Bacteria are members of a diverse and ubiquitous group of prokaryotic, single-celled organisms. They are the only living organisms that use all possible metabolic pathways. Bacteria can be classified based on their shapes: spherical, cylindrical (rods), and helical (spiral). Most bacteria reproduce by binary fission although some reproduce sexually or by budding. Bacteria range in size from 0.5 to 15 p depending on their shape: 0.5-1.0 pfor spherical-shaped species, 1.5-3.0 p for rod-shaped species, and 6-15 p for spiral-shaped species. The interior of a typical bacteria cell—known as the cytoplasm—contains a colloidal suspension of proteins, carbohydrates, and other complex organic compounds. The cytoplasm also houses ribonucleic acid—responsible for protein synthesis—and the nuclear area that contains the DNA—carrying the information necessary for cell reproduction. Bacteria are approximately 80% water and 20% dry material, of which 90% is organic and 10% is inorganic.

Bacteria can be generally classified into two groups, aerobic bacteria and anaerobic bacteria, which is defined later in this section. In the aerobic bacteria class (see Figure 7.22.3), two ecological groups are of concern: the floc-forming microorganisms, which can propagate in an activated-sludge system, and the biofilm-forming microorganisms, which grow attached to surfaces—a feature that is exploited in wastewater treatment processes such as the trickling filter. Apparently, the ability to form bacterial floc is associated with the ability to attach to surfaces, and these two ecological groups overlap to a large extent. Among the well-known names of genera of bacteria that belong to these groups are Pseudomonas, Zooglea, Bacillus, Flavobacterium, and Nocardia.

The anaerobic group (see Figure 7.22.4) includes the fermentative bacteria such as Clostridium, Propionibacter-ium, Streptobacterium, Streptococcus, Lactobacillus, and Enterobacter. Other common genera in the anaerobic group include the sulfate-reducing bacterium, Desulfo-vibrio, and methanogens such as Methanosarcinia and Methanothrix. Anaerobic degradation of organic matter usually requires a complex, interactive community with many different species.

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