Question 1

You take a stool sample from Mrs. Thomas, and send it to the lab for analysis. You tell her that the technicians will do a variety of tests to confirm the identity of the organism causing her illness. The first thing that will be done is to inoculate some of her stool sample onto a medium called hektoen enteric (HE) agar. On this medium, Shigella species produce blue-green colonies, while Salmonella produces blue-green colonies with a black center, and most Gram-positive bacteria do not grow at all. You explain to Mrs. Thomas that this type of medium is called a(n) ________ medium.

Accepted Answer

Hektoen enteric (HE) agar is both selective and differential. It selects for Gram-negative enteric bacteria such as Shigella and Salmonella and differentiates between the two based on colony color and characteristics. Therefore, the correct answer is D, selective and differential.

Question 2

L. pneumophila is widespread and is found in various water sources, including hot tubs, water cooling towers, swimming pools, grocery store misters, and shower water. Select the FALSE statement regarding the organism, given this information.

Accepted Answer

L. pneumophila is not an obligate anaerobe, as it can grow both in the presence and absence of oxygen. The other statements are true: it is a mesophile, it can survive in the presence of chlorine, and it can survive in water pipes.

Question 3

Shigella species and Salmonella species are classified as facultative anaerobes. You know that given this information, the bacteria in your patient's fecal culture will grow best

Accepted Answer

Facultative anaerobes can grow in the presence of oxygen or in the absence of oxygen through anaerobic respiration or fermentation. However, they can metabolize and grow better in the presence of oxygen, which is known as aerobic respiration. Therefore, the best choice for the growth of Shigella and Salmonella species is in the presence of oxygen.

Question 4

The colonies of interest in this case are the blue-green presumptive Shigella colonies. However, growth on hektoen enteric agar is not sufficient for definitive identification. Further tests are needed. First, the lab technicians need to prepare larger numbers of Shigella from your patient's stool sample, and then they need to ensure that they are culturing a single species. The methods used to do this are ________, followed by ________.

Accepted Answer

Enrichment culture is used to increase the numbers of the Shigella bacteria in the sample by providing a favorable environment for their growth. After this step, streak plating is used to obtain isolated colonies of the bacteria for further testing. Therefore, choice E of enrichment culture followed by streak plating is the best option.

Question 5

Eosin methylene blue agar (EMB) is a selective and differential medium. The dyes in the medium inhibit most Gram-positive organisms. Gram-negative organisms such as the coliforms grow on this medium. Coliforms that are fecal in origin produce colonies with a metallic green color on EMB, while those that are non-fecal produce colorless colonies. You plate a urine sample onto an EMB plate and incubate it. Metallic green colonies develop. From this, you can conclude that

Accepted Answer

The metallic green colonies on EMB indicate that the bacteria are Gram-negative and fecal in origin, specifically belonging to the coliform group.

Question 6

A physician sends a stool sample to your lab, and wants to know if there are lactose fermenting microbes in the sample. How might you determine if these microbes are present or not from this mixed-microbe specimen?

Accepted Answer

MacConkey agar is a selective and differential medium that can be used to determine if microbes in a sample are lactose fermenting. If lactose fermenting microbes are present in the sample, they will produce acid byproducts that lower the pH of the agar, causing a color change to pink due to the pH indicator present in the medium. This allows for the selective growth and detection of these microbes. Thayer-Martin agar is selective for Neisseria species, and blood agar is not selective for lactose fermenting microbes.

Question 7

You are culturing a new bacterium in the laboratory. You cannot find any glucose it the lab when you are making growth medium, so you decide to use honey instead. You add twice as much honey than is called for in the medium recipe, because you want to give the organism the best opportunity or growth. To your surprise, the organism does not grow on your modified medium. Select the most likely explanation.

Accepted Answer

Honey is a concentrated solution of sugars, and adding twice as much honey as the recipe called for would result in a hypertonic growth medium. This means that there is a higher concentration of solutes outside the bacterial cells than inside, causing water to diffuse out of the cells and leading to plasmolysis and cell death.

