Lab quiz #2

Streptococci

Enriched blood agar permits the demonstration of the hemolytic properties of what group of microorganisms?

It will give a general idea of the quality of the food.

How does bacterial enumeration help to determine if a food is safe for consumption?

93

If you had fresh lettuce containing 9.3 × 103 organisms/ml, how many colonies would you expect to see on a plate that represents a 10-2 dilution?

Cheese

In which of the following food types would you expect to see high numbers of microbes?

Yogurt

In which of the following foods would you normally expect to find a high number of bacteria?

dark red; colorless

Just after the decolorizer (acid-alcohol) is applied, acid-fast bacteria will appear __________ and non-acid-fast bacteria will appear __________.

Mycobacterium

Members of which genus of bacteria are best stained using the acid-fast method?

phenol

Most bacteria are easily stained by aqueous stains. The lipoidal material that comprises mycobacterial cell walls, however, does not bind to aqueous stains, instead requiring a stain such as carbol fuchsin, which is composed of 5% __________.

Specimen B is motile Specimen A is nonmotile

Two different bacterial samples, A and B, were inoculated into the motility media shown here. Based on your observation of the pictured specimens select ALL appropriate statements.

Specimen A is likely to be Escherichia coli Specimen B is most likely Gram-negative Specimen B is able to ferment lactose Specimen A is able to ferment lactose Specimen A is most likely Gram-negative

Two different bacterial samples, A and B, were inoculated onto Eosin Methylene Blue (EMB) agar. What can you conclude based on your observations? Based on your observation of the pictured specimens that are growing on the EMB agar plate, select ALL appropriate conclusions.

Specimen A is most likely Gram-negative Specimen B is most likely Gram-negative Specimen A is able to ferment lactose

Two different bacterial samples, A and B, were inoculated onto MacConkey agar. What can you conclude based on your observations? Based on your observation of the pictured MacConkey agar plates, select ALL appropriate conclusions.

Specimen A is able to hydrolyze starch

Two different bacterial samples, A and B, were inoculated onto Starch agar. What can you conclude based on your observations? Based on your observation of the pictured specimens that are growing on the Starch agar plates, select ALL appropriate conclusions.

Crystal violet agar: inhibits the growth of gram-negative organisms, used to isolate gram-positive organisms

Which of the following associations is INCORRECT regarding selective media?

2 x 105 organisms/ml

Which of the following bacterial concentrations would produce a TFTC plate (Too Few to Count) at the 10-4 dilution?

Is a particular bacterium susceptible to any of a variety of antibiotics?

Which of the following best describes what a single Kirby-Bauer plate is testing?

The method used to develop a bacterial lawn on the agar plate

Which of the following characteristics does NOT need to be standardized in a Kirby-Bauer assay?

Clusters of blue cocci

Which of the following correctly describes the cellular morphology and appearance of Staphylococcus aureus after performing an acid-fast stain?

Beta hemolysis; results in a greenish halo around the bacterial growth

Which of the following describes an INCORRECT association regarding hemolytic activity?

Dark-red bacilli

Which of the following describes the cellular morphology and appearance of Mycobacterium smegmatis after performing an acid-fast stain?

colonies on EMB agar with a metallic sheen

Which of the following is indicative of the presence of E. coli?

Interpolation

Which of the following terms best describes the enumeration of bacteria you can do with stain and a light microscope in your lab?

A solution containing 1.9 × 104 at a 10-2 dilution

Which of the following would produce a countable plate?

acid-fast; non-acid-fast

While __________ bacteria retain carbol fuchsin after acid-alcohol treatment, __________ bacteria are decolorized by acid-alcohol.

They grow best at body temperature.

Why are the coliform plates incubated at 37° C?

The mycolic acids in the cell walls of acid-fast bacteria make them resistant to acid-alcohol.

Why is acid-alcohol used as a decolorizing agent rather than ethyl alcohol?

All the answers are correct.

Why is it critical to use aseptic technique when transferring the antibiotic disks to the agar?

6.7 x 103 CFU/ml

You found 67 colonies on the ground beef plate with the 10-2 dilution. What is the concentration of bacteria per milliliter of beef?

Beta hemolysis

You inoculate an organism onto blood agar and observe a clear zone surrounding the bacterial growth. What type of hemolysis is indicated by this observation?

