BACT. AEROGENES 679 



Oroivth Requirements. — Bad. coli grows readily on all ordinary laboratory media. The 

 optimum temperature for growth is in the neighbourhood of 37° C, but growth 

 occurs over an extended range, from about 15 to 45° C. 

 Biochemical. — -Produces acid and gas in dextrose, maltose, mannitol, lactose, xylose, 

 rhamnose, and arabinose, but not in dextrin, starch, inositol, or as a rule cellobiose. 

 Sucrose, salicin, raffinose, glycerol, and dulcitol are attacked by some strains, but 

 not by others. Acidifies and clots milk, reduces nitrates, and nearly always forms 

 indole. M.R. + ; V.P. — ; Koser's citrate — ; gas production in MacConkey's 

 broth at 44° C. + . HjS is not usually formed, but the result depends to some 

 extent on the medium. 

 Antigenic Structure. — Bad. coli is an antigenically heterogeneous species, the antigenic 



structure of which has not yet been studied in any detail. 

 Pathogenicity. — Bad. coli is a normal inhabitant of the intestine of man and other animals. 

 In certain circumstances it may play a pathogenic role, sometimes in the intestine 

 itself, more commonly in organs or tissues anatomically related to it, such as the 

 gall-bladder. It is a frequent cause of infection of the urinary tract in man. 

 Varieties. — On the basis of the fermentation reactions. Bad. coli may be divided into the 

 following varieties : — 



var. commune ferments saUcin but not saccharose. 

 var. commnnius ferments saccharose but not sahcin. 

 var. neapolitanum, ferments both saccharose and salicin. 

 var. acidi-ladici ferments neither saccharose nor salicin. 

 Some authorities recognize further varieties which are differentiated by being non- 

 motile. 



Bact. aerogenes 



Bact. aerogenes differs from Bad. coli in the following points. Morphologically the rods 

 are often shorter and plumper and they are occasionally capsulated. They may be motile 

 or non-motile. The colonies on agar are more convex, smoother and often mucoid. The 

 deposit in broth is often more viscous. Growth is rather more abundant at lower tempera- 

 tures, less abundant at temperatures over 37° C, and usually very slight, or absent, at 

 temperatiu-es of 42-44° C. Inositol, cellobiose, dextrin and starch are frequently fer- 

 mented. Most strains fail to form indole. There is a more abundant formation of gas, 

 but a lower acidity. The COj : Hj ratio is high, approximately 2:1. The Voges- 

 Proskauer reaction is positive. The methyl-red reaction is negative. Growth occurs 

 with citrate as the only source of carbon. Gas is not produced in MacConkey broth at 

 44° C. The normal habitat of this species is not known with certainty. It is common 

 on grains and plants, and is often found, though usually only in small numbers, in the 

 faeces of man and animals ; the balance of evidence, however, is against its being primarily 

 an intestinal parasite. 



Bact. cloacae 



Bad. cloacce resembles Bact. aerogenes, except that it is usually motile, is seldom 

 capsulated, liquefies gelatin, but does not ferment glycerol or starch. Its distribution 

 in nature appears to be the same. 



Intermediate Forms 



The existence of strains that are intermediate between Bad. coli and Bact. aerogenes 

 has been noted in the text, and their characters have been described (see p. 663). 

 They are distinguished (1) from Bact. coli mainly in being citrate -positive, in fermenting 

 cellobiose, in being usually indole-negative, and in failing to form gas in MacConkey 

 broth at 44° C, and (2) from Bact. aerogenes in being M.R. +, V.P. — , and in faiUng to 

 utihze uric acid as their sole sovu-ce of nitrogen. In this country they appear to be dis- 

 tributed chiefly in the soil. 



