678 



BACTERIUM 



TABLE 44 



The Fermentation Reactions of some Paracolon Bacilli. 



Notes. — O or S = no fermentation, or very slight. 

 A.G. (S). = slight acid and gas. 

 A (G.v.s.) = acid, very little gas. 



A(S), or alk., or D = slight acid, or alkalinity, or decolorized. 

 A alk. (S) = acid, reverting to slight alkalinity. 



Bact. coli 



Isolation. — Isolated from faeces by Escherich (1885). 



Habitat. — The intestinal tract of man and animals. 



Morphology. — Bacilli with parallel sides and rounded ends, varying in length from almost 

 coccal forms to long rods. The predominating form is a short rod about 2-3 fi long 

 and 0-6 fi in breadth. The bacilli stain evenly, form no spores, and are not usually 

 capsulated. They are Gram-negative and not acid-fast. Some strains are actively 

 motile, with peritrichate flagella. Some are non-flagellated and non-motile. 



Agar Plates. — 24 hours at 37° C. Bad. coli forms circular, low convex, smooth, colourless 

 colonies, about 1-3 mm. in diameter, with a finely granular structure, and an entire 

 edge. The consistency is butyrous, and the growth emulsifies readily. The 

 typical form is often departed from. The surface may be more contoured, and the 

 edge less regular and more effuse. 



Broth. — There is abundant growth with a uniform turbidity, increasing up to 24-72 hours, 

 with a slight, powdery deposit that disperses readily on shaking, and sometimes 

 with a minimal degree of surface growth. 



Gelatin Stab. — There is good growth along the track, and moderate growth on the surface. 

 The medium is not liquefied. 



Blood Agar Plate. — The medium in the neighbourhood of the colonies is discoloured, and 

 there may be haemolysis. Different strains may vary widely in their action on 

 blood- containing media. 



MacConkeys Lactose Agar. — Bact. coli gives rise to circular, smooth, convex colonies, 

 similar to those formed on ordinary agar, but coloured red as a result of the action 

 of the acid formed from the lactose on the neutral red that diffuses into the colony 

 from the medium. 



Potato. — There is slight but obvious growth, with a cream or faintly yellow colour. 



Resistance. — Bact. coli is usually killed by exposure to a temperature of 60° C. for 15 

 minutes, or of 55° C. for 1 hour, but some strains are more resistant. It is less 

 resistant than members of the Salmonella genus to the action of certain green dyes, 

 particularly brilliant green. 



