Lab Exam 2

5

How many result combinations are there for the triple sugar-iron agar test when testing intestinal bacilli?

Escherichia coli

Identify the unknown organism from the following information:

10^-3

If 1 ml of a 10^-1 solution is added to a bottle with 99ml sterile water, the final dilution of that bottle is __________.

False

If E. coli is present in the confirmed test, the water can still be potable.

cytochrome oxidase

If a bacterial species is aerobic, it uses oxygen as its final electron acceptor and must have which enzyme?

A false negative of both red slant and red butt could occur, due to peptone digestion after sugar depletion.

If you inoculate a TSI tube on Monday in lab but don't return until Thursday to read the results, what impact could this have?

a red slant and a yellow butt, no H2S production

If you inoculated a triple iron-sugar slant with Shigella, a glucose fermenter, your results should be what?

True

Partial nitrate reduction results in nitrite, which some bacteria can completely reduce to ammonia.

nitrite and water

Partial nitrate reduction results in the formation of __________.

False

Phenol red will turn yellow in the presence of bacteria.

False

Simmons citrate slants require the addition of a reagent to see results of citrate utilization.

False

Solutions A and B will react with ammonia, which is the product of nitrate reduction.

False

Sulfanilic acid and a-naphthylamine are red reagents that become clear in the presence of nitrite.

H2O2

Superoxide dismutase degrades toxic superoxides into _____.

Yes, the extended period required for the bubbling to occur is probably due to low levels of catalase expression.

You are testing enteric bacteria for catalase activity and some of the bacteria begin bubbling immediately upon addition of the H2O2, while others may take a few minutes before some weak bubbling occurs. Are these late, weak bubbles the result of a positive catalase test?

bacterium B

You have a phenol red lactose Durham tube and a phenol red sucrose Durham tube, both yellow, with a gas bubble in the lactose tube but no bubble in the sucrose tube. Bacterium A can ferment both lactose and sucrose while producing gas in both. Bacterium B can use both sugars also but only produces gas when fermenting lactose. Which bacterium did you inoculate into both Durham tubes?

The purple tube indicates the bacteria grew and utilized lysine, and the yellow tube indicates the second bacterium was only able to utilize glucose and not the amino acid.

You have inoculated two bacteria into two separate tubes of decarboxylase broth with lysine added and have incubated the tubes overnight. The next day, one tube is yellow and one is purple. What do these results indicate?

an actinomycete

You observe a microbial colony on glycerol yeast agar. This microbe is likely to be __________.

The tubes were agitated or inverted and a gas bubble was trapped in them.

Your lab mate inoculates the Durham tubes during Thursday's lab. On Friday you come in to record the results and you find the tubes are all still red with no growth but there are bubbles trapped in the inverted tubes. What is the best possible reason for this?

False

A bacterial species may produce H2S but not turn the but yellow

A yellow broth will form due to acidification and color change of the phenol red, and a gas bubble will be trapped in the inverted tube.

A bacterial species that is a gas producer and has the ability to ferment lactose is added to a Durham tube of phenol red lactose broth. What will the results be the next day?

The organism can convert hydrogen peroxide to water and oxygen. The organism produces catalase.

A Catalase test was done on an unknown specimen. Observe the pictured result and answer the following question. Select ALL appropriate statements regarding the pictured Catalase test result. The organism can convert hydrogen peroxide to hydrogen sulfide. The organism is probably a strict anaerobe. The organism can convert hydrogen peroxide to water and oxygen. The organism does not produce catalase. The organism produces catalase.

The image shows a positive test result. The organism tested is probably an aerobe or a facultative anaerobe. The organism tested is protected against oxygen radicals.

A Catalase test was done on an unknown specimen. Observe the pictured result and answer the following question. Select ALL appropriate statements regarding the pictured Catalase test result. The catalase test can be used to identify genus and species of a test organism. The image shows a positive test result. The organism tested is probably an aerobe or a facultative anaerobe. The organism tested is protected against oxygen radicals. The image shows a negative test result.

5.8 X 10^7 cfu/g

A food microbiologist carried out a serial dilution procedure using a 1-gram sample of ground meat. After incubation, the plates had the results shown in this photograph. What was the bacterial concentration of the original sample of ground meat?

10^-6

A food microbiologist is testing ground meat to estimate the bacterial concentration. He carries out the following serial dilution scheme. What is the total dilution of Plate D's sample?

