Q: A sample is diluted serially and plated. The plate with a $$10^{-4}$$ dilution yields 65 colonies after incubation. If 0.1 ml was plated, what is the estimated number of bacteria per ml in the original sample?

$$65 \times 10^4$ \times 10 = 6.5 \times 10^6$

Question 1

Which of the following best describes a biofilm, and why are biofilms clinically significant in hospital settings?

Accepted Answer

A biofilm is a collection of microbes living on a surface in a complex community; they are clinically significant because microbes in biofilms are often more resistant to antibiotics and disinfectants.

Question 2

Which of the following organisms would be classified as a chemoautotroph based on its energy and carbon sources?

Accepted Answer

A hydrogen, sulfur, or nitrifying bacterium using chemical compounds and carbon dioxide.

Question 3

A bacterial cell is placed in a hypertonic solution. What is the most likely outcome for the cell?

Accepted Answer

The cell will shrivel as water leaves the cell due to osmosis.

Question 4

Which toxic form of oxygen is most likely to cause irreparable damage to cellular components due to its high reactivity?

Accepted Answer

Superoxide radicals

Question 5

A microorganism grows best at pH 5.6 and forms large, fluffy colonies on agar. Which term best describes this organism?

Accepted Answer

Acidophile

Question 6

Which of the following is a major reason why nitrogen fixation by certain bacteria is essential to life on Earth?

Accepted Answer

It converts atmospheric nitrogen into forms usable for amino acid and nucleotide synthesis.

Question 7

A cardiac nurse is concerned about biofilms forming on catheters. What property of biofilms makes them particularly problematic in this scenario?

Accepted Answer

Microbes in biofilms are more resistant to antibiotics and immune responses.

Question 8

Which of the following best describes the process of binary fission in bacteria?

Accepted Answer

Binary fission is a process where a bacterial cell divides into two daughter cells after replicating its chromosome and forming a septum.

Question 9

A culture medium contains peptone, agar, lactose, bile salts, NaCl, neutral red, and crystal violet. Which type of medium is this, and what is its primary use?

Accepted Answer

MacConkey agar; used for selective and differential culture of enteric bacteria based on lactose fermentation.

Question 10

Which equation best represents the number of cells after $$n$$ generations of binary fission, starting with a single cell?

Accepted Answer

$$N = 2^n$$

Question 11

A bacterial culture is growing in a chemostat. What is the primary purpose of using a chemostat in industrial microbiology?

Accepted Answer

To maintain a microbial population in a particular phase of growth by continuously adding fresh medium and removing old medium.

Question 12

Which of the following methods is an indirect way to estimate microbial population size in a liquid culture?

Accepted Answer

Measuring turbidity with a spectrophotometer.

Question 13

A sample is diluted serially and plated. The plate with a $$10^{-4}$$ dilution yields 65 colonies after incubation. If 0.1 ml was plated, what is the estimated number of bacteria per ml in the original sample?

Accepted Answer

$$65 	imes 10^4$ 	imes 10 = 6.5 	imes 10^6$

Question 14

Which of the following best explains why some bacteria can survive in the death phase and regrow when transferred to fresh media?

Accepted Answer

Some cells in the death phase are dormant or have survived environmental stress, allowing them to regrow when conditions improve.

Question 15

Which of the following is NOT a physical requirement for microbial growth?

Accepted Answer

Nitrogen fixation

Question 16

Which type of microorganism would most likely be found growing in deep-sea hydrothermal vents, where pressure is extremely high?

Accepted Answer

Barophile

Question 17

Which of the following statements about selective media is correct?

Accepted Answer

Selective media contain substances that favor or inhibit the growth of particular microorganisms.

Question 18

Which of the following best describes the function of quorum sensing in biofilm formation?

Accepted Answer

Quorum sensing triggers the production of extracellular matrix and changes in cell biochemistry and shape, facilitating biofilm development.

Question 19

Which of the following is a growth factor required by some microorganisms and is a precursor for folic acid synthesis?

Accepted Answer

Para-aminobenzoic acid (PABA)

Microbial Nutrition and Growth: Study Notes CH-6

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