Each of your dilutions will be used for spread plates

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Each of your Dilutions will be used for Spread Plates on two Types of Media: Nutrient Agar (our All-Singing, All-Dancing, Non-Selective Media) and MacConkey Agar (a Medium containing Bile Salts and Crystal Violet that inhibits Gram Positive Bacteria [and some Gram Negative Bacteria] and is Selective for Enteric Bacteria [Atlas Page 13]). Spread Plates are made by transferring 0.1 ml (100 μ l) of each Dilution onto a Nutrient Agar Plate and a MacConkey Agar Plate and then spreading this over the Surface of the Agar using a Sterile Disposable Spreader.
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Lab 6 Page 20 You’ll incubate your Nutrient Agar Plates at 30°C. This will give us a reasonably good Idea of the Total Numbers of Bacteria in our Hamburger Sample. Some of these Bacteria will be Gram Positives (like Bacillus , Micrococcus , and Staphylococcus ) and some of these Bacteria will be Gram Negatives (like E. coli Enterobacter , Salmonella , Shigella ). Some of these Bacteria are harmless Contaminants from the Air (Bacillus , Micrococcus ) and some of these Bacteria are potentially pathogenic Contaminants from the Slaughtered Cow or from a Human who handled the Beef at some Point (Staphyloccus , E. coli , Enterobacter , Salmonella , Shigella ). We can’t readily distinguish these Bacteria on a Nutrient Agar Plate (although you ought to be able to distinguish Bacillus and Micrococcus by Colony Morphology and Colony Color, respectively). You’ll incubate your MacConkey Agar Plates at 37°C. This will give us a reasonably good Idea of the Total Number of Enteric Bacteria in our Hamburger Sample (like E. coli , Enterobacter , Salmonella , and Shigella ). Many of these Bacteria are Pathogenic. And when you consider the original Source of these Bacteria, you cannot help but wonder what other Non-Bacterial Pathogens (Viruses, Protozoa) might also be present. You might want to keep in Mind that we’ve set-up this Exercise for nabbing Aerobic Bacteria. The overwhelming Majority of Bacteria growing in the Human Gut (or the Bovine Intestine) are Anaerobic. So our Bacterial Counts -- both on our Non-Selective Nutrient Agar Plates and on our Selective MacConkey Agar Plates -- will be Underestimates. Organization is the Key to Success in this Experiment. Get all the necessary Materials and bring them to your Lab Bench. Be sure you know what you will be doing and what your Lab Partner will be doing. Remember, no small Part of this Exercise is developing a successful Working Relationship with your Lab Partner. _______________________________________________________ “Dr Jones, wasn't it the Sultan of Madagascar who threatened to cut off your head if you ever returned to his country?” ‘No, it wasn't my head.’ “Then your hands, perhaps?” ‘No, it wasn't my hands. It was... my misunderstanding.’ © 1984 LucasFilm, Ltd.
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Lab 6 Page 21 Water Quality Testing Tests for Water Quality involve enumerating Coliform Bacteria (particularly E. coli and Enterobacter ). Coliform Bacteria are used as Indicator Bacteria because their Presence indicates Sewage Contamination and implies that Pathogens are Present (particularly Hepatitis A Virus). The Operational Definition of Coliform Bacteria for
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