Human Microbiology Lab Exam
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What is the purpose of the phenol red in the carbohydrate fermentation broth?
- It acts as a pH indicator
- Turns yellow as the media turns acidic
- acid indicator
What is the purpose of the Durham tube in the carbohydrate fermentation broth?
The Durham tube indicate whether gases are produced.
Nitrate tubes- Positive and Negative Results characteristics
- Red color throughout the tube after the addition of zinc= negative for nitrate reductase.
- No color change in the tube after the addition of zinc flakes = positive for nitrate reductase
- Red color throughout the tube after the addition of nitrate A and Nitrate B = positive for nitrate reductase.
What is the main product from catalase activity test causing the gas bubbles?
The main product from the catalase test causing the gas bubbles is oxygen.
The oxidase test is used to differentiate between which groups of bacteria?
Oxidase test differentiates between obligate aerobes and facultative anaerobes.
Why is the presence of catalase important to certain bacteria?
- O2 can be reduced into products that are toxic to cells. These products are called Oxygen radicals. Superoxide and hydrogen peroxide are two forms of oxygen radicals.
- Catalase is an enzyme that some bacteria produce to to inactivate and catalyze the destruction of hydrogen peroxide.
Would an obligate anaerobe be positive for catalase? Why or Why not?
- Obligate anaerobe would be negative for catalase because they are killed by the presence of oxygen.
- They do not use O2 so they wouldn't need to break it down.
- They do not have catalase to break down oxygen radicals or they would be able to live in the presence on O2.
Why should a metal (iron) inoculating loop or needle not be used in the oxidase test?
The metal inoculating loop or needle will transfer e- which could give a false positive.
- Uses Simmons citrate agar slant
- pH indicator bromothymol blue
- Positive blue
- Negative green
- Goal is to see if microorganisms can utilize citrate as their sole carbon source.
- bacteria use two enzymes citrate permease and citrase.
- CO2 combines with the sodium in the agar forming sodium bicarbonate.
- pH will increase and turn blue.
What two enzymes used by microorganisms that use citrate as a primary carbon source?
Citrate permease and citrase.
allows the bacteria to transport the citrate into the cell.
breaks down the substrate
Products from Citrate Utilization test
- Products from reaction is pyruvic acid which the bacterium can use as a food source.
- Sodium Citrate Permease
- Citrate---> Citrase ---> Pyruvic Acid+Oxaloacetic acid+CO2
Identification of Enterobacteriacea bacteria requires what battery of tests?
- Methyl Red
- Citrate Utlization test
- Urease Activity
- Related to different pathways for energy production.
- Glucose converted to pyruvic acid during glycolysis
is added to see if cysteine or thiosulfate is reduced
Results for SIM tubes
- -Black precipitate along the stab line of inoculation=
- +Sulfide -Indole
- -Bacterial growth only along the stab line and of inoculation. Deep red color after the addition to Kovac reagent =
- -Sulfide, +Indole, -Motility
- -Black precipitate throughout the tube =
- +Sulfide, + Motility
Two benefits for an organism which produces urease
- 1. They can use urease as a nitrogen source
- 2. Helps raise the pH of their environment
What does it mean to be a mixed fermenter?
- Mixed acid fermenters are positive for the methyl red test
- reduce the pH of the medium lower than organism which are butanediol fermenters
What is the component in the SIM agar tube that makes it possible to detect the production of indole by the bacteria?
The SIM agar contains tryptophan
Reasons VP test could be negative?
1. too much media
Is an organism that is a mixed acid fermenter positive or negative for the methyl red test?
Positive or negative for the Voges-Proskauer test?
Mixed acid fermenter will be positive for MR test and negative for VP
Describe when you would se the urease test to identify bacteria?
A urease test is used to diagnose urinary tract infections. It is used to detect bacteria in the genus proteus.
What two benefits may the bacteria gain by using the reactions that produce H2S as an end product?
Some bacteria can break down cysteine with the enzyme cysteine desulfurase and utilize the amino acid as a nutrient.
Bacteria may be able to use thiosulfate as a final electron acceptor during anaerobic respiration
a-amylase is an enzyme that breaks down starch. What products of this reaction can be transported across the cell membrane?
Glucose, Dextrins, and Maltose
What does a negative starch test look like?
No clear area
What is the importance of exoenzymes to bacteria?
Exoenzymes help break down the larger molecules into smaller components in order that the bacteria can transport them into the cell and use them as food.
Why is milk white?
Casein is the large white protein found in milk that gives it its white color.
Why are some bacteria able to grow on a milk plate even though they are not able to hydrolyze casein?
They use other proteins beside casein for food.
What are two reasons that the production of gelatinase is beneficial to certain bacteria?
- 1. Gelatinase is a virulence factor. Connective tissue is found everywhere so food is plentiful.
- 2. Gelatinase breaks down connective tissue. Once hydrolyzed gelatin can no longer solidify.
- 3. Allows it to move since connective tissue is everywhere.
Why do you place the gelatin cultures at 4* C before observing the results?
- To identify if the gelatin has been hydrolyzed. If solid it has not been.
- @ 37* C both look the same.
What does a positive DNase test look like?
Clear zone around bacteria
What does the presence of Dnase suggest about the pathogenicity of a bacterium?
- DNase is a virulence factor and can degrade the host
- DNA and increases the pathogenicity of the bacteria.
Which of the enzymes from this laboratory are considered virulence factors?
DNase and Gelatinase.
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