The natural environment is generally much more dynamic than the artificial conditions under which bacteria are grown in the laboratory. In a running stream, nutrients are dilute but continuously replaced and waste materials are washed away. Cells may adhere to rocks and other solid surfaces by means of pili and slime layers, but occasionally they are swept away and find themselves in yet a different environment. A natural environment may be similar in many ways to a continuous culture. While bacteria may remain in a prolonged exponential phase, however, they generally multiply more slowly in a natural environment than in artificially favorable laboratory conditions. ■ pili, p. 65 ■ slime layer, p. 63
When in their natural environment, bacteria frequently synthesize structures, such as slime layers, that they may not produce when growing in the laboratory. This occurs because cells can sense various compounds found in the natural environment
and then respond by synthesizing the structures and enzymes useful for growth and survival in that particular environment. Bacteria can grow in complex communities and exhibit behaviors that do not occur when they grow as pure cultures in a test tube.
In nature, bacteria often grow in close association with many other kinds of organisms. For example, the mouth contains aerobes, facultative anaerobes, and anaerobes. The aerobes and facultative anaerobes consume O2, creating microenvironments in which the anaerobes can multiply. The metabolic wastes of one species may serve as a nutrient or energy source of another. For example, members of the genus Syntrophomonas generate propionate as a product of the breakdown of fatty acids. Syntrophobacter uses this compound as an energy source. To complicate matters, both of these organisms are obligate anaerobes that generate energy through an unusual process, creating hydrogen gas as a by-product. This process, however, will only continue if another bacterium is present to consume that gas, making the chemical reactions involved energetically favorable. Thus, Syntrophomonas and Syntrophobacter typically reside in close association with a methanogen, a chemoautotroph that uses hydrogen gas as an energy source and generates methane gas. Understandably, conditions in these close associations are exceedingly difficult to reproduce in the laboratory, which is one reason so few environmental organisms have been isolated in pure culture. ■ fatty acid, p. 34 ■ methanogen, p. 271
Bacteria may live suspended in an aqueous environment, but many attach to surfaces and live in a polysaccharide-encased community called a biofilm (figure 4.20). Biofilms cause the slippery nature of rocks in a stream bed, the slimy "gunk" that coats kitchen drains, and the scum that gradually accumulates in toilet bowls. Biofilm formation begins when a bacterium adheres to a surface, where it multiplies and synthesizes a loose glycocalyx to which unrelated cells may attach and grow. Cells may move within the growing biofilm by twitching motility. ■ glycocalyx, p. 63 ■ pili, p. 65 ■ twitching motility, p. 65
Surprisingly, biofilms are not generally a haphazard mixture of microbes in a layer of slime; rather they have characteristic architectures with open channels through which nutrients and waste materials can pass. Cells communicate with one another by synthesizing and responding to chemical signals, which appears to be important in establishing structure.
Biofilms are more than just an unsightly annoyance. Plaque on teeth, which leads to tooth decay and gum disease, is due to bacteria encased in biofilms. Troublesome persistent ear infections that resist antibiotic treatment and the complications of cystic fibrosis are thought to be due to bacteria growing as a biofilm. In fact, it is estimated that 65% of human bacterial infections involve biofilms. Biofilms are also important in industry, where their growth in pipes, drains, and cooling water towers can interfere with various processes and damage equipment. Biofilms are particularly troublesome because they protect organisms against harmful chemicals such as disinfectants. Bacteria encased in a biofilm may be hundreds of times more resistant to these compounds. ■ disinfectants, p. 110
While biofilms can be damaging, they can also be beneficial. Many bioremediation efforts, which use bacteria to degrade harmful chemicals, are enhanced by organisms present in biofilms. Thus, as some industries are exploring ways to destroy biofilms, others, such as sewage treatment facilities, are looking for ways to foster their development. ■ bioremediation, p. 797
Was this article helpful?