbles, 5 blue colonies with bubble, the result recorded should be 5 other bacteria, 10 total coliform, and 5 E. coli. I hope this helps! Let me know if I can clarify this further.