BACTERIOLOGY 



of growing, say in milk, when their less harmful or non-patho- 

 genic relatives are prevented from growing, and by their action 

 in turning the milk sour, warning the housewife that the 

 milk is not fresh. 



When we have mastered the technique of sterilisation, we 

 are in a position to collect material containing micro-organ- 

 isms for purposes of bacteriological investigation. In doing 

 this, we have to avoid contaminating the material with other 

 micro-organisms, both before, during, and after collection ; to 

 avoid killing the organisms we desire to investigate, e.g. with 

 antiseptics or disinfectants, or by heat, or even, in the case of 

 certain very delicate organisms, by keeping them too long or 

 allowing them to get chilled. 



Fluids and solid material containing organisms can be 

 collected in suitable sterile receptacles such as a test-tube, 

 flask, bottle, suitably stoppered or sealed, or in sterile cotton- 

 wool or gauze. Fluids can also very conveniently be collected in 

 small glass pipettes or tubes, the ends of which can be hermeti- 

 cally sealed in the flame. The specimen should be examined as 

 soon as possible after collection. The systematic examination 

 of such material is then carried out, usually on something like 

 the following lines. Fresh wet preparations are examined 

 under the microscope in order to determine the number, size, 

 shape, and arrangement of the organisms, and especially to 

 investigate whether they possess motility or not. Then smears 

 or films are made on microscopical slides orcover-glasses (usually 

 the former), dried or otherwise fixed, or very thin sections of 

 the tissue containing the organisms are cut by various special 

 methods. These preparations are then stained with suitable 

 aniline or other dyes, often in a very elaborate way, in order 

 to bring out the structure more clearly, or to differentiate one 

 organism from another by such staining reactions. Other 

 means of investigation, such as artificial cultivation, inoculation 

 into animals, etc., will be referred to later. 



Morphology and Life-History of Bacteria. The size, 

 shape, and structure, or as it is usually termed, the morphology 

 of bacteria, may now be considered ; and, as we are dealing 

 with such minute objects, we must have some idea of how they 

 are to be examined and measured. Our microscope for the 

 examination of bacteria must be able to magnify them at least 

 about a thousand diameters or more. The standard of such 

 microscopical measurements is termed a Micromillimetre, 

 usually contracted into the form Micro or Micron, and 

 denominated by the Greek letter [A. A micromillimetre is 



