12 THE NATURE AND IDENTIFICATION OF BACTERIA 



organisms will form spores only if their environment is 

 nutritionally rich, and spore formation then appears to be 

 part of the normal life-cycle of the organism. Whatever 

 may be the cause of their formation, spores constitute a form 

 in which the organism can survive for long periods under 

 adverse conditions. When the environment again becomes 

 suitable for vegetative existence, the spores germinate to 

 form normal cells capable of multiplication as usual. 



Staining reactions 



A useful test that can be applied to bacteria as an aid in 

 diagnosis is their reaction to the staining technique invented 

 by Christian Gram. The dried organisms (in the form of a 

 smear on a microscope slide) are stained with a dye of the 

 pararosaniline series (see p. 203) such as crystal violet and 

 then treated with iodine solution. The preparation is then 

 washed with alcohol until no more violet dye washes off the 

 slide and finally counterstained with a dye of contrasting 

 colour — usually a red dye such as carbolfuchsin. Under 

 this treatment some organisms retain the violet dye and are 

 said to be " Gram-positive," whilst the violet dye is washed 

 out of others by the alcohol, these are stained red by the 

 counterstain and are said to be " Gram-negative." It has 

 been shown recently that the staining complex in Gram-positive 

 organisms is a nucleoprotein which can be extracted from the 

 cells which then stain Gram-negative. For some reason 

 not yet understood, those organisms which are Gram-positive 

 differ in general — there are individual exceptions — from the 

 Gram-negative organisms in being more exacting nutritionally 

 (see Chap. V), having more restricted chemical activities, 

 and in being more sensitive to the action of chemotherapeutic 

 agents such as penicillin, the sulphonamides, the triphenyl- 

 methane dyes, and the acridine dyes. 



Cultural characteristics 



The nature and composition of the medium in which the 

 organism will or will not grow may aid its identification (see 



