Book cover for Nester's Microbiology: A Human Perspective

Nester's Microbiology: A Human Perspective

Denise Anderson, Sarah Salm, Deborah Allen

ISBN #9780073522593

8th Edition

301 Questions

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9,223 Students Helped

Homework Questions

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

This chapter provides an overview of the historical and scientific advances in controlling microbial growth. It emphasizes the use of both physical methods, such as autoclaving and pasteurization, and chemical approaches to effectively manage microbial populations. Special attention is given to the concept of D values for measuring microbial death kinetics and the significant contributions of pioneers like Pasteur and Buchner. Understanding these methods is essential for ensuring safe practices in medicine, industry, and food production.

Learning Objectives

1

Explain the historical development of methods used to control microbial growth.

2

Describe the mechanisms and applications of physical methods such as autoclaving and pasteurization.

3

Understand the calculations involved in determining microbial death kinetics through D values.

4

Identify the contributions of historical figures like Pasteur and Buchner in shaping modern sterilization and preservation techniques.

5

Analyze environmental and situational factors that influence the selection of appropriate microbial control methods.

Key Concepts

CONCEPT

DEFINITION

Microbial Growth Control

The use of physical, chemical, and environmental methods to inhibit, slow, or eliminate the growth of microorganisms.

Autoclaving

A sterilization method that uses pressurized steam at high temperatures (typically 121°C) to kill all microorganisms.

Pasteurization

A heat treatment process that reduces the number of viable microorganisms in a liquid, thereby slowing spoilage and disease transmission.

D Value

The time required at a certain temperature to reduce the microbial population by 90% (one log reduction); a key measure in microbial death kinetics.

Microbial Metabolism

The set of life-sustaining chemical reactions performed by microorganisms, which has been foundational in understanding sterilization and preservation techniques.

Example Problems

Example 1

If there are $10^{3}$ cells per $\mathrm{mL}$ at the middle of log phase, and the generation time of the cells is 30 minutes, how many cells will there be 2 hours later? a) $2 \times 10^{3}$ b) $4 \times 10^{3}$ c) $8 \times 10^{3}$ d) $1.6 \times 10^{4}$ e) $1 \times 10^{7}$

Example 2

Compared with their growth in the laboratory, bacteria in nature generally grow a) more slowly. b) faster. c) at the same rate.

Example 3

Cells are most sensitive to penicillin during which phase of the growth curve? a) Lag b) Exponential c) Stationary d) Death e) More than one of these.

Example 4

Which best describes the intestinal tract? a) an open system with a chemically defined medium b) an open system with a complex medium c) a closed system with a chemically defined medium d) a closed system with a complex medium

Example 5

E. coli, a facultative anaerobe, is grown for 24 hours on the same type of solid medium, but under two different conditions: one aerobic, the other anaerobic. The size of the colonies would be a) the same under both conditions. b) larger when grown under aerobic conditions. c) larger when grown under anaerobic conditions.
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Step-by-Step Explanations

QUESTION

How do you calculate the time required to achieve a desired reduction in microbial count using the D value?

STEP-BY-STEP ANSWER:

Step 1: Determine the initial microbial count and the desired reduction level (e.g., 99.9% kill represents a 3-log reduction).
Step 2: Identify the D value, which is the time required to achieve a 1-log (90%) reduction under specific conditions.
Step 3: Multiply the D value by the number of log reductions needed (e.g., 3 D values for a 3-log reduction).
Step 4: Calculate the total sterilization or inactivation time using the formula: Total Time = D value × Number of log reductions.
Final Answer: The total time needed is the product of the D value and the required number of log reductions to achieve the desired microbial inactivation.

Calculation of D Value

QUESTION

What are the key steps involved in the autoclaving process to ensure effective sterilization?

STEP-BY-STEP ANSWER:

Step 1: Place the items to be sterilized into the autoclave ensuring proper loading for adequate steam penetration.
Step 2: Set the autoclave to the required temperature (typically around 121°C) and pressure (approximately 15 psi).
Step 3: Maintain the temperature and pressure for the designated sterilization time (commonly 15-20 minutes depending on the load and type of items).
Step 4: Allow the autoclave to cool and depressurize gradually before removing the sterilized items.
Final Answer: Following these steps ensures that the autoclaving process effectively sterilizes the items by eliminating all forms of microbial life.

Autoclaving Process

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Common Mistakes

  • Confusing sterilization processes (which eliminate all microorganisms) with disinfection (which only reduces microbial populations).
  • Assuming that all microbial control methods work equally well in every scenario without considering environmental or situational factors.
  • Overlooking the importance of accurately calculating D values, which can lead to ineffective sterilization if not properly determined.
  • Ignoring historical context, which can lead to a misunderstanding of why certain methods are preferred in modern applications.