ORDER V. ACTINOMYCETALES BUCHANAN, 1917. 



(Jour. Bact., 2, 1917, 162.) 



Ac.ti.no.my.ce.ta'les. M.L. mas.n. Actinomyces type genus of the order; -ales ending to 

 denote an order; M.L. pl.f.n. Actinomycetales the Actinomyces order. 



Organisms forming elongated cells which have a definite tendency to branch. These hy- 

 phae do not exceed 1.5 microns and are mostly about 1.0 micron or less in diameter. In some 

 species the cells are acid-fast. In the Mycobacteriaceae the mycelium is rudimentary or ab- 

 sent; no spores are formed. The Actinomycetaceae, Streptomycetaceae and Actinoplanaceae 

 usually produce a characteristic branching mycelium and multiply by means of special 

 spores (oidiospores, conidia or sporangiospores) or combinations of these spores. Special 

 spores are formed by fragmentation of the plasma within straight or spiral-shaped, spore- 

 bearing hyphae; the oidiospores are formed by segmentation or by transverse division of 

 hyphae, similar to the formation of oidia among the true fungi; the conidia are produced 

 singly, at the end of simple or branching conidiophores; the sporangiospores are borne in 

 spherical or variously shaped sporangia. A few species in Nocardia are reported to be motile. 

 In Actinoplanes the sporangiospores have polar fiagella and swim; in Streptosporangium 

 the spores are non-motile. The cell structure is like that of the bacteria proper. The cell 

 wall substance is neither chitin nor cellulose (Avery and Blank, Can. Jour. Microbiol., 1, 

 1954, 140). Thus it differs from the cell wall substance of the true fungi, another indication 

 of a closer relationship with the bacteria than with the fungi. Only a few species are patho- 

 genic. The majoritj' are found in soil or less commonly in fresh water. 



Among the recent systems of classification of this order it is sufficient to mention the 

 following: Baldacci (Mycopath., 2, 1939, 84; and ibid., 4, 1947, 60) divided the order Actino- 

 mycetales into two families: (a) Mycobacteriaceae Chester with two subfamilies, Leptotrichioi- 

 deae Baldacci and Proactinomycoideae Baldacci, each with five genera, and (b) Actinomy- 

 cetaceae Buchanan, with two genera, Micromonospora and Actinomyces. Krassilnikov (Ray 

 fungi and related organisms, Izd. Akad. Nauk, Moskau, 1938) divided the order into (a) 

 Actinomycetaceae, with four genera, Actinomyces, Proactinomyces, Mycobacterium and My- 

 cococcus, and (b) Micromonosporaceae, with one genus, Micromonospora. Waksman (Jour. 

 Bact., 89, 1940, 549) divided the order into four families: Mycobacteriaceae, Proactinomyceta- 

 ceae, Actinomycetaceae and Micromonosporaceae . Later, Waksman and Henrici (Jour. Bact., 

 48, 1943, 339) arranged these organisms into three families: Mycobacteriaceae, Actinomyceta- 

 ceae and Streptomycetaceae. This was the arrangement used in the Manual, 6th ed., 1948, 875. 

 Later Couch (Trans. N. Y. Acad. Sci., /&, 1954, 315) added the family of water-inhabit- 

 ing species {Actinoplanaceae) to this grouping. 



Key to the families of order Actinomycetales. 



I. Mycelium rudimentary or absent; no spores formed. 



Family I. Mycobacteriaceae , p. 695. 

 II. A true mycelium produced. 



* The general arrangement of the families in this Order follows that developed by Prof. 

 S. A. Waksman, New Jersey Experiment Station, New Brunswick, New Jersey, and Prof. 

 A. T. Henrici, University of Minnesota, Minneapolis, Minnesota, as supplemented more 

 recently by the work of Prof. J. N. Couch, University of North Carolina, Chapel Hill, 

 North Carolina, May, 1955. 



