1. This work is licensed under a Creative Commons Attribution 4
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2. Microbial Growth
Module Overview

3. Acknowledgments
This presentation is based on and includes content derived from the following OER resource:
Microbiology
An OpenStax book used for this course may be downloaded for free at:

4. Bacterial Cells Grow in Four Sequential Phases
Most bacterial cells divide by binary fission. Generation time in bacterial growth is defined as the doubling time of the population.
Cells in a closed system follow a pattern of growth with four phases:
lag
logarithmic (exponential)
stationary
death
Image: Microbiology. OpenStax. Fig 9.05

5. Patterns of Microbial Cell Division
The most common mechanism of cell replication in bacteria is a process called binary fission, where a microbe will replicate, forming an exact copy of itself.
In some cyanobacteria, many nucleoids may accumulate in an enlarged round cell or along a filament, leading to the generation of many new cells at once. The new cells often split from the parent filament and float away in a process called fragmentation.
Other species may form a long narrow extension at one pole in a process called budding. The tip of the extension swells and forms a smaller cell, the bud that eventually detaches from the parent cell. Budding is most common in yeast.

6. How Are Cells Plated and Counted?
Cells can be counted by direct viable cell count. The pour plate and spread plate methods are used to plate serial dilutions of agar to allow counting of viable cells that give rise to colony-forming units. Membrane filtration is used to count live cells in dilute solutions. The most probable cell number (MPN) method allows estimation of cell numbers in cultures without using solid media.
Indirect methods can be used to estimate culture density by measuring turbidity of a culture or live cell density by measuring metabolic activity.
Other patterns of cell division include multiple nucleoid formation in cells; asymmetric division, as in budding; and the formation of hyphae and terminal spores.

7. Biofilm Formation
Biofilms are communities of microorganisms in a matrix of extracellular polymeric substance. Biofilms form when planktonic cells attach to a substrate and become sessile. Cells in biofilms communicate through quorum sensing.
Biofilms are commonly found on surfaces in nature and in the human body, where they may be beneficial or cause severe infections.
Image: Microbiology. OpenStax. Fig 9.17

8. Microorganisms Vary in Their Oxygen Usage
Obligate aerobes depend on aerobic respiration and use oxygen as a terminal electron acceptor. They cannot grow without oxygen.
Obligate anaerobes cannot grow in the presence of oxygen. They depend on fermentation and anaerobic respiration using a final electron acceptor other than oxygen.
Facultative anaerobes show better growth in the presence of oxygen but will also grow without it.
Aerotolerant anaerobes do not perform aerobic respiration, but grow in the presence of oxygen and test negative for the enzyme catalase.
Microaerophiles need oxygen to grow, albeit at a lower concentration than 21% oxygen in air.

9. Oxygen and CO2 are Important for Growth
Optimum oxygen concentration for an organism is the oxygen level that promotes the fastest growth rate. The minimum permissive oxygen concentration and the maximum permissive oxygen concentration are the lowest and the highest oxygen levels, respectively, that the organism will tolerate.
A capnophile is an organism that requires a higher-than-atmospheric concentration of CO2 to grow.

10. The Main Enzymes Involved in Detoxification
Peroxidase, superoxide dismutase, and catalase are the main enzymes involved in the detoxification of the reactive oxygen species. Superoxide dismutase is usually present in a cell that can tolerate oxygen.
All three enzymes are usually detectable in cells that perform aerobic respiration and produce more ROS.
Image: Microbiology. OpenStax. Fig 9.23

11. The Effects of pH on Microbial Growth
Bacteria are generally neutrophiles, growing best at neutral pH (7.0).
Acidophiles grow optimally at a pH near 3.0.
Alkaliphiles are organisms that grow optimally between a pH of 8 and
Extreme acidophiles/alkaliphiles grow slowly or not at all at neutral pH (7.0).
Microorganisms grow best at their optimum growth pH. Growth occurs slowly or not at all below the minimum growth pH and above the maximum growth pH.

12. The Effects of Temperature on Microbial Growth
Microorganisms thrive at a wide range of temperatures. Extreme hot and cold temperatures both require evolutionary adjustments to macromolecules and biological processes.
Psychrophiles grow best in the temperature range of 0–15C whereas psychrotrophs thrive between 4C and 25C.
Mesophiles grow best at moderate temperatures in the range of 20C to about 45C. Pathogens are usually mesophiles.
Thermophiles and hyperthemophiles are adapted to life at temperatures above 50C.
Adaptations to hot and cold temperatures require changes in the composition of membrane lipids and proteins.

13. Environmental Conditions that Affect Growth
Halophiles require high salt concentration in the medium, whereas halotolerant organisms can grow and multiply in the presence of high salt but do not require it for growth. Halotolerant pathogens are an important source of foodborne illnesses because they contaminate foods preserved in salt.
Photosynthetic bacteria depend on visible light for energy.
Most bacteria, with few exceptions, require high moisture to grow.

14. Different Kinds of Media Used for Growth
Chemically defined media contain only chemically known components.
Selective media favor the growth of some microorganisms while inhibiting others.
Enriched media contain added essential nutrients a specific organism needs to grow.
Differential media help distinguish bacteria by the color of the colonies or the change in the medium.

15. How to Study this Module
Read the syllabus or schedule of assignments regularly.
Understand key terms; look up and define all unfamiliar words and terms.
Take notes on your readings, assigned media, and lectures.
As appropriate, work all questions and/or problems assigned and as many additional questions and/or problems as possible.
Discuss topics with classmates.
Frequently review your notes. Make flow charts and outlines from your notes to help you study for assessments.
Complete all course assessments.

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