THE DETECTION OF BACTERIAL PATHOGENICITY 229 



inocula containing relatively few bacteria. Toxins generally retain their 

 potency exceedingly well when frozen at — 20°C. Inocula, such as seed 

 cultures, are sometimes lyophilized, and the recommended procedure for 

 this can be found in Chap. V. Particular care should be taken in the 

 labeling of cultures which are to be stored at subzero temperatures, since 

 freezing will often make carelessly written or crayon labels unintelligible. 

 It has been found good practice to typewrite labels onto a piece of 

 adhesive tape, and these, when attached to a glass container, will usually 

 adhere for considerable periods of time. If a crayon pencil is used, it is 

 a good idea to put a piece of transparent plastic tape over the crayoned 

 label. Cultures which are to be stored in the frozen state for any length 

 of time should never be left plugged with cotton alone. A sterile rubber 

 stopper or glass sealing of the tube is necessary to prevent evaporation of 

 of frozen fluid during prolonged periods of storage. When autopsy tissues 

 must be stored pending bacterial examination, it is preferable to retain 

 the tissue in its original structure, since whole tissue fragments retain 

 their infectivity better than the same tissue ground into a suspension. 

 For this reason many workers take double samples of tissue from an 

 autopsy, one portion of which is immediately frozen and the other portion 

 used for workup. Although it is a point well known to the experienced 

 technician, the amateur should be reminded that if a blood sample is 

 intended for serological studies, it should never be frozen before separa- 

 tion of the cells from the serum; otherwise the specimen will be badly 

 hemolyzed. 



SELECTION AND PREPARATION OF LABORATORY 

 ANIMALS FOR PATHOGENICITY STUDIES 



Animals not only are necessary for determining the etiology of specific 

 infectious diseases and the pathogenicity of particular cultures of bac- 

 teria but are also utilized as a means of isolation, to determine in more 

 detail certain specific pathogenic mechanisms, to maintain species that 

 grow best in vivo, to increase pathogenicity, and to produce antibodies. 



The bacterial species and the property to be studied determine the 

 choice of the experimental animals. They should be kept in a well- 

 ventilated and -lighted room with^ properly supervised animal care. 

 Animals inoculated with suspect or known pathogens should, whenever 

 possible, be caged apart from the common stock, and the cages marked in 

 a suitable fashion. Where insects, such as flies, mosquitoes, or ticks, are 

 troublesome, the caging area should be thoroughly sprayed with DDT or 

 a similar insecticide. Do not cage large numbers of animals in a single 

 enclosure. Whenever possible, the larger species, such as rabbits, mon- 

 keys, or dogs, should be kept in individual cages. Marking of laboratory 



