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Transcript
Water activity (aw)= Vapor pressure of a substance or solution Vapor pressure of water at the same temperature Raoult’s law: n aw N n where N: is the number of moles of solute and n: the number of moles of solvent (water). 1/21 w/ m aw w / m W / M w: is the weight of solvent , W: the weight of solute, m: the molar mass of solvent and M: the molar mass of solute. What is the water activity of a sucrose solution containing 5 g in 100 ml water? w/ m aw w / m W / M 2/21 100 / 18 0.997 aw 100 / 18 5 / 342 Oxygen Classes of Microorganisms Microorganisms vary in their need for, or tolerance of, oxygen. In fact, microorganisms can be divided into several groups depending on how oxygen affects them: 3/21 Aerobes and obligate (strict) aerobes Are species capable of growth at full oxygen tensions (air is 21% O2) and respire oxygen in their metabolism. Many aerobes can even tolerate elevated concentrations of oxygen (hyperbaric oxygen>21% O2). The process of energy production involves glycolysis, the Krebs’ cycle and the electron transport system for which O2 acts as a terminal electron acceptor. Energy is generated by the complete oxidation of the organic substrate to carbon dioxide and water. When glucose is the substrate, 38 molecules of ATP are 4/21 generated from ADP for each molecule of glucose oxidized. Examples: Pseudomonas fluorescens, mold fungi (penicillium spp.) and some yeasts. Microaerophiles Are aerobes that can use oxygen only when it is present at levels reduced from that in air (less than 21%) (microoxic conditions). This is because of their limited capacity to respire or because they contain some oxygen-sensitive molecule such as an oxygen-labile enzyme. Example: Campylobacter spp. grow at oxygen 5/21 concentrations between 1% - 10% with an optimum at 6%. Oxygen concentrations above 10% are toxic and will kill the microorganisms. Facultative anaerobes or facultative aerobes These microorganisms can grow in the presence of oxygen (oxic) or absence of oxygen (anoxic). When oxygen is absent, organic electron acceptors are used in the process of energy production (fermentation). Less energy is generated compared with aerobic respiration and organic byproducts are released from the 6/21 cell. Example: Enterobacteriaceae (E. coli, Salmonella spp.) and yeast (Saccharomyces cerevisiae) and Staphylococcus aureus. Anaerobes 1- Aerotolerant anaerobes 2- obligate (strict) anaerobes Can tolerate oxygen and grow in its presence even though they cannot use it. Are inhibited or even killed by oxygen. The reason is mainly because they are unable to detoxify some of the products of oxygen metabolism (H2O2) (they lack catalase enzyme needed for H2O2 7/21 detoxification). Occur in only three groups of microorganisms, a wide variety of prokaryotes, a few fungi, and a few protozoa. The best known group of obligatory anaerobic Bacteria belongs to the genus Clostridium, a group of Gram (+) endospore forming rods (responsible for poisoning and spoilage of canned foods). 8/21 Condition Aerobic respiration Anaerobic respiration Fermentation properties 1- Oxygen acts as the terminal (final) electron acceptor (oxidizing agent). 2- Produces large amount of ATP. 1- Using alternative terminal electron acceptor (NO3- by denitrifiers, SO42by sulfur reducers, Fe3+ by metal reducers and CO2 by methanogens. 2- There are specific ETS like fumarate reduction system which converts fumarate to succinate. 3- Produces less amount of ATP 1- Organic substrates act as the 9/21 terminal electron acceptors. 2- Classified by the end products (lactic acid and ethanol fermentation). 3- Can occur in aerobic and in anaerobic conditions. 4- Produces the least amount of ATP. 10/21 11/17