The Microbial World
Some Basic Biological Principles
Robert Hooke coined the term “cell” in 1665, based on his initial microscopic observations of cork. These observations ultimately gave rise to the cell theory, as postulated by Schleiden, Schwann, and Virchow, about two centuries later. The major points of the cell theory are as follows:
1. All organisms are composed of a fundamental unit, the cell.
2. All organisms are unicellular or multicellular.
3. All cells are fundamentally alike regarding their structure and metabolism.
4. Cells only arise from preexisting cells (“life begets life”).
Life begets life is a refutation of the once popular idea of spontaneous generation, a doctrine that proposed life could arise from non-living components (which was disproved in the late 1800s). Viruses and prions are not composed of cells; they are acellular or subcellular infectious agents. Neither of these two agents is considered to be alive by most biologists.
Living things possess several attributes commonly associated with life, as summarized in Table 2.1. The attributes common to life include the following: being composed of one or more cells, requiring energy, being able to reproduce, being able to respond to stimuli, and having the ability to grow.
Most cells obtain energy through a complex series of biochemical reactions termed metabolism. Cells metabolize organic compounds (proteins, fats, and carbohydrates) in food and capture the energy stored in chemical bonds by forming high-energy bonds in adenosine triphosphate (ATP). Most microbes are heterotrophs, which require an organic source of energy. Some microbes and plants, the autotrophs, do not need to extract energy from organic compounds. Photosynthetic autotrophs obtain energy directly from the sun, while chemosynthetic autotrophs obtain their energy from inorganic compounds. Autotrophs manufacture organic compounds, and some photosynthetic autotrophs produce oxygen (O2), thus, heterotrophs depend upon autotrophs for their energy needs and as a source of carbon (Figure 2.1).
Requirement for Oxygen
Microbes have diverse requirements for O2. Aerobes require O2 for metabolism, while anaerobes do not. Many anaerobic microbes are killed by O2. Bacterial facultative anaerobes actually grow better in the presence of O2, but they can grow in its absence. In clinical microbiology, knowledge of the O2 requirements of bacteria is required, as any bacteria suspected of being anaerobes must be transported and cultured under anaerobic conditions (Figure 2.2).
Genetic Information The genetic information of all cells is stored in molecules of deoxyribonucleic acid (DNA). Genes are segments of the DNA molecule. Each type of organism has its genetic characteristics encoded in DNA molecules that confer its species identity. Thus, DNA acts as the hereditary material for cellular organisms (and DNA viruses).
What Makes a Microbe?
Algae and some fungi are macroscopic; hence, “microscopic” is not an absolute characteristic of microbes. Similarly, the term unicellular does not apply to those algae and fungi that are macroscopic and clearly must be multicellular. There are even macroscopic bacteria, like the recently discovered “monster bacteria” Epulopiscium ﬁshelsoni and Thiomargarita namibiensis (Box 2.1). Microbes are sometimes described as “simple” because many consist of only a single cell or are less than a cell (viruses and prions). Consider, however, that this single cell must fulﬁll all the functions of life. Hence, single-celled organisms, and even those multicellular organisms without specialized cells, are simple only in the sense of numbers and not in a physiological sense.
Microbe, or microorganism, is a term of convenience used to describe biological agents that are generally too small to be seen without...
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