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Microbial Nutrition and Growth

Microbiologists use the term growth to indicate an increase in a population of microbes rather than an increase in size. Microbial growth depends on the metabolism of nutrients, and results in the formation of a discrete colony, an aggregation of cells arising from a single parent cell. A nutrient is any chemical required for growth of microbial populations. The most important of these are compounds containing carbon, oxygen, nilTngen, and/or hydrogen. Nutrients: Chemical and Energy Requirements All cells require three things to conduct metabolism: a carbon source, a source of energy, and a source of electrons or hydrogen atoms. Sources of Carbon, Energy, and Electrons Organisms can be categorized into one of four groups based on their source of carbon and their use of either chemicals or light as a source of energy:• Photoautotrophs use carbon dioxide as a carbon source and light energy from the environment to make their own food. Chcmoautotrophs use carbon dioxide as a carbon source but catabolize organic molecules for energy. Photoheterotrophs are photosynthetic organisms that acquire energy from light and acquire nutrients via catabolism of organic compounds. • Chemoheterotrophs use organic compounds for both energy and carbon.In addition, organotrophs acquire electrons from organic sources, whereasIithotrophs acquire electrons from inorganic sources.Oxygen Requirements Obligate aerobes require oxygen as the final electron acceptor of the electron transport chain, whereas obligate anaerobes cannot tolerate oxygen and use an electron acceptor other than oxygen. Toxic forms of oxygen are highly reactive and cause a chain of vigorous oxidation. Four forms of oxygen are toxic: • Singlet oxygen (l02) is molecular oxygen with electrons that have been boosted to a higher energy state, typically during aerobic metabolism.