Microbial Growth (Part 2) Lecture Notes PDF

Microbial Growth (Part 2) B. Chemical Requirements o Carbon Carbon is the structural backbone of living matter. It is needed for all the organic compounds that make up a living cell. Half the dry weight of a typical bacterial cell is carbon. Chemoheterotrophs (microbes that use organic chemic...

Microbial Growth (Part 2) B. Chemical Requirements o Carbon Carbon is the structural backbone of living matter. It is needed for all the organic compounds that make up a living cell. Half the dry weight of a typical bacterial cell is carbon. Chemoheterotrophs (microbes that use organic chemicals as sources of energy and organic compounds as the main source of carbon) get most of their carbon from the source of their energy—organic materials such as proteins, carbohydrates, and lipids. Chemoautotrophs and photoautotrophs derive their carbon from carbon dioxide. o Nitrogen, Sulfur, and Phosphorus Microorganisms need other elements to synthesize cellular material. For example: protein synthesis requires N as well as some S. synthesis of DNA and RNA require N and P. Nitrogen makes up about 14% of the dry weight of a bacterial cell, and sulfur & phosphorus together constitute about another 4%. Sources of N  Many bacteria obtain N by decomposing protein- containing materials and other nitrogen-containing compounds.  Other bacteria use nitrogen from ammonium ions (NH4+), which are already in the reduced form.  Other bacteria are able to derive nitrogen from nitrates {compounds that dissociate to give the nitrate ion (NO3-) in solution}.  Some bacteria (nitrogen-fixing bacteria) use gaseous nitrogen directly from the atmosphere. This process is called nitrogen fixation. Two types: a. One group is free living, found mostly in the soil. Example: species of Azotobacter, Bacillus, Clostridium, and Klebsiella.  b. Other group live cooperatively in symbiosis with the roots of legumes such as soybeans, alfalfa, beans, and peas. Example: include Rhizobium, Bradyrhizobium, Sinorhizobium, Mesorhizobium, and Azorhizobium Sources of S Important natural sources of sulfur include the sulfate ion (SO42-), hydrogen sulfide, and the sulfur-containing amino acids. Sources of P A source of P is the phosphate ion (PO43-). Potassium, magnesium, and calcium are also elements that microorganisms require, often as cofactors for enzymes. o Trace Elements Microbes require very small amounts of other mineral elements, such as iron, copper, molybdenum, and zinc; these are referred to as trace elements. Most are essential for functions of enzymes, usually as cofactors. Although these elements are sometimes added to a laboratory medium, they are usually assumed to be naturally present in tap water and other components of media. o Oxygen Microbes that use molecular oxygen (aerobes) extract more energy from nutrients than microbes that do not use oxygen (anaerobes). Organisms that require oxygen to live are called obligate aerobes. Obligate aerobes are at a disadvantage because oxygen is poorly soluble in the water of their environment. Therefore, many of the aerobic bacteria have developed the ability to continue growing in the absence of oxygen. Such organisms are called facultative anaerobes. Facultative anaerobes can use oxygen when it is present but are able to continue growth by using fermentation or anaerobic respiration when oxygen is not available. However, their efficiency in producing energy decreases in the absence of oxygen. An example of facultative anaerobes is the familiar Escherichia coli that are found in the human intestinal tract. Obligate anaerobes are bacteria that are unable to use molecular oxygen for energy-yielding reactions. The genus Clostridium (causing tetanus and botulism) is the most familiar example. In fact, most are harmed by the presence of oxygen. There are several toxic forms of oxygen: 1. Singlet oxygen (1O2-) is normal molecular oxygen (O2) that has been boosted into a higher-energy state and is extremely reactive. 2. Superoxide radicals (O2-), are formed during the normal respiration of organisms that use O2 as a final electron acceptor, forming water. In the presence of oxygen, obligate anaerobes appear to form superoxide radicals, which are toxic to cellular components. All organisms attempting to grow in atmospheric oxygen must produce an enzyme, superoxide dismutase (SOD), to neutralize them. Their toxicity is caused by their great instability, which leads them to steal an electron from a neighboring molecule, which in turn becomes a radical and steals an electron, and so on. 3. The hydrogen peroxide produced in this reaction contains the peroxide anion (O22-) and is also toxic. It is the active component in the antimicrobial agents hydrogen peroxide and benzoyl peroxide. The most familiar enzyme catalase converts hydrogen peroxide into water and oxygen: The other enzyme that breaks down hydrogen peroxide is peroxidase, which differs from catalase in that its reaction does not produce oxygen: 4. The hydroxyl radical (OH·) is another intermediate form of oxygen and probably the most reactive. It is formed in the cellular cytoplasm by ionizing radiation. Most aerobic respiration produces traces of hydroxyl radicals, but they are transient. Obligate anaerobes usually produce neither superoxide dismutase nor catalase. Because aerobic conditions probably lead to an accumulation of superoxide radicals in their cytoplasm, obligate anaerobes are extremely sensitive to oxygen. The End