ELEMENTARY BACTERIOLOGY LABORATORY EXERCISES 35 
SECTION 12: VARIABILITY OF BACTERIA AND 
RELATED PHENOMENA 
EXERCISE 51 
DISSOCIATION OF BACTERIA 
By “bacterial dissociation” is meant the appearance of two or more variant 
forms from the same species. This phenomenon may be manifested by smooth 
(“s”) and rough (“r”) colonies; by slimy and non-slimy colonies; by motile and 
non-motile cells; and in other ways. 
Inoculate a culture of Bacterium aerogenes into beef infusion broth having 
a pH of 7.8. Incubate for two weeks at room temperature. 
After incubation, plate the culture on nutrient agar. At the same time plate 
as a control a culture of the same organism which has not been held in an alka- 
line medium. Incubate the plates at room temperature for three days. 
Examine the plates for different types of colonies. 
EXERCISE 52 
FILTERABLE FORMS OF BACTERIA 
It now appears to be fairly well established that some, and perhaps all, 
species of bacteria undergo a life cycle which includes forms small enough to 
pass through fine earthenware filters which remove bacteria of ordinary size. 
The “filterable forms” of bacteria appear to be quite different in their morpho- 
logical, cultural and physiological properties from the ordinary forms of the 
same species. 
Filter a broth culture of Bacterium aerogenes through a Berkefeld filter. 
Inoculate two glucose-beef infusion broth tubes with 1 c.c. each of the filtrate. 
Incubate at 37° C. for one day and then continue the incubation at 30° C. for 
two weeks. Also incubate one sterile tube of broth for a control. 
After incubation, examine the tubes for the absence of ordinary bacterial 
growth. From one tube, place about two drops of the broth on the surface of each 
of two glucose-infusion agar plates. Add about 0.5 c.c. of sterile glucose-infusion 
broth to each plate and spread over the surface of the agar. Incubate the plates 
at 37° C. for two days. Make a control plate in the same way from the control 
tube of sterile broth. 
After incubation, examine the plates for the presence of barely visible sur- 
face colonies. Make stained preparations with gentian violet from the surface 
growth and examine microscopically for the presence of definite cells. 