Intermediate

You measure the zone of inhibition around a rifampin disk as 17 mm. How would you report the susceptibility of this organism to rifampin?

Susceptible

You measure the zone of inhibition around a tetracycline disk as 19 mm. How would you report the susceptibility of this organism to tetracycline?

Resistant

You measure the zone of inhibition around an erythromycin disk as 12 mm. How would you report the susceptibility of this organism to erythromycin?

The organism is able to ferment mannitol.

You observe a yellow zone surrounding growth on mannitol salt agar. What does this indicate about the organism?

E. coli

You observe colonies growing on EMB agar that are blueblack with a metallic green sheen. What species of bacteria is this likely to be?

The organism is able to ferment lactose.

You observe red colored colonies growing on MacConkey agar. What does this indicate about the organism?

It is impossible to tell from the information given.

You perform a Kirby-Bauer assay with two antibiotics. Antibiotic X has a zone of inhibition of 3 mm. Antibiotic Y has a zone of inhibition of 5 mm. Which antibiotic is better at killing this particular microorganism?

No, the contaminant adds too many variables to make this assay usable.

You perform a Kirby-Bauer assay, but you neglect to use aseptic technique. Your plate becomes contaminated with a bright red organism. You can easily tell this organism apart from the one you intended to inoculate (which has grown into a nice lawn all over the plate). In order to conserve time, you measure the zone of inhibition for your intended organism, ignoring the contaminants. Are you assured to get the correct answer using this method?

Cefoxitin

You perform a Kirby-Bauer test for a patient with a bladder infection caused by a Gram-positive organism. Considering the following data, which antibiotic would you prescribe? Chloramphenicol: 12 mm zone of inhibition Cefoxitin: 18 mm zone of inhibition Streptomycin: 14 mm zone of inhibition Ampicillin: 26 mm zone of inhibition

5 µg = 12 mm, 10 µg = impossible to predict

You perform a Kirby-Bauer test with a ciprofloxacin-resistant microorganism. You place two ciprofloxacin-containing disks on the agar: one with 5 µg and one with 10 µg. What would you expect the zones of inhibition to be for these two disks?

The bacteria is most likely Gram-negative The bacteria is unable to ferment lactose The bacteria is probably an enteric organism

A bacterial sample was grown on Eosin Methylene Blue (EMB) agar. What can you conclude based on your observations? Based on your observation of the pictured specimens that are growing on the EMB agar plate, select ALL appropriate conclusions.

Mycolic acids (lipoidal)

A thick waxy layer of what substance in the cell wall constitutes the major portion of the mycobacterial cell wall, separating it from other microorganisms?

dark red; blue

After methylene blue is applied as a counter-stain, acid-fast bacteria will appear __________ while non-acid-fast bacteria will appear __________.

A low number of organisms does not indicate the absence of pathogens

After running an experiment, collecting samples from raw carrots and counting the bacteria in the samples, you find that the carrots you tested had only 15 bacteria/ml. Your lab partner states that this number is so low that the carrots are safe to eat. What is your interpretation?

The organism tested ferments mannitol. The organism tested is possibly a type of Staphylococci. The organism tested is able to grow in the presence of high salt.

An unknown specimen was grown on Mannitol Salt agar (MSA). Observe the pictured result and answer the following question. Select ALL appropriate statements regarding the pictured Mannitol Salt agar (MSA) test result.

The media used in this test is good for growing Staphylococci. The tested organism is likely to be coagulase positive. The media used in this test is selective. The media used in this test is differential.

An unknown specimen was grown on Mannitol Salt agar (MSA). Observe the pictured result and answer the following question. Select ALL appropriate statements regarding the pictured Mannitol Salt agar (MSA) test result.

Ampicillin diffuses much faster/farther into the agar than does chloramphenicol.

Compare the zone of inhibition chart for ampicillin (when testing Gram-positive organisms) and chloramphenicol. What might you predict based on these data? Chloramphenicol: 12 mm zone of inhibition Ampicillin: 26 mm zone of inhibition

Tuberculosis and leprosy

The acid-fast stain has diagnostic value in identifying bacteria that contribute to which of the following infectious diseases?

The plate was not incubated long enough; this could be a false negative. The result is negative for starch hydrolysis.

