The Genus Zoogloea
Bacteria of the genus Zoogloea have historically been considered members of the family Pseudomonadaceae but have been differentiated from other obligately aerobic, Gram-negative, nonsporeforming, rod-shaped bacteria that grow in aquatic habitats on the ba
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The Genus Zoogloea PATRICK R. DUGAN, DAPHNE L. STONER AND HARVEY M. PICKRUM
Bacteria of the genus Zoogloea have historically been considered members of the family Pseudomonadaceae but have been differentiated from other obligately aerobic, Gramnegative, nonsporeforming, rod-shaped bacteria that grow in aquatic habitats on the basis of their production of a characteristic gelatinous matrix. The “zoogloeal matrix” surrounds clumps of cells found in natural aquatic habitats or when grown in unshaken liquid culture in the laboratory (Butterfield, 1935; Butterfield et al., 1937; Wattie, 1943). The name Zoogloea, which was derived from the Greek word meaning animal glue, refers to the primary trait, the zoogloeal matrix, that is used to distinguish Zoogloea from other metabolically similar bacteria. The two species of Zoogloea, Z. ramigera and Z. filipendula, have been isolated from sewage or from sewage-treatment systems that employ oxidative methods, i.e., activated sludge systems, trickling filters, or oxidation ponds (Butterfield et al., 1937; Butterfield and Wattie, 1941; McKinney and Horwood, 1952; McKinney and Weichlein, 1953). Both species produce branched, finger-like projections or outgrowths from the floc (i.e., clumps of cells that grow in an aggregated form that settles or “flocs” from aqueous suspension) when grown under quiescent culture conditions but have been differentiated from each other on the basis of cell size, biochemical reactions, and appearance of the zoogloeal matrix.
Characteristics of the Zoogloeal Matrix The zoogloeal matrix is a capsular envelopment that surrounds several cells and commonly results in a flocculent growth habit in liquid media. That is, cell flocs settle from suspension, leaving a relatively clear supernatant. Unz and coworkers (Unz, 1974; Farrah and Unz, 1975; Unz and Farrah, 1976a) have convincingly demonstrated that floc formers of the Zoogloea type grow in an aggregated state and that the individual cells are entrapped within the zoogloeal matrix while cell division occurs. Floc formation of the zoogloeal type implies cell growth in an aggregated form (Friedman and Dugan, 1968b), whereas the term “flocculation” (Peter and Wuhrmann, 1971) has been defined as the aggregation of suspended bacterial cells after growth of cells has occurred, i.e., the “flocculaThis chapter was taken unchanged from the second edition.
tion” of colloidal material. Biologically, the two processes are considerably different although the same physical and chemical laws apply to both. Although there can be confusion relative to the distinction between zoogloea formation and flocculent growth, there is a consensus that microbial floc formation results from the presence of adherent extracellular fibrils (Busch and Stumm, 1968; Deinema and Zevenhuizen, 1971; Finstein, 1967; Friedman and Dugan, 1968b; Friedman et al., 1968, 1969; Tago and Aida, 1977). The presence of polysaccharide and the ability to flocculate have been demonstrated with mutants lacking polysaccharide and which do
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