8: Microbial Growth
We are all familiar with the slimy layer on a pond surface or that makes rocks slippery. These are examples of biofilms—microorganisms embedded in thin layers of matrix material (Figure \(\PageIndex{1}\)). Biofilms were long considered random assemblages of cells and had little attention from researchers. Recently, progress in visualization and biochemical methods has revealed that biofilms are an organized ecosystem within which many cells, usually of different species of bacteria, fungi, and algae, interact through cell signaling and coordinated responses. The biofilm provides a protected environment in harsh conditions and aids colonization by microorganisms. Biofilms also have clinical importance. They form on medical devices, resist routine cleaning and sterilization, and cause health-acquired infections. Within the body, biofilms form on the teeth as plaque, in the lungs of patients with cystic fibrosis, and on the cardiac tissue of patients with endocarditis. The slime layer helps protect the cells from host immune defenses and antibiotic treatments.
Studying biofilms requires new approaches. Because of the cells’ adhesion properties, many of the methods for culturing and counting cells that are explored in this chapter are not easily applied to biofilms. This is the beginning of a new era of challenges and rewarding insight into the ways that microorganisms grow and thrive in nature.
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- 8.1: How Bacteria Grow
- The bacterial cell cycle involves the formation of new cells through the replication of DNA and partitioning of cellular components into two daughter cells. In prokaryotes, reproduction is always asexual, although extensive genetic recombination in the form of horizontal gene transfer takes place, as will be explored in a different chapter. Most bacteria have a single circular chromosome; however, some exceptions exist.
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- 8.2: Factors that Affect Bacterial Growth
- Bacteria have a minimum, optimum, and maximum temperature for growth and can be divided into 3 groups based on their optimum growth temperature: psychrophils, mesophils, thermophils, or hyperthermophils. Bacteria show variation in their requirements for gaseous oxygen and can be placed in one of the following groups: obligate aerobes, microaerophils, obligate anaerobes, aerotolerant anaerobes, or facultative anaerobes.
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- 8.3: Bacterial Quorum Sensing
- Pathogenicity is the ability of a microbe to cause disease and inflict damage upon its host; virulence is the degree of pathogenicity within a group or species of microbes. The pathogenicity of an organism is determined by its virulence factors. Virulence factors enable that bacterium to colonize the host, resist body defenses, and harm the body. Most of the virulence factors are the products of quorum sensing genes.
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- 8.5: Growth Characteristics
- Provided with the right conditions (food, correct temperature, etc) microbes can grow very quickly. It’s important to have knowledge of their growth, so we can predict or control their growth under particular conditions. While growth for muticelluar organisms is typically measured in terms of the increase in size of a single organism, microbial growth is measured by the increase in population, either by measuring the increase in cell number or the increase in overall mass.
Thumbnail: Heavy rains cause runoff of fertilizers into Lake Erie, triggering extensive algal blooms, which can be observed along the shoreline. Notice the brown unplanted and green planted agricultural land on the shore. (credit: NASA)