The Genus Brevibacterium
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The Genus Brevibacterium MATTHEW D. COLLINS
Introduction The genus Brevibacterium was established by Breed (1953), with B. linens as the type species, for a number of Gram-positive, short, unbranching, rod-shaped bacteria formerly classified in the genus Bacterium. Owing to the poor delimitation of the genus, it soon became a repository for a broad range of organisms with very diverse chemical, biochemical and physiological features. Taxonomic studies, in particular those based on cellular chemical analyses, demonstrated the extreme heterogeneity of the genus and resulted in the reclassification of many Brevibacterium species in other coryneform genera (including Arthrobacter, Aureobacterium, Corynebacterium, Curtobacterium, Exiguobacterium, Microbacterium, Nocardioides, Oerskovia and Rhodococcus; see Jones and Keddie, 1985). The description of the genus was subsequently tightened and restricted to the species Brevibacterium linens and B. iodinum (formerly Chromobacterium iodinum; Collins et al., 1980). In addition to B. linens and B. iodinum, several other species have been assigned to the genus in recent years viz., B. casei (Collins et al., 1983), B. epidermidis (Collins et al., 1983), B. mcbrellneri (McBride et al., 1993), B. otitidis (Pascual et al., 1996) and B. avium (Pascual and Collins, 1999). The seven currently described species form a phenotypically coherent group of organisms, and 16S rRNA gene sequencing studies have shown the genus to be phylogenetically distinct from other actinomycete taxa (Pascual et al., 1996). The intrageneric phylogenetic relationships of the seven currently described species of Brevibacterium inferred from 16S rRNA are shown in Fig. 1.
General Characteristics Brevibacteria exhibit a distinct rod-coccus cycle during growth on complex media. Cells from older cultures (3–7 days) are composed of coccoid cells (ca. 0.6–1.0 µm diameter), whereas irregular, slender rods are characteristic of expo-
nential phase cultures. Both rod and coccoid forms are Gram-positive, but some strains and older cultures decolorize readily. Members are considered nonmotile although sparse flagellation has been reported for B. iodinum (Colwell et al., 1969). Brevibacteria are nonfastidious, chemoorganotrophic, obligately aerobic (oxidative or indifferent toward sugars), and catalasepositive. All brevibacteria are proteolytic, with most strains hydrolyzing casein, gelatin and milk. Brevibacteria grow well the presence of 8% NaCl, and many strains also grow in 15% NaCl. The cell-wall peptidoglycan is the directly cross-linked type based on meso-diaminopimelic acid (A2pm; Schleifer and Kandler, 1972). The walls of brevibacteria contain complex teichoic acids consisting of neutral sugars, amino sugars and sugar alcohols (Anderton and Wilkinson, 1980; Fiedler et al., 1981). The long-chain cellular fatty acids are primarily of the anteisoand iso-methyl branched types, with 12methyltetradecanoic and 14-methylhexadecanoic acids predominating (Collins et al., 1980; C
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