Ch. 7 - The Control of Microbial Growth

Tortora - Microbiology: An Introduction 14th Edition

Tortora14th EditionMicrobiology: An IntroductionISBN: 9780138200398Not the one you use?Change textbook

Tortora 14th Edition

Ch. 7 - The Control of Microbial Growth

Problem 3

Chapter 7, Problem 3

Thermal death point is not considered an accurate measure of the effectiveness of heat sterilization. List three factors that can alter thermal death point.

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Understand that the thermal death point (TDP) is defined as the lowest temperature at which all microorganisms in a liquid culture are killed in 10 minutes. It is a specific measure but can be influenced by various factors.

Identify that one factor altering TDP is the type of microorganism, as different species and strains have varying heat resistances due to their structural and physiological differences.

Recognize that the composition of the medium or environment where the microorganisms are present affects TDP; for example, the presence of organic matter, fats, or proteins can protect microbes from heat.

Consider the pH of the environment, since acidic or alkaline conditions can influence microbial heat sensitivity and thus change the thermal death point.

Note that the initial number of microorganisms (microbial load) also impacts TDP because a higher number of cells may require longer or higher heat exposure to achieve complete sterilization.

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Key Concepts

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

Thermal Death Point (TDP)

Thermal Death Point is the lowest temperature at which all microorganisms in a liquid culture are killed in 10 minutes. It is used to assess heat sterilization but can vary depending on conditions, making it an inconsistent measure of sterilization effectiveness.

Microbial Resistance and Population Characteristics

Different microorganisms have varying heat resistance, influenced by species, spore formation, and population size. Larger or more resistant populations require higher temperatures or longer exposure times to achieve sterilization.

Environmental and Physical Factors Affecting Heat Sterilization

Factors such as pH, presence of organic matter, moisture content, and the medium's composition can alter heat transfer and microbial survival. These variables impact the thermal death point by protecting microbes or affecting heat penetration.

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Textbook Question

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

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

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Textbook Question

Which of the following is most effective for sterilizing mattresses and plastic Petri dishes?

a. Chlorine

b. Ethylene oxide

c. Glutaraldehyde

d. Autoclaving

e. Nonionizing radiation

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Textbook Question

Which of these disinfectants does not act by disrupting the plasma membrane?

a. Phenolics

b. Phenol

c. Quats

d. Halogens

e. Biguanides

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Textbook Question

The thermal death time for a suspension of Bacillus subtilis endospores is in dry heat and less than in an autoclave. Which type of heat is more effective? Why?

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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

b. Ethylene oxide

c. Supercritical fluids

d. Autoclaving

e. Short-wavelength radiation

Textbook Question

If pasteurization does not achieve sterilization, why is pasteurization used to treat food?

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