CULTURAL REACTIONS 



839 



siderable length, or in groups. Long, unjointed filaments are characteristic of 

 some species, notably of the anthrax bacillus. Irregular forms, consisting mainly 

 of poorly-stained thin bacilli, or of club- or bottle-shaped bacilli, are not uncommon. 

 With a few exceptions, of which B. anthracis is the most important, all the mem- 

 bers are motile by about 4-12 peritrichous flagella. Spores are present in all ; 

 and are formed only in the presence of oxygen ; they vary in shape from spherical 

 to ellipsoidal, and may appear at the equator, subterminally, or at the very end 

 of the bacillus. In some members their diameter does not exceed that of the 

 bacillus, but in others the rod is swollen to resemble a Clostridium. Capsules 

 are met with in only one member, B. anthracis, and then only when it is growing 

 in the animal body, or in media rich in animal protein. The organisms are usually 

 Gram-positive, but considerable variation may be shown ; some are strongly 

 positive, others weakly positive, and a few frankly negative. When stained 

 with various dyes, it is generally possible to distinguish areas of uneven staining ; 

 in large bacilli a number of small particles are seen, quite distinct from spores. 

 Some of these particles appear to consist of fat (see Burdon et al. 1942), others 

 of volutin or glycogen. By a few authors the volutin granules have been regarded 

 as nuclear material scattered diffusely 

 through the cell ; the evidence against 

 this contention has already been given in 

 Chapter 2. None of the vegetative 

 bacilli is acid-fast, though, in the sporing 

 condition, they resist decolorization for a 

 short time with weak acids and alcohol. 



Cultural Reactions. — Growth is free on 

 all the ordinary media. Single colonies 

 on agar are generally large, varying 

 from 2 to several millimetres in dia- 

 meter. Some have a finely granular, 

 mealy appearance, others are mem- 

 branous and thrown into wrinkles. In 

 broth there is a tendency towards the 

 formation of a surface scum, with or 

 without turbidity, or of a heavy floccu- 

 lent or membranous deposit. Gelatin is 



usually liquefied rapidly. In a stroke agar culture the growth is raised and 

 confluent, and generally of membranous consistency, rendering emulsification 

 difficult. Growth is not improved by the addition of blood, serum, or glucose. 

 Variant colonial types have been described for several members of the group. 

 Some members form motile colonies (see B. rotans p. 855). 



Resistance. — In the vegetative condition the bacilli are killed by moist heat 

 at a temperature of 55° C. in 1 hour. The spores vary greatly in resistance ; 

 some, like those of B. anthracis, are destroyed by boiling for about 10 minutes ; 

 others, like those of B. subtilis, may withstand boiling for hours. AU are killed 

 by steam under pressure at 120° C. in 40 minutes. Similarly with disinfectants 

 the resistance varies ; HgClg even in a 1/1000 solution may fail to kill anthrax 

 spores in less than 70 hours (Poppe 1922). Potassium permanganate, on the 

 other hand, in a 4 per cent, solution kills them in 15 minutes, and a 3 per cent, 

 solution of hydrogen peroxide in 1 hour. Generally speaking, the spores are 



Fig. 176. — B. megatherium. 

 From an agar slope culture ( x 1000). 



