This set of Microbiology Questions and Answers for Freshers focuses on “Quantitative Measurement of Bacterial Growth”.
1. Which of the following is an indirect method for measuring bacterial growth?
a) Cell count
b) Cell mass
c) Cell activity
d) Both Cell mass and Cell activity
Explanation: Cell activity is an indirect method for measuring bacterial growth by relating the degree of biochemical activity to the size of the population.
2. Which of the following instrument is used for the bacterial count?
a) Petroff-Hausser counting chamber
Explanation: Bacteria can be counted easily and accurately with the Petroff-Hausser counting chamber. This is a special slide accurately ruled into squares that are 1/400 mm2 in area. A suspension of unstained bacteria can be counted in the chamber, using a phase contrast microscope.
3. Which of the following method is used for a viable count of a culture?
a) Direct microscopic count
b) Plate-count method
c) Membrane-filter count
d) Plate-count method and membrane-filter count
Explanation: The main disadvantage of direct counting of cell numbers is that there is no way to determine whether the cells being counted are viable. To determine the viable count of culture, we must use a technique that allows viable cells to multiply, such as the plate-count method or membrane-filter method.
4. The number of bacteria per ml depends on the dilution of the sample.
Explanation: In the plate count method, the formula used is as follows:
Number of bacteria per ml = Number of colonies counted on plate X dilution of sample.
Thus number of bacteria directly depends on the dilution of sample.
5. Which of the following is the relationship between optical density and cell mass?
a) exponentially proportional
b) linearly proportional
c) inversely proportional
d) not related
Explanation: The photoelectric colorimeter used for measuring bacterial population, measures optical density (a function of light intensity) which is almost linearly proportional to cell mass.
6. How many cells present per milliliter in a bacterial culture can make the culture turbid?
c)1 lakh cells
Explanation: Bacteria in a suspension absorb and scatter the light passing through them so that a culture of more than 107 to 108 cells per milliliter appears turbid to the naked eye. Then a spectrophotometer or colorimeter can be used for turbidimetric measurements.
7. A dead cell does not contribute to turbidity in the culture medium.
Explanation: Both dead as well as living cells contribute to turbidity. However, turbidity cannot be measured for cultures grown in deeply colored media or cultures that contain suspended material other than bacteria.
8. Which of the following is a direct measurement of growth?
a) Determination of nitrogen content
b) Turbidimetric methods
c) Determination of Dry weight of cells
d) Measurement of a specific chemical change produced on a constituent of the medium
Explanation: To measure the dry weight of cells is the most direct approach for quantitative measurement. All others are indirect methods and is applicable only in special circumstances.
9. Which of the following method is used for enumeration of bacteria in vaccines and cultures?
a) Microscopic Count
b) Membrane filter
c) Plate count
d) Dry weight determination
Explanation: Membrane count is used for the enumeration of bacteria in vaccines and cultures. Even electronic enumeration is used in this application.
10. Colony-forming units per ml is the unit of _____________
a) Microscopic count
b) Electronic enumeration
c) Plate count
d) Turbidimetric measurement
Explanation: Since the plate count method is used in the enumeration of bacteria in milk, water, foods, soil, etc, the unit in which growth is measured is colony-forming units per ml as the bacteria forms colonies in the petri dish.