2 ACTINOMYCETES 



is sterile and abortive; on the other hand, when a streptomyces has lost 

 the capacity of producing aerial mycelium, a form analogous to that of a 

 nocardia may result, except for the structure of the mycelium and the 

 capacity of the degenerated streptomyces to regain the lost capacity. 4. 

 Another difference between the nocardias and the streptomycetes is the 

 acid-fastness or partial acid-fastness of some of the former when grown in 

 certain media. 



The differentiation between species of Streptomyces, and those of Micro- 

 monospora is not very difficult, since both formation of aerial mycelium and 

 sporulation are markedly distinct for both genera. Some difficulty may be 

 experienced in differentiating species of these two genera from those of Ther- 

 moactinomyces, since the latter also produce aerial mycelium, similar to 

 species of Streptomyces, and they produce single spores, similar to Micro- 

 monospora, but such spores are produced in the aerial mycelium rather 

 than in the vegetative mycelium as in the case of the Micromonospora. 



Classification of Actinomycetes According to Bergey 



According to the latest edition of Bergey's Manual, the order ACTINO- 

 MYCETALES is divided into three families, Mycobacteriaceae, Actino- 

 mycetaceae, and Streptomycetaceae. The first family comprises the genus 

 Mycobacterium, which is beyond the scope of this treatise. The second and 

 third families comprise the true actinomycetes, with which this treatise is 

 primarily concerned. The genera Nocardia and Actinomyces fall into the 

 second family, and the genera Streptomyces and Micromonospora belong to 

 the third. Another genus has recently been added, Thermoactinomyces, 

 comprising largely certain thermophilic forms. 



Of the various species belonging to the five genera of true actinomycetes, 

 those of the genus Actinomyces have not, so far, been found capable of pro- 

 ducing antibiotics. Among the species belonging to the other four genera, 

 those of the genus Streptomyces are by far the most important; hence a 

 detailed consideration of the nature of this genus is justified. 



The Genus Streptomyces 



Species of Streptomyces produce a well-developed mycelium. The diam- 

 eter of the hyphae seldom exceeds 1.0 p and is usually only 0.7-0.8 /z. The 

 hyphae vary greatly in length. Some are long with limited branching; 

 others are short and much branched. The vegetative mycelium does not 

 form cross walls; it does not break up into rod-shaped and coccus-like 

 bodies. The streptomycetes reproduce by means of conidia or by bits of 

 mycelium. The spores or conidia are formed in special spore-bearing hyphae 

 or sporophores which arise from the aerial mycelium, either monopodially 

 or in the form of tufts or whorls. The sporulating hyphae are straight or 



