• What is an enteric? (Know the scientific definition and their 4 defining characteristics) • Why do we care about studying the Enterobacteriaceae? • What's a "coliform" in general? • Which bacteria is considered as the prototypical coliform? • Why are we interested in testing our water supplies for the presence of coliforms? • Which three genera within Enterobacteriaceae are considered obligate pathogens? • Why was Pseudomonas aeruginosa included in the enteric bacteria lab? • What's the primary test to distinguish normal flora(opportunistic) from the true pathogenic members of the family Enterobacteriaceae? What type of media could we use for this purpose? • Which media/test can be used to tell if the bacteria are coliforms? • Know if the following media are selective/ differential/ both. What makes it selective/ differential? What is the indicator? How does each work to be selective &/or differential? How to interpret the results (what characteristics of the bacteria can be told by observing the results) o PR Broth o EMB Agar o Simmon’s Citrate agar o SIM Media • Can a result in PR broth with glucose tell you whether it is a coliform or not? • Can a result on EMB tell you if it produces gas as an end product of lactose fermentation? • If you want to know if gas was produced as an end product of lactose fermentation, which media would you use? Why? How would you know? • Know how the functioning of a media would be affected if you forget to add a particular critical constituent of the media (we have discussed similar questions in the lab discussions- eg would the EMB media still be selective if you forget to add Eosin and Methylene blue?)
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- An enteric bacterium is a type of bacteria that inhabits the gastrointestinal tract. - The four defining characteristics of enteric bacteria are: 1. They are Gram-negative. 2. They are rod-shaped. 3. They are facultatively anaerobic. 4. They possess Show more…
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Lab 15: Gram-negative bacilli 1. What is the structure of Gram negative cellular envelope? 2. Name at least three genera belonging to Enterobacteriaceae family. 3. Characterize Enterobacteriaceae: a) What is their Gram reaction? b) What is their shape? c) What body part is inhabited by these bacteria? d) What is the transmission route for these bacteria? e) What type of metabolism do they have? f) What are their oxygen requirements? g) Are they motile? h) What kinds of infections are caused by these bacteria? 4. What does EMB stand for? 5. What is the purpose of EMB (what kind of medium is it)? a) What kind of organisms are growing on this medium? b) What indicator do we use? c) What inhibitor do we use? d) What is the main sugar in the EMB medium and why? e) What color indicates sugar fermentation? 6. What is the purpose of Tetrazolium salt medium? a) What color indicator do we use in Tetrazolium salt medium? b) How do we determine motility using Tetrazolium salt medium? c) What are some other ways to check for bacterial motility? 7. What final products of sugar fermentation could be determined in Phenol red medium? a) What is the pH indicator for acid? b) What color indicates positive result for fermentation? c) What color indicates use of peptone (protein) instead of sugar nutrients? d) What is the indicator for gas? 8. What is the function of oxidase? What color of the oxidase reagent indicates positive result? How could you interpret positive oxidase test? 9. What is the substrate of Urease enzyme? 10. Which bacterium is positive for Urease? 11. What color indicates positive result for Urease and why? 12. What does SIM stand for? a) What indicator do we use for sulfur? b) What color indicates sulfur reduction? c) What indicator do we use for Indole? d) What color indicates Indole presence? e) What indicates bacterial motility in SIM medium?
Adi S.
What would the consequence be of raising the starting pH of EMB agar to 7.8 (normal pH = 7ish)? Would it alter the selective nature of the media, the differential nature of the media, or both? Would it be more or less sensitive (more = detects coliforms more easily, less = less likely to detect coliforms)? Explain your answer. 2. All bacteria are able to use citrate for energy or as a carbon source. That being said, only some bacteria can grow on citrate media. Explain why bacteria that can use citrate for energy and carbon may not be able to grow on this media. 3. Imagine that I made a phenol red broth with lactose instead of glucose or sucrose and inoculated it with all of the organisms used in this lab today. Which ones do you think would turn yellow? Can you predict if they would make a bubble? Explain your answer. 4. In the SIM media, which ingredients could be eliminated if the medium were used strictly for testing for motility and indole production? What if I were testing only for motility and sulfur reduction? 5. One characteristic of Enterobacteriaceae is that they all ferment glucose. Coliforms, on the other hand, are only coliforms if they ferment glucose AND vigorously ferment lactose. Bacteria can be both an Enterobacteriaceae and a Coliform. a. Which media do you look at to tell if the bacteria is able to ferment glucose? b. Which media do you look at to tell if the bacteria is able to ferment lactose? c. Look at your results and answer the following questions. Be sure to base this off of your results and NOT an internet search. · Which of the bacteria are true members of Enterobacteriaceae? Are there any that are not Enterobacteriaceae? · Which of the bacteria are true Coliforms? Which are not coliforms?
Josee P.
Q4. Proteus species and certain species of Salmonella produce a black precipitate when growing on Hektoen. Chemically, what is occurring to cause the black precipitate to form? What ingredients in the media are being used by the bacteria to produce the precipitate? Is this a selective or differential aspect of the media? Q5. With respect to MacConkey agar, what would be the possible consequence of: a) Replacing the lactose with glucose? Would you be able to distinguish EC from PV if there is glucose instead? Explain. (Note: Results of a later exercise will help here with this question). b) Replacing the neutral red with phenol red (yellow when acidic; red or pink when alkaline)? Q6. These questions relate to blood agar, which is considered to be a type of differential agar besides being an enriched agar. a) What happens to blood agar when an alpha-hemolytic reaction occurs? b) What happens when a beta-hemolytic reaction occurs? c) What happens when a gamma-hemolytic reaction occurs? Other questions to help you work your way through the results. These do NOT need to be included in the report, but some students do find them helpful to answer and include: 7. Which microorganism(s) that you observed in class is/are beta hemolytic? 8. Which microorganism that you observed in class produces hydrogen sulfide (or at least theoretically)? How would this trait be observed with the media we used? (Also useful to think about: which amino acid can provide sulfur to bacteria?) 9. What is the meaning of the color change to the media and/or colony when E. coli is grown on MacConkey agar? Or Hektoen? How about Pseudomonas aeruginosa on MacConkey or Hektoen? How about Proteus on MacConkey or Hektoen? How about Staph. aureus on Mannitol Salt Agar (MSA) in comparison to Staph. epidermidis or Micrococcus luteus on MSA (in this case, you have to look at the image emailed to you)? Did you see any swarming of Proteus on blood agar or other media?
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