The following image is of a specimen that was grown on Starch agar for 8 hours. Following incubation, the plate was flooded with iodine. Based on this information, as well as your observation of the pictured sample, select all appropriate statements. Select all accurate statements regarding the presented scenario and the pictured test result.

Susceptible, resistant, or intermediate

What are the three possible outcomes for each antibiotic: microorganism pair in a Kirby-Bauer assay?

The pictured bacteria are possibly a type of mycobacteria The pictured bacteria have mycolic acid in their cell walls The pictured bacteria are acid-fast

What can you conclude about the Acid-Fast stained specimen? Based on your observation of this acid-fast stained specimen, select ANY & ALL appropriate statement(s).

If a Gram stain was performed on this bacterium within 24 hrs of a fresh culture, it would most likely be Gram-positive. The pictured bacteria are bacilli The indicated structure is an endospore

What can you conclude about the pictured bacteria? Based on your observation of this stained specimen, select ALL appropriate statements.

The pictured spores could have been made by Rhizopus. A multicellular fungus formed these spores. This is an image of zygospores. The fungal spores shown are produced sexually.

What can you conclude about the pictured fungal spores? Based on your observation of the picture fungal spores select ALL appropriate statements.

The pictured spores were most likely made by an Aspergillus mold. This is an image of conidia. The fungal spores shown are produced asexually.

What can you conclude about the pictured fungal spores? Based on your observation of the picture fungal spores select ALL appropriate statements.

This is an image of sporangia. The fungal spores shown are produced asexually The pictured spore could have been made by Rhizopus.

What can you conclude about the pictured fungal spores? Based on your observation of the picture fungal spores select ALL appropriate statements.

This is an image of conidia. The pictured spores were most likely made by a Penicillium mold. The fungal spores shown are produced asexually.

What can you conclude about the pictured fungal spores? Based on your observation of the picture fungal spores select ALL appropriate statements.

The yeast shown is in the process of reproducing asexually. The yeast shown is in a diploid state.

What can you conclude about the pictured yeast? Based on your observation of this yeast specimen select ALL appropriate statements.

The diameter of the zone of inhibition surrounding the antibiotic disks

What characteristic of the Kirby-Bauer test helps scientists to determine if a microorganism is susceptible to an antibiotic?

Blue

What color do you expect Escherichia coli to be after performing the acid-fast stain?

Red

What color do you expect Mycobacterium smegmatis to be after performing the acidfast stain?

The susceptibility of an organism to a set of antibiotics

What does the Kirby-Bauer test assess?

Fecal contamination of the milk occurred somewhere in the production process.

What does the presence of enteric organisms in milk indicate?

The area around the antibiotic disk in which bacteria cannot grow

What is a zone of inhibition?

A chemical that inhibits microbial growth

What is an antibiotic?

Differential media incorporate chemical compounds that produce characteristic changes in the appearance of the bacterial colonies

What is the definition of differential media?

Enriched media have been supplemented with highly nutritious materials for the purpose of cultivating fastidious organisms.

What is the definition of enriched media?

Selective media incorporate chemical substances that inhibit the growth of one type of bacteria while permitting the growth of another.

What is the definition of selective media?

Eosin and methylene blue are dyes that precipitate on the surface of acid-producing colonies of enteric lactose fermenters, creating a metallic green sheen.

What is the purpose of adding eosin and methylene blue to eosin-methylene blue agar (EMB)?

Phenol red permits the differentiation of bacteria based on their ability to ferment mannitol.

What is the purpose of adding phenol red to mannitol salt agar (MSA)?

High concentrations of salt inhibit the growth of non-halophilic organisms but allow for the growth of bacteria from the genus Staphylococci.

What is the purpose of adding salt to mannitol salt agar?

Allows for further penetration of the stain through the lipoidal wall and into the cytoplasm

What is the purpose of heating the slide during the staining process?

You need to find the initial concentration of bacteria in the food.

When you are testing food for bacterial contamination, why must you perform serial dilutions of the samples you collect? * Serial dilution is a series of sequential dilutions used to reduce a dense culture of cells to a more usable concentration. Each dilution will reduce the concentration of bacteria by a specific amount.

In the guts of mammals

Where would you be most likely to find members of the genera Escherichia?

In soils and water

Where would you be most likely to find the fungus Penicillium?

In the soil

Where would you expect to find members of the genus Pseudomonas?