Question 8

The lab technicians culture the sample on Thayer-Martin agar. This complex medium contains 5% sheep blood and a combination of antibiotics. This medium is

Accepted Answer

Thayer-Martin agar is selective because it contains antibiotics that inhibit the growth of unwanted bacteria, allowing only the growth of Neisseria species and a few other organisms.

Question 9

If you are asked to make 200 mL of Thayer-Martin agar, who much sheep's blood would you add to the medium? Recall that this complex medium contains 5% sheep blood.

Accepted Answer

Thayer-Martin agar contains 5% sheep's blood. To make 200 mL of the medium, you need to add 10 mL of sheep's blood (5% of 200 mL = 10 mL). Therefore, the correct choice is C) 10 mL.

Question 10

Clostridium paradoxum grows optimally at 55°C, pH 9.3; it will not grow in the presence of O2. Please select the TRUE statement about this organism.

Accepted Answer

The statement mentions that Clostridium paradoxum "will not grow in the presence of O2," indicating that it is an obligate anaerobe. Therefore, options A, B, D, and E are incorrect.

Question 11

The causative organism of gonorrhea is Neisseria gonorrhoeae, a fastidious, Gram-negative diplococcus. What does fastidious mean in this context?

Accepted Answer

Fastidious in this context means that Neisseria gonorrhoeae has complex nutritional requirements and is difficult to grow in laboratory cultures without proper nutrients and conditions.

Question 12

In addition to the few Shigella colonies that are noted, there are also abundant salmon pink colonies, around which the medium has turned yellow, losing its original blue color. You know that hektoen enteric agar must contain ________.

Accepted Answer

Hektoen enteric agar contains a pH indicator, which is responsible for the color change in the medium. When fermentation of lactose occurs, it leads to the production of acids, which causes the pH indicator to change from blue to yellow or pink. Therefore, option B is the correct answer. Options A, C, D, and E do not accurately describe the composition of hektoen enteric agar.

Question 13

Diagnosis of legionellosis depends partly on culture of the causative agent from medical samples. Typically, the organism is grown on buffered charcoal yeast extract (BCYE) agar. What essential component must this medium have to ensure growth of L. pneumophila?

Accepted Answer

Cysteine is an essential component of BCYE agar for the growth of Legionella pneumophila. It is required for the synthesis of proteins and nucleic acids. Red blood cells and NaCl are not essential components for the growth of Legionella pneumophila, although they can be added to the medium. Chlorine is actually toxic to Legionella pneumophila and should be avoided during culture.

Question 14

Thayer-Martin agar typically contains a combination of antibiotics: vancomycin, colistin, nystatin, and trimethoprim. Please select the TRUE statement.

Accepted Answer

Neisseria gonorrhoeae is naturally resistant to many antibiotics, but Thayer-Martin agar is specifically designed to inhibit the growth of normal oral and gut flora so that pathogenic Neisseria species can be isolated from clinical samples. Thayer-Martin agar contains antibiotics that selectively inhibit Gram-negative bacteria and certain species of Gram-positive bacteria, including Streptococcus pneumoniae and some staphylococcal species. This allows N. gonorrhoeae to grow on the media.

Question 15

You are in charge of water quality for your city's water treatment plant. Of the methods at your disposal, which will be the most efficient and cheapest method of determining the number of viable bacteria in the water coming out of your plant?

Accepted Answer

Membrane filtration followed by placing the membrane in a growth medium for colony counts after incubation is both efficient and cost-effective for determining the number of viable bacteria in water. This method concentrates the bacteria present in large volumes of water onto a filter, which is then placed on a growth medium. After incubation, the colonies that grow can be counted, providing a direct measure of the number of viable bacteria in the water sample. This method is preferred for water quality testing because it is specific for viable bacteria, relatively simple, and does not require expensive equipment.

