Obligate aerobes obtain energy using respiration exclusively; none can use fermentation as an alternative.
Members of the genus Micrococcus are Gram-positive cocci found in soil and on dust particles, inanimate objects, and skin. Because they are often airborne, they easily contaminate bacteriological media. There, they typically form pigmented colonies, a characteristic that aids in their identification. The colonies of M. luteus, for example, are generally yellow (figure 11.12). Like members of the genus Staphylococcus, which will be discussed later, they tolerate arid conditions and can grow in 7.5% NaCl.
Mycobacterium species are widespread in nature and include harmless saprophytes, which live on dead and decaying matter, as well as organisms that produce disease in humans and domestic animals. Although they have a Gram-positive type of cell wall, these bacteria stain poorly because of a waxy lipid in their cell wall. Special procedures can be used to enhance the penetration of the stain. Once they are stained, they resist destaining, even when acidic decolorizing solutions are used. Because of this, Mycobacterium species are called acid-fast, and the acid-fast staining procedure is an important step in their identification
(see figure 3.15). Nocardia species, a related group of bacteria that commonly reside in the soil, are also acid-fast. ■ acid-fast, p. 47 Mycobacterium species are generally pleomorphic rods; they often occur in chains that sometimes branch, or bunch together to form cordlike groups. Two species are notable for their effect on human health—M. tuberculosis, as the name implies, causes tuberculosis, and M. leprae causes Hansen's disease (leprosy). Mycobacterium species are more resistant to disinfectants than most other vegetative bacteria. In addition, they differ from other bacteria in their susceptibility to antimicrobial drugs.
Pseudomonas species are Gram-negative rods that are motile by polar flagella and often produce pigments (figure 11.13). Although most are strict aerobes, some can grow anaerobically if nitrate is available as a terminal electron acceptor. The fact that they do not ferment and are oxidase positive serve, in the laboratory, as important characteristics to distinguish them from members of the family Enterobacteriaceae, which will be discussed in a later section. ■ oxidase, p. 148
As a group, Pseudomonas species have extremely diverse biochemical capabilities. Some can use more than 80 different substrates, including unusual sugars, amino acids, and compounds containing aromatic rings. Because of this, Pseudomonas species play an important role in the degradation of many synthetic and natural compounds that resist breakdown by most other microorganisms. The ability to carry out some of these degradations is encoded by plasmids.
Pseudomonas species are widespread, typically inhabiting soil and water. While most are harmless, some cause disease in plants and animals. Medically, the most significant species is P. aeruginosa. It is a common opportunistic pathogen, meaning that it primarily infects people who have underlying medical conditions. Unfortunately, it can grow in nutrient-poor environments, such as water used in respirators, and it is resistant to many disinfectants and antimicrobial medications. Because of this, hospitals must be diligent to prevent its spread among patients. Many
Figure 11.13 Pigments of Pseudomonas Species Cultures of different strains Figure 11.12 Micrococcus luteus Colonies of Pseudomonas aeruginosa. Note the different colors of the water-soluble pigments.
282 Chapter 11 The Diversity of Prokaryotic Organisms species that were formerly classified as Pseudomonas have recently been removed and placed in one of several new genera. This revision was prompted in part by ribosomal RNA studies. ■ Pseudomonas aeruginosa, p. 697
Thermus and Deinococcus are related genera that have scientifically and commercially noteworthy characteristics. Thermus species are thermophilic, as their name implies. This trait has proven to be extremely valuable because of their heat-stable enzymes, including DNA polymerase. An integral part of the polymerase chain reaction (PCR) is Taq polymerase, the DNA polymerase of T. aquaticus. Although the organisms have an unusual cell wall, they stain Gram-negative. Deinococcus species are unique in their extraordinary resistance to the damaging effects of gamma radiation. For example, D. radiodurans can survive exposure to a dose several thousand times that lethal to a human being. The dose literally shatters the organism's genome into many fragments, yet enzymes are able to repair the extensive damage. Scientists anticipate that through genetic engineering, Deinococcus species may help clean up the soil and water contaminated by the 10 million cubic yards of radioactive waste that have accumulated in the United States. Although their unusual cell wall has multiple layers, they stain Gram-positive. ■ PCR, pp. 229,239
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