False

A gap in the but of the agar of a TSI agar slant after 24 hours of growth indicates liquefaction by extracellular enzymes.

True

A mannitol salt agar plate is useful for differentiating between Staphylococcus aureus and Staphylococcus epidermidis.

True

A potent nitrate reductase may reduce nitrates to ammonia.

Staphylococcus

A simple test to differentiate between Staphylococcus and Streptococcus is the addition of H2O2 to a colony to see if bubbles form, which are positive for catalase. Which organism would be positive?

The pH of the agar decreased following incubation. The bacterium is able to ferment glucose. Hydrogen sulfide (H2S) was produced.

A specimen was inoculated onto the Triple Sugar Iron (TSI) medium pictured here. Based on your observation of the pictured TSI (Triple Sugar Iron) medium select ALL appropriate conclusions. The pH of the agar decreased following incubation. The bacterium is able to ferment glucose. The bacterium is unable to ferment lactose and/or sucrose. Hydrogen sulfide (H2S) was produced. The bacteria is Gram-positive.

The bacterium is able to ferment glucose. The bacterium is able to ferment lactose and/or sucrose. To conduct this test the slant must be streaked. To conduct this test the butt of the tube should be stabbed.

A specimen was inoculated onto the Triple Sugar Iron (TSI) medium pictured here. Based on your observation of the pictured TSI (Triple Sugar Iron) medium, select ALL appropriate statements. The bacterium is able to ferment glucose. The bacterium is able to ferment lactose and/or sucrose. To conduct this test the slant must be streaked. The bacteria is Gram-positive. To conduct this test the butt of the tube should be stabbed. Hydrogen sulfide (H2S) was produced. The pH of the agar increased following incubation.

It takes into account only viable cells.

A student is counting the number of bacteria in a sample. She uses two methods: 1) Counting bacterial cells with a microscope and 2) A plate count method using serial diluted sample. Why does the plate count have an advantage?

The agar was not at the correct temperature when poured.

A student is learning how to perform serial dilutions, using a stock solution of E. coli. He was instructed to use the dilution scheme shown in the diagram. The stock solution was turbid, as shown below. He delivered 1 ml from each tube into a separate sterile Petri dish and poured melted agar into each dish. When he examined the plates after 48 hour incubation at 37oC, he found no growth in any of the seven plates. What could explain these results?

10^-4

A student is testing a milk sample, and carries out the dilution scheme in the diagram below. What is the total dilution of Tube C?

yellow or orange, due to the acidification of the broth

A urease-negative bacterial species is allowed to grow in a urea broth overnight. What color would the broth be the next day?

The bacteria were able to ferment the sugar source and produce CO2.

After 24 hours of incubating your bacterial isolate to test for carbohydrate fermentation capabilities, your Durham tube shows a yellow broth with a trapped bubble. What does this indicate?

False

All bacteria have the ability to ferment any sugar available.

False

All enteric bacteria have the decarboxylase enzymes for each amino acid

10^-5

Each tube in a set of tubes is a dilution of the previous tube.

Staphylococcus aureus

Based on the results of this Gram stain and the ID matrix below, which of the following microbes could this possibly be?

False

Both the decarboxylase and the deaminase tests are performed under anaerobic conditions.

addition of biologically available N to an ecosystem.

Nitrogen fixation results in the

hydrogen peroxide

Catalase breaks down which compound into water and oxygen gas?

fecal

Coliforms in a water sample indicate ________ contamination.

True

Due to its ability to ferment mannitol, Staphylococcus aureus would cause yellow coloration of the agar when grown on mannitol salt agar.

False

Fermentation of the sugars in a Durham tube will always result in gas production.

Indole

From your first day of observations, your unknown bacteria appear to be gram-positive cocci that produce yellow/golden growth on TSA. Based on these first few observations, and the ID matrix below, the unknown is either Micrococcus luteus or Staphylococcus aureus. Which of the following tests would NOT distinguish these two species?

the number of tubes exhibiting gas following the incubation period

In a standard qualitative analysis of water, the MPN provides an estimate of the number of coliforms present based on __________.

hydrogen sulfide production

In the TSI test, sodium thiosulfate acts as a substrate for the measure of what reaction?

True

In the catalase test, if possible, you should not test colonies from a blood agar plate because they may give a false positive due to the erythrocytes having catalase activity.

False

In the citrate utilization test, citrate metabolism decreases the pH, which results in a yellowing of the blue agar to generate a green positive color.

No nitrate was reduced by the bacteria.

