TECHNIQUE 21 



M: COLONY PREPARATIONS 



(5, 6, 7) 



Although a bacterial surface colony upon solid medium is a scmi-artificial 

 formation, the study of its minute structure is often of biological interest 

 and diagnostic importance. It is capable of providing evidence of the natural 

 relationship of the bacteria to one another, and, especially in the study of 

 dissociation, may indicate differences in structure and behaviour which are 

 not always obvious by other methods of examination. 



Entire colonies may be embedded and sectioned, like portions of tissue, 

 but as the colonies are usually exceedingly thin and flat (much more so than 

 they appear to the unaided eye), whole mounts may be made upon slides or 

 coverslips. These are usually termed impression preparations. They are 

 best made from very small colonies, although quite large ones can be mounted 

 if the growth is sufficiently tough. 



A small piece of medium bearing the desired colonies, is cut out with 

 the point of a knife and placed, face downward, upon a slide or covershp. 

 Surface tension will suffice to keep the medium, and the attached colonies, 

 firmly pressed to the glass. It is then fixed, in its entirety, preferably in 

 Bouin's solution, until the medium is blanched throughout, and can be 

 peeled away from the glass, leaving the colonics adhering to the surface of 

 the covcrslip. The preparations may then be washed, stained and mounted. 

 Sometimes they are of great beauty. 



The best medium for this purpose is blood agar. It is very adhesive when 

 fixed, and becomes firmly attached to the glass, so that it can be peeled away 

 without danger of sliding the medium laterally and destroying the colony. 

 Plates should be inoculated with the rounded tip of a glass rod, to avoid 

 scratching the surface, they should be perfectly dry, and free from bubbles 

 and other irregularities. 



N: SUMMARY 



The examination of bacteria in dried, heat-fixed smears, stained by the 

 <usual diagnostic methods, fails to give a true picture of their morphology. 



