Conditions In order to grow bacteria under laboratory conditions it is essential to fulfill two requirements: suitable nutrients must be supplied; the physical conditions must be as near optimum as possible for the organism under consideration. BACTERIAL NUTRITION Nutrition = the ways by which an organism assimilates from the environment the substances necessary for its metabolism. Nutrients = substances for which their solutions can cross the cytoplasmic membrane to be used in the cell’s metabolism. Bacteria feed by absorption. In bacteria, the digestion is extracellular. BACTERIAL NUTRITION The main elements required for growth are carbon, hydrogen, oxygen and nitrogen, with sulfur and phosphorus required in somewhat smaller amounts, and other elements such as sodium, potassium, magnesium, iron and manganese in considerably smaller amounts. Hydrogen and oxygen can be supplied in the water that is essential for any growth. 1. Energy source Fototrophic bacteria use the energy of light. Chemotrophic bacteria use the energy produced by chemical oxidation reactions. These reactions use an electron donor (the energy source) and a electron acceptor. * The electron donor. The chemotrophic bacteria are litotrophs (the electron donor is an inorganic compound) or organotrophs (the electron donor is an organic compound). ** The electron acceptor. ** The electron acceptor The oxygen is used in aerobic respiration. The respiratory chain of electron transport associated to the cytoplasmic membrane is implicated. Organic compounds (the carbon source or its metabolites) are used in fermentative processes. Fermentation results in a mixture of final products, some more reduced, some more oxidized. It is less efficient than aerobic respiration for obtaining the energy. Classification of bacteria Bacteria that use fermentation are called anaerobes; those which grow both in the presence or in the absence of the oxygen are called facultative anaerobes. Microaerophilic bacteria grow only at low concentrations of oxygen. 2. Carbon source Autotrophic bacteria use CO2 as a carbon source and heterotrophic bacteria, an organic compound (carbohydrate or aminoacid), also used as energy source. Heterotrophic bacteria which need for growth CO2 concentrations bigger than those in atmosphere are called carboxyphilic bacteria. 3. Nitrogen source The main inorganic form of nitrogen used in biosynthesis is ammonia, usually in the form of an ammonium salt. A few bacteria can use gaseous nitrogen or organic nitrogen (aminoacids or polypeptides) as a nitrogen source and reduce it to ammonia. 4. Other inorganic salts Microorganisms require a supply of inorganic salt for growth, particularly the anions phosphate and sulfate, and the cations sodium, potassium, magnesium, iron, and calcium. Some ions such as cobalt, zinc, manganese, copper are needed in trace amounts and are supplied by the tap water or impurities from the culture medium or glassware. 5. Growth factors Certain organic compounds such as aminoacids, nucleotides, monosaccharides, lipids and co-enzymes must be either synthesized by the microorganism or provided as nutrients in the environment. Some microorganisms can synthesize by themselves all the aminoacids and they are called prototrophs. Auxotrophs have lost, by mutation, a biosynthetic ability. Therefore these essential nutrients, called growth factors, must be supplied in the growth medium. 5. Growth factors Pathogenic bacteria are heterotrophs dependent to several growth factors. Some bacteria are so dependent to their natural environment that they cannot be grown in vitro (e.g., Treponema pallidum, Mycobacterium leprae). Auxotrophs have selective advantages: in the presence of the necessary growth factor, bacteria can multiply quicker than the parental strain. PHYSICAL CONDITIONS REQUIRED FOR GROWTH 1. Temperature 2. Hydrogen ion concentration (pH) 3. Osmotic pressure 4. Oxygen It is necessary to provide the correct atmosphere for the growth of aerobes and anaerobes. Aerobic and facultative anaerobic bacteria can grow in the presence of oxygen. For anaerobic bacteria, the presence of oxygen is toxic. The tolerance to oxygen is related to the ability of the bacterium to detoxify superoxide and hydrogen peroxide, produced by aerobic respiration. 1. Temperature The optimal temperature for growth of some human pathogens is rather unique and can be a simple way to select those organisms. For example, Campylobacter species grow best at 42C, a temperature that is excessive for the growth of most other human pathogens. 1. Temperature Mesophilic bacteria grow best at temperature ranging from 20C-40C. Most human pathogens are mesophilic. Thermophilic bacteria grow best at 50C-60C. Psychrophilic bacteria grow best at temperatures ranging from 0C-10C. 2. Hydrogen ion concentration (pH) The majority of commensals and pathogenic bacteria grow best at a neutral or very slightly alkaline reaction (pH 7.2-7.6). Some bacteria grow in the presence of a considerable degree of acidity and are termed acidophilic, e.g. Lactobacillus species. Some species are very sensitive to acid, but tolerant of alkali, e.g. Vibrio cholerae. 3. Osmotic pressure Bacteria are relatively tolerant of changes in the osmotic pressure of their environment and can grow in media with widely varying contents of salt, sugar and other solutes. This is partly a reflection of the mechanical strength of their cell walls. Halophilic species can grow at higher concentrations up to saturation; few halophilic species are pathogens (e.g., Vibrio parahaemolyticus). 4. Oxygen It is necessary to provide the correct atmosphere for the growth of aerobes and anaerobes. Aerobic and facultative anaerobic bacteria can grow in the presence of oxygen. For anaerobic bacteria, the presence of oxygen is toxic. The tolerance to oxygen is related to the ability of the bacterium to detoxify superoxide and hydrogen peroxide, produced by aerobic respiration.
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