Bacteria are grouped in a number of different ways. Most bacteria are of one of three typical shapes—rod-shaped (bacillus), round (coccus, e.g., streptococcus), and spiral (spirillum). An additional group, vibrios, appear as incomplete spirals. The cytoplasm and plasma membrane of most bacterial cells are surrounded by a cell wall; further classification of bacteria is based on cell wall characteristics (see Gram's stain). They can also be characterized by their patterns of growth, such as the chains formed by streptococci. Many bacteria, chiefly the bacillus and spirillum forms, are motile, swimming about by whiplike movements of flagella; other bacteria have rigid rodlike protuberances called pili that serve as tethers.
Some bacteria (those known as aerobic forms) can function metabolically only in the presence of free or atmospheric oxygen; others (anaerobic bacteria) cannot grow in the presence of free oxygen but obtain oxygen from compounds. Facultative anaerobes can grow with or without free oxygen; obligate anaerobes are poisoned by oxygen.
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