For more solid samples, such as foods, swabs are a good tool for recovering any coliforms on the sample. The swab can be inserted into the food sample directly or the sample can be ground up before swabbing. For the presumptive test, the swab is then placed in a tube containing lactose broth and a Durham tube. Usually this test is done in triplicate, meaning three swabs will be used on the sample and placed in individual tubes. For testing liquids, pipetting a certain amount of the sample into the tube is preferred to the swab method.
The Presumptive test looks for the presence of gas in the tube. As the possible coliforms break down, the lactose gas is given off as a by-product and it is trapped inside the Durham tube. Absence of gas indicates there aren't coliforms present in the sample tested. A positive result would be gas bubbles after 24 hours of incubation; if gas bubbles are only seen after 48 hours, this is considered a doubtful positive result. The gas may not be due to coliforms and further testing is needed.
An Eosin Methylene Blue (EMB) plate is streaked with both the positive and the doubtful positive broths from the Presumptive test. A negative EMB plate will have pink colonies and may indicate that the gas seen in the Presumptive test was not due to coliforms but other types of bacteria. A positive EMB plate will have purple or shiny green colonies; this confirms that coliforms may be in the sample.
Colonies from the positive EMB plate are picked and grown in lactose broth as well as on a nutrient agar slant. The Completed test doublechecks the results of the previous lactose broths. After a 24 hour incubation, bacteria from the nutrient agar slant are stained using the Gram stain. If the bacteria are Gram - they will be pink; if they are Gram + they will appear purple. If the organisms present are Gram -, have a rod shape when looked at under a microscope, and also produced gas in the lactose broth, then recovering of coliforms was successful.