SUGGESTIONS FOR USE OF MANUAL 3 



sugars with the formation of organic acids. This again corresponds with our 

 organism. We turn next to f. Produce little or no acid from litmus milk. 

 This does not correspond with the characters we have determined for our 

 organism. We proceed to ff. Produce CO2 and frequently visible gas (CO2 + 

 H2) from glucose. Reduce nitrates, etc. 



Our organism produces visible gas from glucose and reduces nitrates. 

 This indicates that it belongs to Family X. Enterohacteriaceae, p. 443. 



This appears to fit our unknown organism. We now refer to page 443 

 on which the key to the Family Enterohacteriaceae is found. In this key we 

 ascertain the Tribe to which our organism belongs. 1. Ferment glu- 

 cose and lactose with the formation of acid and visible gas. Usually do 

 not liquefy gelatin. Tribe I. Eschericheae. 



This corresponds with the characters exhibited by our organism. We 

 refer to the key for Tribe I. Eschericheae on the same page. 1. Methyl 

 red test positive. Voges-Proskauer test negative. Salts of citric acid may or 

 may not be used as sole source of carbon. Genus I. Escherichia, p. 444. 



This description appears to correspond with that of our unknown or- 

 ganism. We find the key to the species of Genus Escherichia follows the 

 key to the Tribe Eschericheae. On tracing our organism in this key we find 

 that it corresponds to Escherichia coli. A brief description of this or- 

 ganism is found on the same page. 



* In the use of keys for identifying bacteria, the student is confronted 

 with two difficulties, both based primarily on lack of knowledge and ex- 

 perience. The first is insufficient knowledge concerning the morphology, 

 physiology, possible pathogenicity and habitat of the microorganisms that 

 are to be identified. This may be due to careless observations or to poor 

 training in the special techniques that must be used in determining the 

 identity of a given bacterium. 



The second difficulty in the use of a key comes from inexperience in the 

 use of technical terms; that is, the student may not thoroughly understand 

 the meaning of the statement in the key and so cannot follow a route 

 through the key with certainty. For example in the keys used here, the 

 student must know the difference (1) between chains of cells which are 

 composed of dividing cells which do not separate at once, and (2) filaments 

 which are composed of dividing cells which remain more permanently to- 

 gether and are normally flattened against each other on adjacent sides. 

 They may show some differentiation into hold fast cells and reproductive 

 cells (conidia), (3) Both chains of cells and filaments are to be distinguished 

 from the mycelial threads found in Actinomycetaceae. These are unseptate 

 and branching with a true branching. 



* Condensed and paraphrased from Hitchcock's Descriptive Systematic Botanj^, 

 New York, 1935. 



