Each method of counting bacteria has advantages and disadvantages; non

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. 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. 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. 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. 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