THE IDENTIFICATION OF MICB0-0R0ANI8MS 357 



was injected into a field mouse, the streptococci proliferated, invaded the blood- 

 stream, and caused death, while Ery. muriseptica did not grow at all ; the strepto- 

 coccus was therefore obtained in pure culture from the blood. This principle 

 is of wide application. It is used particularly to isolate organisms that are 

 pathogenic to a certain laboratory animal from other closely similar organisms 

 that are not pathogenic. Thus B. anthracis can easily be separated from 

 B. subtilis or B. megatherium by the injection of a mouse or a guinea-pig. 

 It is also used to isolate pathogenic organisms which are not easy to grow in 

 culture, or which are readily over-grown by contaminating organisms. As 

 examples, we may quote the tubercle bacillus in pus, or the pueumococcus in 

 sputum. The contaminating organisms are rapidly killed in the animal body, 

 whereas the pathogenic organism multiplies and can be recovered in pure culture 

 from the tissues. 



N. Single Cell Methods. — The aim of these methods is to obtain a culture of a 

 given organism from a single bacterial cell. If this can be carried out successfully, 

 then the resultant culture must obviously be pure. If the technique for single- 

 cell isolation was simple and flawless, this would be the ideal method for the puri- 

 fication of cultures ; in fact, however, several of the methods advocated for this 

 purpose suffer from optical or other defects, which seriously detract from their 

 value. In Barber's (1908) method the culture is diluted, and a series of tiny 

 droplets prepared. These are placed on the under surface of a cover-slip forming 

 the roof of a special chamber, and examined under the microscope. When a drop 

 is found containing only one organism, it is picked off with a special capillary 

 pipette and transferred to a fluid medium. This method has been widely used, but 

 it suffers from the defect that in viewing a spherical droplet the optical conditions 

 are such as to render accurate observation of particles at the water-air interface 

 very difficult or impossible. Hence there is no absolute certainty that a single 

 cell has been picked. A method devised by Topley, Barnard and Wilson (1921) 

 eliminates these particular optical defects. A loopful of a young gelatin culture 

 at 37° C. is placed on a slide, and covered with a quartz cover-glass. Under dark- 

 ground illumination a single organism is picked out which is well removed from 

 any other organisms, and is covered with a minute droplet of mercury. The 

 preparation is exposed for a short time to ultra-violet irradiation with the object 

 of destroying all organisms except the single one that has been protected by the 

 mercury droplet. After incubation overnight the preparation is again examined, 

 and, if successful, a colony will be observed at the site previously occupied by the 

 protected organism. This can then be transferred to a liquid medium. Adequate 

 controls are necessary to prove that the irradiation was sufficient to kill all non- 

 protected organisms. Several other methods have been described. 



The Identification of Micro-organisms 



Having once obtained a pure culture of a particular organism it is necessary 

 to establish its identity by an appropriate series of tests. This may require a few 

 weeks, or it may take several months to complete. Many of the reactions may 

 have to be tested three or four times to make sure of their consistency. It is often 

 desirable to prepare photographs recording the morphology and colonial appearances 



