50 ELEMENTARY BACTERIOLOGY LABORATORY EXERCISES 
DIFFERENTIATION OF BACTERIUM COLI AND BACTERIUM 
AEROGENES ON EOSINE-METHYLENE BLUE AGAR 
Size 
Confluence 
Elevation 
Appearance 
by Trans- 
mitted 
Light 
Appearance 
by Reflected 
Light 
(1) 
Bacterium coli 
Well isolated colonies are 
2-3 m.m. in diameter. 
Neighboring colonies show 
little tendency to run _to- 
gether. 
Colonies slightly raised; sur- 
face flat or slightly concave, 
rarely convex. 
Dark, almost black centers 
which extend more than 34 
across the diameter colony; 
internal structure of central 
dark portion difficult to dis- 
cern. 
Colonies dark, button-like, 
often concentrically ringed 
with a _ greenish metallic 
sheen. 
(2) 
Bacterium aerogenes 
Well isolated colonies are 
larger than coli; usually 
4-6 m.m. in diameter or more. 
Neighboring colonies run to- 
gether quickly. 
Colonies considerably raised 
and markedly convex; occa- 
sionally the center drops pre- 
cipitately. 
Centers deep brown; not as 
dark as Bacterium coli and 
smaller in proportion to the 
rest of the colony. Striated 
internal structure often ob- 
served in young colonies. 
Much lighter than Bacterium 
coli, metallic sheen not ob- 
served except occasionally 
in depressed center when 
such is present. 
(1) Bacterium coli. Two other types have been occasionally encountered: 
One resembles the type described, except that there is no metallic sheen, the col- 
onies being wine colored. The other type of colony is somewhat larger (4 m.m.), 
grows effusely, and has a marked crenated or irregular edge, the central portion 
showing a very distinct metallic sheen. These two varieties constitute about 2 
or 3 per cent of the colonies observed. 
(2) Bacterium aerogenes. A small type of aerogenes colony, about the size 
of the colon colonies, which show no tendency to coalesce, has been occasionally 
encountered. 