In the nitrate reduction test, there is only a color change after the addition of zinc to your 24-hour incubated solutions A and B. What does this indicate?

No color would form.

In the nitrate reduction test, what would the result be after solutions A and B were added to a culture that had reduced nitrate to ammonia?

Yes, a possible false positive may occur due to the zinc reducing any available nitrate, which will react with solutions A and B.

In the nitrate reduction test, you mistakenly add the zinc powder before the addition of solutions A and B. Will this change your results? Why?

True

In the oxidase test, incubating bacteria for 10 minutes after the addition of the oxidase reagent in order to see a color change is too long and should be considered a false positive.

a negative result due to no urease activity

In the urease test, a red coloration of the broth after 24 hours of incubation is considered __________.

False

In the urease test, urea degradation will result in a yellow coloring of the phenol red indicator.

True

The catalase test should have the same result whether the bacteria are on a slant, on a nutrient agar plate, or have been transferred to a glass slide.

amine or diamine and CO2

The decarboxylation of an amino acid results in the production of a(n) __________.

Escherichia coli or Proteus vulgaris

The following observations, and the ID matix below, suggest that your unknown microbe is one of two possible species. Which two?

Escherichia coli

The image below shows the results of fermentation tubes inoculated with an unknown microbe. Based on the results and the ID matrix below, which of the following microbes could this unknown be?

degrade tryptophan

The indole production test determines the ability of microorganisms to __________.

produces

The metabolic process of ammonification ________ ammonia.

True

The methyl red and the Voges-Proskauer tests use the same medium, while the indole and citrate utilization tests use different media.

A red coloration of the methyl red indicates that the bacteria are maintaining an acidic environment, while a yellow coloration indicates that this bacterial species converts these acidic products in other metabolic pathways.

The methyl red test is based on the glucose utilization of Enterobacteriaceae, resulting in acidic end products. The coloration of the reagent is based on what further reactions?

False

The nutrient agar is used to culture actinomycetes.

True

The only carbohydrate in a phenol red lactose broth tube is the sugar lactose.

False

The oxidase test determines if a bacterial species has a means of dealing with hydrogen peroxide.

True

The pH indicator phenol red is yellow at acidic pH and red to pink in alkaline solutions.

yellow for the methyl red and reddish-pink for the Voges-Proskauer tests

The production of acetylmethylcarbinol by E. aerogenes would result in a pH of 6.5 in the MR-VP broth. What results would you guess for the methyl red and the Voges-Proskauer tests?

False

The removal of an amino group from an amino acid is performed by the enzyme aminase.

True

The triple sugar-iron agar medium contains lactose, sucrose, glucose, sodium thiosulfate, and ferrous sulfate.

Specimen A does not produce cytochrome oxidase. Specimen B produces cytochrome oxidase.

Two different bacterial samples, A and B, were tested with an Oxidase dry slide. This is a picture of the test results. Select ALL appropriate statements regarding the pictured Oxidase test results. Specimen A fermented cytochrome oxidase. Specimen A produces cytochrome oxidase. Specimen A does not produce cytochrome oxidase. Specimen B fermented cytochrome oxidase. Specimen B produces cytochrome oxidase. Specimen B does not produce cytochrome oxidase.

True

Under acidic conditions, both the methyl red and phenol red solutions would turn yellow.

CO2, H2O, and NH3

Urease is the enzyme that breaks down urea into what compounds?

H2S production

What creates the black precipitate that is found in the but of a TSI agar slant inoculated with Salmonella?

True

When you add hydrogen peroxide to a culture of bacteria, if the bacteria in the culture cannot produce catalase, they will most likely be killed.

Each tube in a set of tubes is a dilution of the previous tube.

Which of the following best describes serial dilution?

resistant to chlorination

Which of the following is NOT characteristic of a coliform?

gas and acid produced from lactose fermentation

Which of the following is consistent with a positive presumptive test for coliforms?

to avoid artificially raising the numbers of bacteria present

Why must water blanks for dilutions be sterile?

Yes, this increased incubation means that the bacteria express a low level of the enzyme or their enzyme has a low level of activity.

Would a bacterial species be considered urease positive if you have to incubate it for 48 hours to get the pink positive reaction?

Indole

Which of the following tests would be useful in distinguishing Escherichia coli from Pseudomonas aeruginosa?

Sometimes a group of cells forms a single colony.

Why do we use the term colony-forming units, instead of cells, when expressing concentration of bacteria in a sample?