Question 16

A halophilic, aerobic mesophile has a generation time of 20 minutes. You place 1 × 104 of these organisms on an appropriate solid medium, and incubate the plate for 100 minutes. Select the FALSE statement.

Accepted Answer

According to the given data, the generation time of the organism is 20 minutes. Therefore, after 100 minutes of incubation, there will be (100/20) = 5 generations of growth. Starting with 1 × 104 cells, this would result in a final population size of 32 × 104 or 3.2 × 105 cells. The statement in option C is true. Option A states that the growth medium contains NaCl, which is consistent with the organism being halophilic (salt-tolerant). Option B states that the organism will grow at 40oC, but there is no information provided in the question about the temperature range for growth. Option E states that the organism produces superoxide dismutase and catalase, which are common enzymes produced by many aerobic bacteria to detoxify oxygen radicals.

Question 17

You are determining the number of colony forming units (CFUs) for Susan, who has a urinary tract infection (UTI). You added 1 mL of Susan's urine sample to 99 mL of diluent, and spread 0.1 mL of that dilution on a culture medium. After appropriate incubation, you count 143 colonies on the plate. What was the concentration of bacteria in the original urine sample?

Accepted Answer

To determine the concentration of bacteria in the original urine sample, we need to do the following calculations:

1. We know that 0.1 mL of the 1:1000 dilution was plated on the culture medium.
2. We observed 143 colonies on the plate.
3. Therefore, there were 143 CFUs in 0.1 mL of the 1:1000 dilution.
4. To determine the concentration of bacteria in the original sample, we need to multiply the number of CFUs by the dilution factor.
5. The dilution factor for this sample is 1:1000.
6. To convert the 0.1 mL of the diluted sample to the original volume of the sample, we multiply by the reciprocal of the dilution factor:
      0.1 mL x (1/1000) = 0.0001 L
7. We can now calculate the concentration of bacteria in the original sample:
      CFUs / volume of the diluted sample x reciprocal of the dilution factor
      143 CFUs / 0.0001 L x (1/1000)
      = 1.43 x 10^5 CFUs/L or cells/mL

Therefore, the correct answer is D) 1.43 x 10^5 cells/mL.

Question 18

You take absorbance readings on a spectrophotometer across a 6-hour culture of E. coli cells growing in tryptic soy broth (TSB). Your absorbance readings clearly indicate a lag phase, a log phase, and a stationary phase. You come back in and take readings at 8, 10, 12, 14, and 16 hours, but the absorbance number remains the same. Shouldn't it start coming down as the closed batch culture enters death phase? What's the most likely thing that is happening?

Accepted Answer

When using absorbance readings to track bacterial growth, there is no way to distinguish between live and dead cells. As a result, the absorbance stays constant as dead cells remain in the culture even as live cells enter death phase. This is a common phenomenon when using absorbance to track bacterial growth.

Question 19

On HE medium, Shigella species produce blue-green colonies, while Salmonella produces blue-green colonies with a black center. There are two noticeable types of colonies growing on the hektoen enteric (HE) medium in Mrs. Thomas' test. One type of colony is greenish in color. This leads you to conclude that the bacteria in these colonies

Accepted Answer

The greenish colonies on HE medium indicate the presence of a Gram-negative bacteria. Since these colonies do not have a black center, they are most likely to be Shigella which produce blue-green colonies on HE medium. Therefore, the correct answer is A) is Gram-negative and is likely Shigella.

Question 20

In some labs, samples containing suspected L. pneumophila are cultured on traditional BCYE medium as well as BCYE medium lacking L-cysteine. How would L. pneumohila be identified in this situation?

Accepted Answer

Legionella pneumophila requires L-cysteine for growth, so it will grow on traditional BCYE medium that contains L-cysteine but not on BCYE medium lacking L-cysteine.

Quiz 4: Dynamics of Microbial Growth