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• Some grains are susceptible to spoilage. – Aspergillus flavus produces aflatoxins that accumulate in grains, nuts, and corn. • Aflatoxins are linked to liver and colon cancers. – Ergotism occurs when Claviceps purpurea deposit toxins in rye, wheat and barley. • They may induce convulsions and hallucinations. • Milk and dairy products sometimes sour. – Milk is an excellent source of nutrition for humans and microbes. – Milk sours when bacterial enzymes digest fats into fatty acids. – Curdling occurs when bacteria ferment lactose into: • lactic acid, • acetic acid. – Some bacterial species cause curdling by breaking down casein proteins • This is a critical step in cheese production. • Drying and osmotic pressure help preserve foods. – Dry foods cannot support microbial growth. – Lyophilization (freeze-drying) involves deep-freezing then vacuum pumping off water. – Osmotic pressure causes water to diffuse out of cells, causing dehydration and death in highly salted or sugared foods. • Many milk products are the result of fermentation. – Fermentation of lactose by bacteria makes: • buttermilk. • acidophilus milk. • sour cream. – Dry milk solids containing active bacterial cultures are added to boiled milk to form yogurt. – Cheese production begins when casein curdles out of milk. • The curd (unripened cheese) is sold as – cottage cheese. – pot cheese. – cream cheese. 26.1 Water Pollution • Unpolluted and polluted water contain different microbial populations. – Unpolluted water contains low organic nutrients, thus low numbers of microbes. – Water can be polluted with • sewage. • agricultural runoff. • industrial pollutants. – Polluted water is high in • organic matter. • coliform and noncoliform bacteria. • Biofilms are prevalent in the environment. – A biofilm is an immobilized population of microbes tangled together in fibers adhering to a surface. – Microbes in a biofilm work together for • nutrient storage production • predator protection • communicate with each other by quorum sensing – Biofilms are used in bioremediation to degrade toxic wastes. – Biofilms can form in the human body and on medical instruments. FIGURE 08: Biofilm contamination Courtesy of Dr. Rodney M. Donlan and Janice Carr/CDC • The bacteriological analysis of water tests for indicator organisms. – Presence of indicator organisms shows that water has been contaminated by feces. – Coliform bacteria live in mammalian intestines but can survive in water. – The membrane filter technique and standard plate count (SPC) are used to determine numbers of bacteria in a water sample. – The most probable number (MPN) test determines number of bacteria by observing carbon dioxide gas production. • Nitrogen-fixing microbes live symbiotically with legume roots. – They provide the plant and surrounding soil with usable nitrogen. – Streptokinase breaks down blood clots formed during a heart attack. – Hyaluronidase facilitates absorption of fluids injected under the skin. – Lactones produced by species of fungi are used for flavor and aroma enhancement. © Maximilian Stock LTD/Phototake/Alamy Images Figure 01B: A pharmaceutical technician monitors a series of fermentors. 27.3 Other Microbial Products • Many antibiotics are the result of industrial production. – Penicillin was the first antibiotic to be produced on an industrial scale. – Antibiotic production can involve fermentation, producing natural antibiotics or semisynthetic drugs. – Bacillus thuringiensis produce a toxin (Bt-toxin) during sporulation that is deposited on leaves. • If ingested by caterpillars, the toxin lyses the gut wall causing paralysis and bacterial invasion. • It seems to be harmless to plants and humans. • Through biotechnology cotton and corn plants carrying the Bt-toxin gene have been developed that kill only insects that eat them. • Fungal organisms are also being commercially developed. – Yeast cells are grown in fermentors. • They are sold as dry yeast or compressed yeast cakes. – Many species of mushrooms are grown on farms. • High humidity and cool temperatures are required for mushroom growth. FIGURE 08b: A mushroom farm © Mashkov Yuri, Itar-Tass/Landov • Bioremediation helps clean up pollution naturally. – Naturally occurring Pseudomonas species can be stimulated to degrade oil in spills. – Species of Arthrobacter degrade oil and allow plant growth in spill regions. FIGURE 09: The Exxon Valdez oil spill Courtesy of the Exxon Valdez Oil Spill Trust Council/NOAA