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A series of questions and answers related to growth media, selective media, differential media, and the process of spreading bacterial cultures. It also includes information on the Gram status of microbes, the use of specific agar plates to detect pathogenic strains, and the importance of generating pure cultures. likely intended for students studying microbiology or related fields.
Typology: Exams
1 / 22
True or False: Growth media is best suited for distinguishing between two similar species of bacteria. True False Growth media is designed to simply support (and not restrict) microbial growth.
Question 1 2 / 2 pts
A researcher is asked to determine if a sample contains Neisseria meningitides. Knowing Neisseria meningitides is slow growing and other foreign microbes may also be present in the culture, which type of media would be best suited? Growth media Correct! Selective media Selective media. If additional foreign microbes may be present in your sample and you know Neisseria meningitides is slow growing, you must restrict the unwanted microbes while supporting the growth of Neisseria meningitides. Thus, you are selectively encouraging one microbes growth, while selectively restricting others.
Question 2 2 / 2 pts
Selective and Differential media Differential media What are the requirements of a fastidious microbe? A fastidious microbe is an organism with complex growth requirements such that if absent it will not grow. Enriched medias thus contain these specific and essential nutrients required for the growth of a particular subset of microorganisms.
Question 3 10 / 10 pts Question 4 2 / 2 pts
True or False: LB agar is classified as a non-selective, non- differential media. Correct! True LB agar is the most basic type of agar and like LB media supports the growth of virtually all microbes without restriction. False The polysaccharide-based hardening agent added to LB media to produce LB agar is derived from what? Your Answer:
Question 5 5 / 5 pts
Seaweed extract Seaweed (algae) extract. Blood agar is which type of medium? Select all that apply. Correct! Differential Blood agar can be used to differential between species based on its hemolytic activity. Selective Correct!
Question 6 2 / 2 pts
Enriched Selective and Differential Match the following hemolytic class with its description of activity.
Question 7 5 / 5 pts
Columbia CNA agar is used to isolate: Correct Answer Gram-Positive
A C B Question 8 0 / 2 pts
Gram-Negative Mycobacteria Gram-Positive and Gram-Negative True or False: Chocolate (cocoa) is not a component of Chocolate agar plates. Correct! True The name is derived simply based on the color that actually comes from the presence of ‘cooked’ (lysed) red blood cells in the media.
Question 9 2 / 2 pts
False An unknown microbe is streaked onto a MacConkey agar plate. After an overnight incubation at 37°C, growth is observed. Would a Gram stain be necessary? Why or why not? Your Answer: No, a gram stain would not be necessary because only gram negative growth would be observed on MacConkey agar plates. No. A Gram stain would not be necessary, as only Gram-Negative microbes will grow on MacConkey agar.
Question 10 5 / 5 pts
In an attempt to detect the presence of the pathogenic strain of E. coli O157:H7, a researcher spread a culture onto a MacConkey agar with failed results. What type of agar should they (correctly) try next? Why? Correct Answer: The microbe should be plated on SMAC (Sorbitol-MacConkey agar) as it is specifically formulated to detect O157:H7. Pathogenic E. coli (O157:H7) cannot ferment sorbitol while non- pathogenic E. coli can ferment both soribitol and lactose. Therefore, colonies that ferment (acidic conditions; non-pathogenic) can be differentiated from non-fermenters (neutral to basic conditions; pathogenic).
Question 11 7.5 / 10 pts Question 12 5 / 5 pts
What is the Gram status (positive or negative) of microbes growing on Eosin Methylene Blue (EMB) agar plates? Your Answer: gram negative Gram-Negative. EMB plates specifically restrict the growth of Gram-Positive bacteria. Mannitol salt agar will turn what color in the presence of the pathogenic strain Staphylococcus aureus?
Question 13 0 / 2 pts
Correct Answer Yellow Pathogenic Staph aureus will turn the agar from red to yellow. What is the process of spreading a bacterial culture onto a petri dish called? Plating
Question 14 3 / 3 pts Question 15 10 / 10 pts
Describe the primary advantage of using a petri dish over growing a liquid culture? Your Answer: When using a petri dish you can observe the growth of a sample in a specified region. The primary advantage is that cells are held into place. When grown in a nutrient broth, bacterial cells can multiply but are free to move around in solution. When grown on agar within a petri dish the fixed in such as way as to form colonies. True or False: The visualization of colonies on a petri dish represents bacterial cells that have often multiplied a million times over.
Question 16 2 / 2 pts
Correct! True To form a bacterial colony the initial cell must have multiplied many times over, often greater than a million, in order for the naked eye to resolve it. False True or False. The purpose of a quadrant streak is to expand a bacterial population. True Correct!
Question 17 2 / 2 pts
False The purpose of the quadrant streak is to generate individual colonies such that a single (pure) bacterial sample can be isolated. To be considered a pure culture, the sample (1) can be traced back to a single cell and (2)?
Question 18 5 / 5 pts
The culture must also be free from external contaminants. Simply put, a pure sample would never contain multiple bacterial species (ie) Strep and Staph. When performing a quadrant streak, the sample is spread across the plate in such as way as to form what? A dilution gradient is formed. The resulting gradient should always contain within it the growth of individual colonies. In what phase of a dilution streak would you expect to find the highest concentration of bacteria, P2 or P4?
Question 19 4 / 5 pts Question 20 5 / 5 pts
P2 (Phase 2) would contain a higher concentration of bacteria than Phase 4 (P4). The phases rank (from highest to lowest), P1 > P2 > P3 > P4.
Question 21 2 / 2 pts
True or False. When performing a dilution streak a new (or sterilized) loop must be used for each phase. Correct! True Failure to do so would prevent the establishment of a dilution gradient, as the same bacterial concentration would be spread across both phase regions. False The number of phases (3 vs. 4) and/or the number of times a loop passes through a
Question 22 5 / 5 pts
Either deviation is acceptable in practice provided the resulting gradient contains within it the growth of individual colonies—if not, the experiment must be repeated.
Question 23 2 / 2 pts
True or False. Pathogenic strains of bacteria tend to grow slower than normal non-pathogenic bacterial strains. True Correct! False Pathogenic strains of bacteria tend to grow faster than non-pathogenic strains at 37°C, which is why researchers may set incubators at 25°C to restrict its growth. When given an unknown bacterial sample the first step is to expand the current bacterial population. Which form of media best suites this need? Why?
Question 24 0 / 5 pts
it is, most importantly, a non-selective media. Given the alternatives, this is the best option.