Carefully swirl the dish to cover the surface.Pipette 1 ml of the 1-in-10 hamburger solution directly into the Petri dish marked “10.”.Inoculate Petri Dishes (see Student Lab Sheet: Coliform Counts).Add 9 ml of sterile saline solution to each of the 3 test tubes.The concentration of the hamburger is 1 in 10. Add the hamburger to the sterile saline solution in the blender.Weigh out 10 grams of hamburger on sterile aluminum foil.Add 90 ml of sterile saline solution to the blender.A water bath set at this temperature would be ideal. The best temperature for pouring is 111–115° F (44–46° C). Monitor the agar temperature as it cools. Caution: It will harden if cooled too long. The agar should be translucent with no undissolved granules of agar on the sides of the flask.Be very careful, as it can flash boil over the top very quickly. Once you have the VRBA agar in the flask, bring it to a slow boil.This will tell you about how many ml of agar to prepare. A general rule is that it takes about 20 ml per Petri dish.In a flask, add 41.5 grams of agar to 1 L of water (use the best water available, e.g., distilled, etc.).Prepare the VRBA as per instructions on the label.Preparation of the Violet Red Bile Agar (VRBA) Have students assist with the preparation of the VRBA and hamburger solution.Distribute Student Lab Sheet: Coliform Counts to each student.Divide the class into teams of 4 students each.Outbreak Investigation- Salmonella MuenchenĮvaluation: Lose a Million Bacteria (The Game).Cooking Right: The Science of Cooking a Hamburger.Blue's the Clue: Souring Milk for Science.Use the following links to see the remaining lessons: This lesson was developed as a portion of an entire unit of lessons focusing on food safety from farm to table. ![]() ![]() If the students see a halo around a colony, it’s likely to be bile precipitate. So, the color change indicates their presence.)Ĭrystal Violet allows coliforms to grow by inhibiting gram-positive bacteria. (Coliforms give off CO 2, which combines with water to form carbonic acid which causes a color change in the agar. Neutral Red is a dye, which acts as a pH indicator. This reduces competition and allows the gram-negative bacteria, which are the coliforms, to grow more readily. We have all eaten hamburgers and enjoyed them and we are all okay! You might remind the students that hamburgers purchased at most fast food franchises are carefully cooked and safe to eat.īile salts inhibit the growth of gram-positive bacteria. Note: Please be sensitive that some students can be really turned off from eating hamburger if we overstate the case and alarm the students unnecessarily. Sometimes there are as many as 104 to 106 coliforms in 1 gram of hamburger. They are also normal constituents of plant products. ![]() Coliforms are not disease producers themselves, but they indicate that food may have been contaminated with fecal contamination, which may contain pathogens. They can be found in untreated water and find their way into food from fecal contamination resulting from unsanitary processing conditions or human food handlers. The presence of coliforms in food does not mean that the food is not consumable - it means that proper precautions must be taken to reduce their presence in the food before it is eaten.Ĭoliforms are bacteria of great concern because they indicate the potential presence of pathogenic microorganisms such as Cryptosporidium, Salmonella, or Giardia.
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