Organic compounds (sugars, amino acids, etc.)
amounts of these trace elements are found in most natural environments, including water.
Some bacteria cannot synthesize some of their cell constituents, such as amino acids, vitamins, purines, and pyrimidines, from the major elements. Consequently, these organisms can only grow in environments where such compounds are available. Those low molecular weight compounds that must be provided to a particular bacterium are called growth factors. ■ purines, p. 31 ■ pyrimidines, p. 32
Microorganisms display a wide spectrum in their growth factor requirements, reflecting differences in their biosynthetic capabilities. The fewer enzymes an organism has for the biosynthesis of small molecules such as amino acids, the more growth factors must be provided. For example, E. coli is quite versatile and does not require any growth factors. It grows in a medium containing only glucose and six different inorganic salts. In contrast, species of Neisseria require at least 40 additional ingredients, including 7 vitamins and all of the 20 amino acids. Bacteria such as Neisseria that require many growth factors are called fastidious.
Fastidious bacteria are exploited to determine the quantity of specific vitamins in food products. To do this, a well-characterized species of Lactobacillus is grown in a medium that lacks a specific vitamin, but has been supplemented with a measured amount of the food product. The amount of growth of the bacterium is related to the amount of test vitamin in the product.
Organisms derive energy either from sunlight or by metabolizing chemical compounds. These processes will be discussed in chapter 6. Organisms that harvest the energy of sunlight are called phototrophs (photo means "light"). These include plants, algae, and photosynthetic bacteria. Organisms that obtain energy by metabolizing chemical compounds are called chemotrophs (chemo means "chemical"). Mammalian cells, fungi, and many types of bacteria metabolize organic compounds such as sugars, amino acids, and fatty acids. Some prokaryotes can extract energy from seemingly unlikely sources such as hydrogen sulfide, hydrogen gas, and other inorganic compounds, an ability that distinguishes them from eukaryotes. Microbiologists often group prokaryotes according to the energy and carbon sources they utilize (table 4.4).
Prokaryotes can thrive in virtually every conceivable environmental habitat, because they are able to use diverse sources of carbon and energy.
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