spores. Aerobic. Gram-positive. The colonies are similar in gross appearance to those of the genus Mycobacterium. Paraffin, phenol and m-cresol are frequently utilized as a source of energy. In their early stages of growth on culture media (liquid or solid), the structure of no- cardias is similar to that of actinomycetes in that they form a typical mycelium; hyphae branch abundantly, the branching being true. The diameters of the hyphae vary between 0.5 and 1 micron, usually 0.7 to 0.8 micron, according to the species. The mycelium is not septate. However, the further development of nocardias differs sharply from that of ac- tinomycetes : the filaments soon form transverse walls and the whole mycelium breaks up into regularly cylindrical short cells, then into coccoid cells. On fresh culture media, the coccoid cells germinate into mycelia. The whole cycle in the development of nocardias con- tinues for 2 to 7 days. Most frequently the coccoid cells are formed on the third to fifth day, but in certain species (e.g., Nocardia rubra) they can be found on the second day. Numerous chlamydospores may be found in older cultures of nocardias. They are formed in the same way as the chlamydospores in true fungi: the plasma inside the filaments of pinger (Beitr. Path. Anat. u. AUg. Path., 9, 1891, 287-328) whose observation has not been corroborated. Nocardia-like motile organisms were later mentioned by Rullman (quoted by von Magnus, Undersgelser over en Gruppe Actinomyceter isolerede fra Mennskets Sva- elg., Thesis, 1936, Univ. Copenhagen), Schurmayer (Cent. f. Bakt., I Abt., Orig., 37, 1900, 49-61, 101-106), Luginger (Montash. f. Prakt. Tierheilk., 15, 1904, 289-336) and Hunte- miiller (Beitr. Path. Anat. u. Allg. Path., 69, 1921, 110-121). Luginger's observation is par- ticularly interesting because it refers to a microaerophilic organism. In no case was the presence of flagella demonstrated, and the statements seem to have attracted little atten- tion. Colien (Jour. Bact., SO, 1935, 301-322) saw a stage of cocci motile by a single flagellum in what appears to have been a non-acid-fast Nocardia, and Topping (Zent. f . Bakt., II Abt., 97, 1937, 289-304) and 0rskov (Zent. f. Bakt., II Abt., 98, 1938, 344-357) found instances of motility among nocardias as well as coryneform bacteria from soil. The forty strains stud- ied by Topping included acid-fast as well as non-acid-fast forms and showed both granular and turbid growth in liquid media. Stained preparations showed the presence of flagella: polar or short and lateral on longer branched cells. One of 0rskov's motile strains was examined by Jensen and was found to agree essentially with Nocardia citrea (Krassilnikov) Waksman and Henrici. It produced a soft, lemon-yellow growth on nutrient agar and a diffuse turbidity in broth. The cells were Gram-positive but not acid-fast (fully decolorized by 5 per cent sulfuric acid in 10 seconds). Nitrate was re- duced to nitrite, starch was hydrolj^zed, gelatin was slowly liquefied and casein was very slowly digested. Direct microscopic examination showed well developed initial mycelia with mere traces of aerial hyphae. The mycelial structure persisted for a considerable time below the agar surface, but after 24 to 40 hours some of the surface hyphae began to divide into rod-shaped cells that were very actively motile ; this was best seen when a drop of wa- ter and a coverslip were placed on top of the agar colonies. In broth cultures the motility was much less obvious. Staining of the motile cells showed one to four (or more) stout fla- gella. Single flagella were often attached to the corner of the cell. Rods with both polar and lateral flagella were sometimes seen, but not branched flagellated filaments as pictured by Topping. Motility may not be common among the nocardias (it was not observed in the numerous strains studied by von Magnus (op. cit., 1936), Krassilnikov (Proaktinomitseti., Bull. Acad. Sci., U.S.S.R., S6r. Biol. No. 1, 1938, 138-182), Erikson (Ann. Rev. Microbiol., 3, 1948, 23-54) or previously by the present writer), but its existence, at least in Nocardia, is indisputable and this really is not surprising in view of the numerous observations on motility in the • closely related coryneform bacteria (Jensen, Ann. Rev. Microbiol., 6, 1952, 77-90). The species in the order Actinomycetales cannot any longer be regarded as constantly non-mo- tile."
