220 LABORATORY EXERCISES IN BACTERIOLOGY, 



movement is noticed, supposed to be due to the presence of an undulating membrane 

 attached to the organism, as in case of the oscillatoria. 



To this power of movement must in part be referred the active approach or retreat 

 of motile bacteria to foci of various substances, as points of disease in the body (chemo- 

 taxis) ; the similar change of location of non-motile forms probably depending upon 

 growth-progression or passive convection by currents in the surrounding fluids. 



Exercise 52. Rub up a little carmine with distilled water. Arrange a 

 drop as a hanging drop, and examine, noting the dancing movement of the 

 grains. Arrange similarly a drop of water, after diffusing in it a few bac- 

 teria from a growth of Micrococcus pyogenes. Has it a similar movement? 

 With a needle touch a tiny drop of carbolic acid to the drop and again ex- 

 amine. Does it continue to move ? What is the source of movement ? 



In the same way prepare a hanging drop of Bacillus typhosus. Note 

 the movement of the individual rods. How does it differ from that in the 



previous preparation? Add again a trace of 



A o G> co ec carbolic acid. What effect has it had upon the 



an rr> a\ CD o& oc movement? 



)W VJ t> gj Tljr V Q 



e Exercise 53. In a similar hanging-drop pre- 



paration of Bacillus typhosus suspend the smallest 

 possible fragment of sterile, solid agar. Note 

 for fifteen to thirty minutes the approach and 

 FIG. 64. REPRODUCTION OF attachment of the bacilli to the fragment as 

 A. Division^coJc'us into dip- illustrative of positive chemotaxis. (Can there 

 lococci or two cocci. B. be any relation between this experiment and 

 C t0 Into ra sardoa f0 or Tghi the agglutination phenomenon of the organism?) 



cocci. D. Division of rod 



form by transverse separa- 6 - Reproduction of Bacteria. Under ordinary 



tion into two rods. circumstances the majority of bacteria multiply as far as 



is known by direct cell division, a delicate extension of the 



cell -wall extending from the sides through the cell-body in one, two, or all three planes. 

 The globular forms may divide in all three planes, but the rod forms are limited to 

 a single, lateral, division. Thus a coccus, before dividing, becomes slightly elongated 

 (Fig. 64) into an oval; and the division occurring in one direction, it is separated into 

 two. These two may not be entirely parted, and remaining together constitute a 

 diplococcus. If in the subsequent division of these and their offspring in the same 

 manner the separation of the individuals should not be complete, a chain of strepto- 

 cocci results. Sometimes after division in one direction, or coincidently with it, a 

 second division occurs at right angles to the first plane of division, whence four indi- 

 viduals, or two diplococci, or one tetrad, may result. Or, again, a third division at 

 right angles to the other two may take place, whence a sarcina form or a zooglea 

 of separate individuals may arise. It is estimated that complete division of Micro- 

 spira comma (Asiatic cholera) requires twenty minutes, leading to the possibility of 

 billions of individuals within a single twenty-four hours from an original organism; 

 and it may be said that under fair conditions of growth most bacteria will divide within 

 thirty or forty minutes. 



