Microbiology is the study of microorganisms, a large and diverse group of microscopic organisms that exist as single cells or cell clusters; it also includes viruses, which are microscopic but not cellular. Microorganisms have a tremendous impact on all life and the physical and chemical makeup of our planet. They are responsible for cycling the chemical elements essential for life, including carbon, nitrogen, sulfur, hydrogen, and oxygen; more photosynthesis is carried out by microorganisms than by green plants. Furthermore, there are 100 million times as many bacteria in the oceans (13 × 1028) as there are stars in the known universe. The rate of viral infections in the oceans is about 1 × 1023 infections per second, and these infections remove 20–40% of all bacterial cells each day. It has been estimated that 5 × 1030 microbial cells exist on earth; excluding cellulose, these cells constitute about 90% of the biomass of the entire biosphere. Humans also have an intimate relationship with microorganisms; 50–60% of the cells in our bodies are microbes (see Chapter 10). The bacteria present in the average human gut weigh about 1 kg, and a human adult will excrete his or her own weight in fecal bacteria each year. The number of genes contained within this gut flora outnumber that contained within our genome by 150-fold; even in our own genome, 8% of the DNA is derived from remnants of viral genomes.
BIOLOGIC PRINCIPLES ILLUSTRATED BY MICROBIOLOGY
Nowhere is biologic diversity demonstrated more dramatically than by microorganisms, cells, or viruses that are not directly visible to the unaided eye. In form and function, be it biochemical property or genetic mechanism, analysis of microorganisms takes us to the limits of biologic understanding. Thus, the need for originality—one test of the merit of a scientific hypothesis—can be fully met in microbiology. A useful hypothesis should provide a basis for generalization, and microbial diversity provides an arena in which this challenge is ever present.
Prediction, the practical outgrowth of science, is a product created by a blend of technique and theory. Biochemistry, molecular biology, and genetics provide the tools required for analysis of microorganisms. Microbiology, in turn, extends the horizons of these scientific disciplines. A biologist might describe such an exchange as mutualism, that is, one that benefits all contributing parties. Lichens are an example of microbial mutualism. Lichens consist of a fungus and phototropic partner, either an alga (a eukaryote) or a cyanobacterium (a prokaryote) (Figure 1-1). The phototropic component is the primary producer, and the fungus provides the phototroph with an anchor and protection from the elements. In biology, mutualism is called symbiosis, a continuing association of different organisms. If the exchange operates primarily to the benefit of one party, the association is described as parasitism, a relationship in which ...