Classification of Bacteria

Bacteria are now classified into two major groups, the Eubacteria and the Archaebacteria (which were formerly grouped under the Protista). The majority of bacteria involved with food systems are Eubacteria. The Archaebacteria presently house the unique halobacteria, which are obligate halophiles and can cause the red tainting of salted fish. All bacteria fall into two convenient groups, those that stain purple with the Gram stain (Gram-positive) and those that stain red with the Gram stain (Gram-negative). In Gram-positive bacteria, there is a bi-lipid membrane between the cell wall and cytoplasm, with the cell wall consisting mainly of peptidoglycan linked with teichoic acids. The cell envelope of Gram-negative bacteria is more complex, consisting of three layers, often referred to collectively as the sacculus. The innermost layer (i.e., the inner cytoplasmic membrane) is adhered to the peptidoglycan layer (otherwise referred to as the murein sacculus (7)), which is covalently linked to elongated lipoprotein molecules. This peptidoglycan-lipoprotein complex partly occupies the periplasmic space between the two (inner and outer) hydrophobic membranes. The outermost layer is an outer membrane consisting of phospholipids, lipopolysaccharides (LPS), and proteins. The LPS content of the sacculus of Gram-negative bacteria constitutes an impermeable barrier to many polar and nonpolar molecules, including dyes and surface-active agents such as bile salts. This difference in permeability to dyes and surface-active agents is used in the selective isolation of Gram-negative organisms with the complete exclusion of Gram-positive organisms, as with the use of MacConkey agar.

There are three general metabolic groups of bacteria: (1) obligate aerobes, (2) facultative anaerobes, and (3) obligate anaerobes. Representative members of each of these groups are found among both the Gram-positive and Gram-negative bacteria.

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