Chapter 3 



The Genus Nocardia 



Characterization of Genus 



The genus Nocardia represents a group 

 of aerobic actinomycetes which includes both 

 pathogens and saprophytes. The relation- 

 ship of this genus to, and possible overlap- 

 ping with, the genus Mycobacterium, on the 

 one hand, and the genus Streptomyces, on the 

 other, have already been discussed (Volume 

 I i. Numerous cultures of nocardiae have 

 been isolated from human and animal infec- 

 tions, and claimed to be the causative agents 

 of the particular disease. The fact, however, 

 thai a culture of an organism has been iso- 

 lated from a lesion of a man or an animal is 

 no proof thai it is primarily responsible for 

 the particular disease; it may actually be a 

 secondary invader or a member of a mixed 

 infection. Some specie- of Nocardia are def- 

 initely associated, however, with certain 

 diseases, or have at least been isolated from 

 infected tissues. This gave origin to the term 

 "nocardiosis," descriptive of these disease 

 conditions. 



The colonies produced by nocardiae are 

 cither smooth, or rough and much folded; 

 they are either of a sofl or dough-like con- 

 sistency, or compact and leathery, especially 

 in early stages of growth. Many specie.- of 

 Nocardia do not form any aerial mycelium; 

 Mime give rise to a limited aerial mycelium 

 which may structurally be similar to that of 

 the substrate mycelium; still others may 

 produce aerial hyphae and spores which may 

 be indistinguishable from those of Strepto- 

 myces and are thus responsible for various 



case- of overlapping between these two 

 genera. 



Nocardias multiply by concentration 

 and segmentation of the protoplasm within 

 a filamentous cell, followed by dissolution oi 

 the cell membrane. The fragmented portions 

 of the mycelium usually develop into fresh 

 mycelium under favorable conditions, either 

 by germ tubes or by lateral budding. Strep- 

 tomycetes produce true spores or conidia, the 

 vegetative mycelium not segmenting spon- 

 taneously into bacillary or coccoid forms, but 

 remaining nonseptate and coherent even in 

 old cultures, thus producing the characteris- 

 tic tough textured, leathery growth. In no- 

 cardiae, the aerial mycelium represent- an 

 extension upward of the vegetative myce- 

 lium; it does not exhibit any differentiated 

 protoplasm and is sterile and abort ive. When 

 a streptomycete has lost the capacity of 

 producing aerial mycelium, a form analogous 

 to thai of a nocardia may result, except for 

 the structure of the mycelium and the 

 capacity of the degenerated streptomycete 

 to regain the lost capacity. It is occasionally. 

 therefore, a matter of personal preference 

 whether to place a freshly isolated culture in 

 one genus or another. Some nocardiae are 

 acid-fasl or partially acid-fast, and others 

 are not . 



The mode of branching of the substrate 

 mycelium (see Volume I, Chapter 5), the 

 biochemical properties (proteolytic and sero- 

 logical activities), and chemical nature of the 

 cell walls of nocardiae appear to distinguish 



them from the streptomycetes. Boare and 



