Ch. 7 - The Control of Microbial Growth

Tortora - Microbiology: An Introduction 14th Edition

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Tortora 14th Edition

Ch. 7 - The Control of Microbial Growth

Problem 5

Chapter 7, Problem 5

A bacterial culture was in log phase in the following figure. At time x, an antibacterial compound was added to the culture. Draw the lines indicating the addition of a bactericidal compound and a bacteriostatic compound. Explain why the viable count does not immediately drop to zero at x.

Verified step by step guidance

Step 1: Understand the graph before time x. The graph shows a bacterial culture in the log phase, where the number of cells increases exponentially. This is represented by a straight line with a positive slope on a log scale, indicating rapid bacterial growth.

Step 2: At time x, an antibacterial compound is added. To represent the effect of a bacteriostatic compound, draw a horizontal line starting at time x. This indicates that the bacterial growth stops, so the number of viable cells remains constant over time, but cells are not killed.

Step 3: To represent the effect of a bactericidal compound, draw a line starting at time x that slopes downward on the log scale. This shows a decrease in the number of viable cells over time, as the compound kills bacteria, reducing the viable count.

Step 4: Explain why the viable count does not immediately drop to zero at time x. Even with a bactericidal compound, bacteria do not die instantaneously because the killing process takes time. Some cells may be in different physiological states, and the compound needs time to penetrate cells and disrupt vital processes.

Step 5: Summarize that the bacteriostatic compound halts growth without killing cells, resulting in a flat line, while the bactericidal compound reduces viable cells over time, shown by a downward slope. The delay in cell death explains why the viable count does not instantly fall to zero.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Bacterial Growth Phases

Bacterial growth occurs in distinct phases: lag, log (exponential), stationary, and death. The log phase is characterized by rapid cell division and exponential increase in cell number, which is crucial for understanding how bacteria respond to antibacterial agents added during this phase.

Bactericidal vs. Bacteriostatic Agents

Bactericidal agents kill bacteria, leading to a decline in viable cell count, while bacteriostatic agents inhibit bacterial growth without killing, causing the cell number to plateau. Differentiating these effects helps interpret changes in bacterial counts after treatment.

Viable Count and Delayed Effect of Antibacterials

Viable count measures living bacteria capable of reproduction. After adding an antibacterial, the count does not drop immediately due to factors like bacterial cell cycle stages, time needed for drug action, and presence of resistant or dormant cells, causing a lag before observable effects.

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Which of the following is used to control microbial growth in foods?

a. Organic acids

b. Alcohols

c. Aldehydes

d. Heavy metals

e. All of the above

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The antimicrobial effect of gamma radiation is due to:(a) ________.

The antimicrobial effect of ultraviolet radiation is due to: (b) ________.

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How do autoclaving, hot air, and ultra-high-temperature pasteurization illustrate the concept of equivalent treatments?

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Textbook Question

Use the following information to answer questions 6 and 7. The data were obtained from a use-dilution test comparing four disinfectants against Salmonella Choleraesuis.

G = growth, NG = no growth

Which disinfectant is the most effective?

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Textbook Question

Use the following information to answer questions 6 and 7. The data were obtained from a use-dilution test comparing four disinfectants against Salmonella Choleraesuis.

G = growth, NG = no growth

Which disinfectant(s) is (are) bactericidal?

a. A,B,C, and D

b. A,C, and D

c. A only

d. B only

e. None of the above

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Textbook Question

Which of the following cannot be used to sterilize a heat-labile solution stored in a plastic container?

a. Gamma radiation