The Genus Arthrobacter
Conn (1928) described a group of bacteria, extremely numerous in certain soils, which were unusual in that they appeared as Gram-negative rods in young cultures and as Gram-positive cocci in older cultures. For these bacteria, Conn (1928) created the spec
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The Genus Arthrobacter DOROTHY JONES AND RONALD M. KEDDIE
Conn (1928) described a group of bacteria, extremely numerous in certain soils, which were unusual in that they appeared as Gram-negative rods in young cultures and as Grampositive cocci in older cultures. For these bacteria, Conn (1928) created the species Bacterium globiforme, which, as Arthrobacter globiformis, was later to become the type species of the genus Arthrobacter. The abundance in soil of bacteria similar to Conn’s organism, and of other coryneform bacteria, was confirmed later by Jensen (1933, 1934) and Topping (1937, 1938), who, however, referred to them as soil corynebacteria, and by Taylor and Lochhead (1937), who used the name Bacterium globiforme. Jensen (1934) considered that these soil bacteria should be classified in the genus Corynebacterium because of their morphological resemblance to corynebacteria of animal origin. However, Conn (1947) vigorously opposed this view and created the genus Arthrobacter (by reviving an old name), with A. globiformis as the type species and with two of Jensen’s soil corynebacteria as additional species (Conn and Dimmick, 1947). In addition to their characteristic morphology and staining reactions, members of the genus Arthrobacter were originally described as being highly aerobic, nutritionally nonexacting, and capable of liquefying gelatin slowly (Conn and Dimmick, 1947). These features were chosen mainly to distinguish Arthrobacter from Corynebacterium as represented by C. diphtheriae and similar animal parasitic species. However, because of its poor circumscription (see Gibson, 1953; Jensen, 1952), the genus Arthrobacter was not widely accepted until it was included as a member of the family Corynebacteriaceae in the seventh edition of Bergey’s Manual of Determinative Bacteriology (Breed et al., 1957). But by that time, the genus had been extended to include the two nutritionally exacting species A. terregens (Lochhead and Burton, 1953) and A. citreus (Sacks, 1954), and shortly afterwards two others were added (Lochhead, 1958a). Indeed, one of Conn’s strains of A. globiformis was shown subsequently to require biotin for growth (Chan and Stevenson, 1962; Morris, 1960). Thus the concept had developed of Arthrobacter as a genus of soil bacteria whose major distinguishing feature was a growth cycle in which the irregular rods in young cultures were replaced by coccoid forms in older cultures; these coccoid forms, when transferred to fresh medium, produced outgrowths (“germinated”) to give irregular rods again, and so the cycle was repeated (Fig. 1). This dependence on morphological features and habitat in the circumscription led to a great deal of confusion in the classification of the genus Arthrobacter and thus created considerable problems in the identification of new isolates as arthrobacters. Thus, isolates from soil and, more especially,
This chapter was taken unchanged from the second edition.
those from other habitats have frequently been referred to in the
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