• 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 thehuman body and onmedical instruments.

Courtesy of Dr. Rodney M. Donlan and Janice Carr/CDC

FIGURE 08: Biofilm contamination

• 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.

Figure 01B: A pharmaceutical technician monitors a series of

fermentors.

© Maximilian Stock LTD/Phototake/Alamy Images

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 beendeveloped 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.

Courtesy of the Exxon Valdez Oil Spill Trust Council/NOAA

FIGURE 09: The Exxon Valdez oil spill