Each method of counting bacteria has advantages and disadvantages; non

Question

Each method of counting bacteria has advantages and disadvantages; none is 100 percent accurate. Cell counts may be made with a counting chamber, a slide marked with a grid to facilitate counting of cells and to determine the volume of liquid in the area counted. Counts are made under a microscope and calculations made to determine the number of cells per ml of the original culture. Electronic cell counters can be used to count cells suspended in a liquid medium which passes through a hole small enough to allow the passage of only one bacterial cell at a time. Smear counts are similar to cell counts: A known volume of culture is spread over a known area (1 cm2) of a slide and then stained. Counts are made from several microscope fields, and calculations are made. In membrane filter counts, a known volume of a culture is passed through a filter, which is then examined microscopically for cells. The advantage of cell counts, smear counts, and membrane filter counts is that they are quickly accomplished with little complicated equipment; however, both living and dead cells are counted.

The serial-dilution method involves the making of a series of dilutions, usually by a factor of 10, into a nutrient medium. The highest dilution producing growth gives a rough indication of the population of the original culture; for example, if the highest dilution to produce growth is the 1:100 dilution, the original culture had between 100 and 1,000 cells per ml.

Plate counts are made by making serial dilutions (usually in sterile tap water or an isotonic solution) of the original culture. Samples of known volume of the dilutions are transferred to petri dishes and mixed with nutrient agar. After a suitable incubation period, the colonies on the plates with between 30 and 300 colonies are counted. Because each colony is assumed to have arisen from a single cell, calculations can be made to determine the original population size. Plate counts have the advantage of not including dead cells, and they can be used when the population is so low as to make other methods impractical, but they require more time than direct counts, and they detect only those organisms that can grow under the conditions of incubation; the development of one colony from more than one cell is also a source or error.    1 
 1. The author’s purpose in this passage is to

A. argue for the development of a fully accurate counting method.
B. discuss the advantages of several methods of counting cells.
C. show that new counting methods have surpassed those used in the past.
D. give instruction in the performance of cell counts.
E. describe a variety of methods of counting bacteria.

2 
 2. We can infer that no method of bacteria counting is wholly accurate because

I. the number of cells is likely to be so large.
II. the cells are microscopic in size.
III. both living and dead cells are counted.

A. II only
B. I and II only
C. I and III only
D. II and III only
E. I, II, and III

3 
 3. If we know the total bacteria cell volume in a sample, to determine the bacteria cell count we must also know

I. the volume of a single cell.
II. the volume of the nutrient culture.
III. the volume of the calibrated centrifuge tube.

A. I only
B. II only
C. III only
D. I and II only
E. II and III only

4 
 4. Which of the following best describes the audience to which this passage is probably addressed?

A. advanced students in microbiology
B. casual readers of a scientific magazine
C. elementary school students 
D. introductory college biology students
E. high school mathematics students

RC Butler 2022  -  Practice Two RC Passages Everyday.   
Passage # 170 Date: 27-Apr-2022
 This question is a part of RC Butler 2022 .  Click here for Details

Source: Cliffs Test Prep.

thakurarun85 · Answer

Please provide the solutions.

Ajay5068 · Answer

Hi can you please explain the last question how can a person from non science background possibly determine who is this passage directed to according to GMAT all the questions asked must have an answer in the para or can be inferred from the para the last question doesn't help in either. Kindly let me know that how can the answer be derived or have i miss read something ???

Sajjad1994 · Answer

Official Explanation

4. Which of the following best describes the audience to which this passage is probably addressed?

Difficulty Level:   Hard

Explanation

To answer this question, one should not neccessarily be a student of science. You need to concentarte on the overall content of the passage and then use the process of elimination to reach out to the answer.

The passage is too advanced for options  (B)  or  (C)  but not so technical as you would expect for advanced students  (A) . The subject of the passage is not mathematics  E.  So best option is  (D) .

Answer: D

Talkdwai · Answer

How do we sure about that this passage is not addressed "advanced students in microbiology"? The passage depicts all about the technical thing. For a moment if I agree with D is the answer, how do you addressed from the passage that A is not the answer. If there is any conspicuous process of elimination then please share. I don't find any proper logic in your explanation.

thakurarun85 · Answer

Please provide solution for question 2 and 3. The OA doesn't reflect any thing from passage.

Sajjad1994 · Answer

Official Explanation

2. We can infer that no method of bacteria counting is wholly accurate because

Difficulty Level:   700

Explanation

Though some methods count both living and dead cells, one method that is described counts only living cells.

Answer: B

Sajjad1994 · Answer

Official Explanation

3. If we know the total bacteria cell volume in a sample, to determine the bacteria cell count we must also know

Difficulty Level:   700

Explanation

If you know the total volume of the bacteria cells and the volume of one cell, you can divide the former by the latter to determine the total number of cells.

Answer: A

Each method of counting bacteria has advantages and disadvantages; non

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GMAT Reading Comprehension (RC) Questions

Each method of counting bacteria has advantages and disadvantages; non

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