Differentiation of Certain Genera of Gram Negative Bacteria 505 



Pseudomonas species isolated from marine environment and see 

 how far these fit into tlie four groups just postuhited al)ove. Ap- 

 proximately 60 of these organisms, isolated by various workers at 

 Tony, were classified by them as Pseudomonas sp. (55). Their 

 overall similarities in about 80 tests, for each organism, were 

 calculated, using the formula of Sneath ( 51 ) . The results ( Fig. 1 ) 

 show that at least three well defined groups can be differentiated, 

 although there is some overlapping, and that these correspond 

 generally to Groups I, If, III and IV, the latter two being almost 

 indistinguishable. It might just be mentioned here that of the 

 Pseudomonas sp. recently deposited in one or other of the Na- 

 tional Tvpe Culture Collections now housed at Torry, Pesu- 

 domonas halocrenaea fits into Group I; Pseudomonas fragi into 

 Group II; Pseudomonas aiirefaciens (36) into Group III and Pseu- 

 domonas ruhescens (44) into Group IV. 



Aeromonas and Vibrio groups:- According to our scheme the 

 Aeromonas and Vibrio sp. are differentiated from the Pseti- 

 domonas sp. by acting upon glucose fermentatively. Aeromonas 

 sp. are characterised by the production of acid and gas from 

 glucose (at 20 C), whereas the Vibrio sp. ferment glucose 

 anaerogenically. Moreover, only the latter are sensitive to 2:4 

 diamino - 6:7 di-isopropyl pteridine (0/129) (45). 



The validity of the genus Aeromonas has recently been ques- 

 tioned by Stevenson (54) who suggested that all its members 

 could be regarded as non-chromogenic species of the genus 

 Serratia. Several authors, Stevenson points out, have stated that 

 flagellation in the genera Aeromonas and Serratia may vary ac- 

 cording to conditions of culture, and he believed that his Aero- 

 monas margarita was in reality a non-chromogenic strain of 

 Serratia marcescens. It is not surprising that there should be some 

 confusion between the Aeromonas sp. and some members of the 

 Enterobacteraceae, owing to the similarities of their biochemical 

 reactions. Differentiation depends then on the accurate deter- 

 mination of their type of flagellation. Careful studies by electron 

 microscope at Torrv Research Station has shown that Aeromonas 

 margarita is in fact peritrichous (Figs. 2 and 3) and this, along 

 with its biochemical propensities, including its being oxidase -ve, 

 indicates that it is a t\pical member of the Enterobacteriaceae. 



