Soil Microbes and the Five-Second Rule PDF

Have you ever dropped a bite of food on the ground and then picked it up and ate it because it had not passed the "five-second rule"? This popular urban legend states that if food is dropped on the ground, it remains safe to eat as long as it is picked up within five seconds. Today's experiment will...

Have you ever dropped a bite of food on the ground and then picked it up and ate it because it had not passed the "five-second rule"? This popular urban legend states that if food is dropped on the ground, it remains safe to eat as long as it is picked up within five seconds. Today's experiment will cause you to think twice before consuming food that has been dropped on the ground. The two major habitats for microbes on the Earth are soils and freshwater, including lakes, ponds, and streams. The most prevalent type of soil in terrestrial environments is known as mineral soil, while organic soil forms from sediment that accumulates in bogs and marshes. It is the initial colonization of exposed surfaces of rocks by microbes, in addition to physical and chemical forces, that begins the formation of mineral soils. As organic matter accumulates, plant growth begins. Interestingly, a distinct zone known as the rhizosphere develops around the plant roots. The rhizosphere contains dense population of microbes growing on the surfaces of soil particles. Although there are many different types of living organisms that inhabit soils, bacteria, fungi, and actinomycetes are among the most numerous. Clearly, then, there must be some reason why these organisms congregate in the rhizosphere. Soil microbes carry out a variety of functions, some of which are absolutely essential for life on Earth. You learned in Exercise 8 that fungi are usually the primary decomposers within their habitat and break down highly complex substrates such as cellulose, starch, and lignins. Recall that this initial degradation occurs outside of the fungal cell. Many of the smaller nutri- ents are then transported into the fungus, but some are also made available for plant roots within the rhizosphere. The actinomycetes, a group of gram-positive, filamentous and/or branching bacteria, are also involved in the decomposition of complex organic substances and are responsible for giving soil its characteristic earthy smell. Although the actinomycetes are better known for their production of antibiotics, these organisms play a vital role in the decomposition of compounds that are less easily degraded. Just imagine if organic materials could not be degraded-our planet would soon become covered in organic trash. Additionally, plants and, in turn, humans, would cease to exist because the nutrients necessary for growth would not be available in a usable form. Another responsibility of the soil microbes involves the conversion of inorganic components from one form to another in a process known as biogeochemical cycling. The carbon, nitro- gen, sulfur, and phosphorus cycles all depend on soil microbes during one or more steps. Perhaps the best known example of this phenomenon is the ability of some bacteria to fix nitrogen. Nitrogen (Ni} is an essential element required by all organisms, including humans, for the synthesis of proteins and nucleic acids. Although nitrogen is the most abundant gas in the atmosphere (approximately 79%), few organisms can use it as a source of nitrogen. Most living organisms utilize nitrogen in the form of ammonia (NH3), and nitrogen fixation is the process by which certain bacteria convert atmospheric nitrogen into ammonia.  Nitrogen fixation can be accomplished by two microbial groups. The first are the free-living nitrogen-fixing bacteria that are found in high concentrations within the rhizosphere. The second and more important type of nitrogen-fixing bacteria are the symbiotic nitrogen-fixing bacteria. As you learned in Exercise 25, these bacteria infect the roots of leguminous plants, such as soybeans, beans, peas, alfalfa, clover, and peanuts, and form root nodules, or growths on the plant roots. Nitrogen is fixed in symbiotic process between the plant and the bacteria. The bacteria contained within the root nodules fix nitrogen for the plant, while the plant harbors the organisms and provides nutrients in the form of plant sap. When the bacteria or their host plant dies or is eaten, the ammonia becomes available to other organisms, including humans. Soil varies greatly with respect to its physical features, including oxygen concentration, water availability, pH, temperature, salt concentration, and nutrient availability. Thus, soil microbes must be capable of growth and metabolism along a gradient of physical conditions. However, extremes of any of the above physical features can influence the microbial populations present. For example, acidic soils will contain a larger number of fungal species because fungi are more tolerant of lower pH values than bacteria. As the usable nutrients and suitable environmental conditions change, so do the populations of microorgansms. Typically, however, the concentration of microorganisms is directly proportional to the  concentration of organic matter present in the soil. Clearly, then, the five second rule is just a myth. Any time food contacts soil, regardless of the length of time, food will become contaminated with microbes. 1 ml 1 ml Soil Sample 10-2 99ml H20 99 ml H20 Dilution: 10-4 Dilution: 1o--t. GYE plus cycloheximide 1 ml 0.1 ml 1 ml 0.1 ml Dilution: 10-2 Dilution: 10-3 Dilution: 10-4 Dilution: 1o-s SDA 0.1 ml 1 ml 0.1 ml 1 ml Dilution: 1o-3 Dilution: 10-4 Dilution: 1o-s Dilution: 1Q--6 TSA 1 ml 0.1 ml 1 ml 0.1 ml Dilution: ,0-4 Dilution: 1o-s Dilution: 1Q--6 Dilution: 1o-7 Figure 26.1. Enumeration of soil microbes.