SEPARATION OF STRAINS 21 



Salmonella species are food-poisoning organisms which produce 

 toxins when growing in protein- containing media such as meat 

 products, egg powders, etc. 



Shigella species cause dysentery if they become established in the 

 intestinal flora. 



Eberthella typhosa is the causal organism of typhoid, 



Fanuly XIII 



Bacillus subtilis is a common air and soil inhabitant. Organisms 

 of this and related species are now proving fruitful sources of new 

 antibiotics. 



Bacillus anthbacis is the causal organism of anthrax. 



Clostridium tetani is the causal organism of tetanus. 



Clostridium Botulinum is the food-poisoning organism causing 

 botulism. Its exotoxin is the most toxic substance known. 



Clostridium welchii is an intestinal organism which, if it becomes 

 established in a wound, produces a number of toxins and gives rise 

 to the condition known as gas gangrene. 



Clostridium acetobutylicum is non-pathogenic and is used com- 

 mercially for the production of acetone and butyl alcohol. 



STRAINS 



Using the chemical, morphological, and cultural tests 

 described above it is possible to divide organisms into genera 

 and species, but we cannot identify any particular strain of 

 organism with certainty. For example, any organism having 

 the characteristics in Table II (plus such others as are com- 

 monly used for finer differentiation) of, say, Esch. coli will be 

 called ''Esch. coli'' But organisms conforming to the Esch. 

 coli test form the bulk of the flora of faecal matter and 

 we can isolate millions of ''Esch. coli " from a particle of 

 faeces. Although these organisms all give the characteristic 

 tests it does not follow that they are all identical or have 

 sprung from one common stock. When we come to examine 

 enzyme systems other than those involved in the systematic 

 tests, or if we examine the rates at which the various sugars, 

 etc., are attacked, we shall find wide differences between the 

 various organisms that have been isolated and called "Esch. 

 coli " and we say that these different organisms are different 

 " strains " or " variants " of Esch. coli. Strains are often 



