The importance of bacteria to humans
- Singular:
- bacterium
News •
Bacteria in food
Milk from a healthy cow initially contains very few bacteria, which primarily come from the skin of the cow and the procedures for handling the milk. Milk is an excellent growth medium for numerous bacteria, and the bacteria can increase rapidly in numbers unless the milk is properly processed. Bacterial growth can spoil the milk or even pose a serious health hazard if pathogenic bacteria are present. Diseases that can be transmitted from an infected cow include tuberculosis (Mycobacterium tuberculosis), undulant fever (Brucella abortus), and Q fever (Coxiella burnetii). In addition, typhoid fever (Salmonella typhi) can be transmitted through milk from an infected milk handler. Pasteurization procedures increase the temperature of the milk to 63 °C (145 °F) for 30 minutes or to 71 °C (160 °F) for 15 seconds, which kills any of the pathogenic bacteria that might be present, although these procedures do not kill all microorganisms.
Certain bacteria convert milk into useful dairy products, such as buttermilk, yogurt, and cheese. Commercially cultured buttermilk is prepared from milk inoculated with a starter culture of Lactococcus (usually L. lactis or L. lactis cremoris). Yogurt and other fermented milk products are produced in a similar manner using different cultures of bacteria. Many cheeses are likewise made through the action of bacteria. Growth in milk of an acid-producing bacterium such as L. lactis causes the casein to precipitate as curd. Following the removal of moisture and the addition of salt, the curd is allowed to ripen through the action of other microorganisms. Different bacteria impart different flavors and characteristics to foods; for example, the mixture of Lactobacillus casei, Streptococcus thermophilus, and Propionibacterium shermanii is responsible for the ripening of Swiss cheese and the production of its characteristic taste and large gas bubbles. In addition, Brevibacterium linens is responsible for the flavor of Limburger cheese, and molds (Penicillium species) are used in the manufacture of Roquefort and Camembert cheeses. Other types of bacteria have long been used in the preparation and preservation of various foods produced through bacterial fermentation, including pickled products, sauerkraut, and olives.
The toxins of many pathogenic bacteria that are transmitted in foods can cause food poisoning when ingested. These include a toxin produced by Staphylococcus aureus, which causes a rapid, severe, but limited gastrointestinal distress, or the toxin of Clostridium botulinum, which is often lethal. Production of botulism toxin can occur in canned nonacidic foods that have been incompletely cooked before sealing. C. botulinum forms heat-resistant spores that can germinate into vegetative bacterial cells that thrive in the anaerobic environment, which is conducive to the production of their extremely potent toxin. Other food-borne infections are actually transmitted from an infected food handler, including typhoid fever, salmonellosis (Salmonella species), and shigellosis (Shigella dysenteriae).
| substance | action |
|---|---|
| hyaluronidase | increases permeability of tissue spaces to bacterial cells |
| coagulase | increases resistance of bacteria to phagocytosis (engulfment by defense cells, or phagocytes) |
| hemolysins | destroy red blood cells |
| collagenase | dissolves collagen, a connective tissue protein |
| leukocidin | kills white blood cells (specifically leukocytes) and hence decreases phagocytic action |
| exotoxins and endotoxins | interfere with normal metabolic processes |
Bacteria in industry
Anaerobic sugar fermentation reactions by various bacteria produce different end products. The production of ethanol by yeasts has been exploited by the brewing industry for thousands of years and is used for fuel production. Specific bacteria carry out the oxidation of alcohol to acetic acid in the production of vinegar. Other fermentation processes make even more valuable products. Organic compounds, such as acetone, isopropanol, and butyric acid, are produced in fermentation by various Clostridium species and can been prepared on an industrial scale. Other bacterial products and reactions have been discovered in organisms from extreme environments. There is considerable interest in the enzymes isolated from thermophilic bacteria, in which reactions may be carried out at higher rates owing to the higher temperatures at which they can occur.
| product | bacterium | application or substrate |
|---|---|---|
| amylases | thermophilic Bacillus species | used in brewing to break down amyloses to maltoses |
| cellulases | Clostridium thermocellus | release of sugars from cellulose in waste from agriculture and papermaking |
| proteases (thermolysin, subtilisin, aqualysin) | Thermus aquaticus, Bacillus species | used in brewing, baking, cheese processing, removal of hair from hides in the leather industry, and laundering |
| glucose isomerase | Bacillus coagulans | conversion of glucose to fructose as a sweetener in the food industry |
| beta-galactosidase | Thermus aquaticus | hydrolysis of lactose in milk whey to glucose and galactose |
| cobalamin (cyanocobalamin) | Pseudomonas stutzeri | |
| vinegar | Acetobacter species | from alcohol |
| monosodium glutamate | Micrococcus species | from sugar |
| dextran | Leuconostoc mesenteroides | from sucrose |
Deposits of insoluble ferric iron (iron in the +3 oxidation state) are common in many environments. Bacterial reduction of ferric iron is common in waterlogged soils, bogs, and anaerobic portions of lakes. When the soluble ferrous iron (iron in the +2 oxidation state) thus formed reaches aerobic regions under neutral pH, the ferrous iron spontaneously oxidizes to insoluble, brown ferric deposits. In an acidic environment, iron does not readily oxidize from the ferrous to the ferric state. However, this reaction is tremendously accelerated by the acidophilic lithotrophic bacterium Acidithiobacillus ferrooxidans. When pyritic (ferrous sulfide) deposits are exposed to the air by mining operations, there is slow spontaneous oxidation of pyrite to ferrous ions and sulfuric acid. When the production of sulfuric acid causes conditions to reach a certain level of acidity, A. ferrooxidans thrives and oxidizes ferrous iron to the ferric form, which in turn oxidizes more pyrite in a continuously increasing fashion, with the formation of substantial amounts of sulfuric acid. The acidity of the environment may increase to a level near a pH of 2, which is a better environment for the solubilization of many other metal ions, particularly aluminum. Some of the ferrous iron generated by the bacteria is carried away by groundwater into surrounding streams, making them acidic and loaded with iron, which precipitates and forms deposits of iron some distance downstream from the mine. Acid mine drainage would not develop in the absence of bacterial activity, and the only practical way to prevent its occurrence is to seal or cover the acid-bearing material to prevent exposure to air. (For more information of oxidation and reduction reactions, see oxidation-reduction reaction.)
Although bacterial oxidation of sulfide materials results in the undesirable formation of acid mine drainage, the same reaction has been put to use for microbial leaching of copper, uranium, and other valuable metals from low-grade sulfide-containing ores. These metals are released from the ore after their conversion to more soluble forms by the direct oxidation of the metal by the bacterium and by the indirect oxidation of the metals in the ore by the ferric iron that was formed by bacterial action.
Microbial decomposition of petroleum products by hydrocarbon-oxidizing bacteria and fungi is of considerable ecological importance. The microbial decomposition of petroleum is an aerobic process, which is prevented if the oil settles to the layer of anaerobic sediment at the bottom (natural oil deposits in anaerobic environments are millions of years old). Hydrocarbon-oxidizing bacteria attach to floating oil droplets on the water surface, where their action eventually decomposes the oil to carbon dioxide. It is becoming a common practice to spray such bacteria and their growth factors onto oil spills to enhance the rate of degradation of the nonvolatile aliphatic and aromatic hydrocarbons.
