134 Microbes and You 



genes culture is V.-P. positive and Escherichia coli is V.-P. nega- 

 tive. 



Citrate Test 



In the chapter on media making various substances were classi- 

 fied into natural, derived, and synthetic media, and it was pointed 

 out that a synthetic medium is one of which we know the exact 

 composition. Koser developed a synthetic mixture in which the 

 sole source of carbon is sodium citrate. 



All living cells require carbon for growth and metabolism, but 

 unless the carbon is in an available form, the organisms cannot 

 utilize this element. Escherichia coli is incapable of growing in 

 Koser's citrate medium while Aerohacter aerogenes flourishes with 

 sodium citrate as its sole source of carbon. An organism which 

 grows is said to be citrate positive in contrast to the citrate nega- 

 tive species which are incapable of growing in this synthetic 

 medium. 



There is at least one precaution that should be stressed relative 

 to Koser's medium. It should be inoculated very lightly with the 

 test organisms. If too many cells are transferred to the broth, 

 nutrients may be carried over and false interpretations may result. 

 Bacteria might also cannibalize each other for their carbon. It is 

 recommended that before transferring the bacteria to citrate broth, 

 a light suspension of the organisms should be prepared in sterile 

 distilled water, and a small loopful of the water blank suspension 

 should be used as the inoculum for the citrate broth. 



These last four tests are the accepted reactions for separating 

 the closely related coli and aerogenes species, and for convenience 

 we refer to these tests as the IMViC Reactions tabulated as follows: 



The small letter i in the word IMViC is inserted purely to make 

 the word more pronounceable; it does not refer to any particular 



