352 A MANUAL OF BACTERIOLOGY 



inoculated. If no incubator is available, the culture may be kept 

 in a warm place, or in an inside pocket. 



Many laboratories now undertake the examination of material. 

 Culture outfits are supplied by some, consisting of a sterilised 

 tube containing a sterilised swab. Failing this, a piece of mem- 

 brane may be forwarded in a tube or bottle which has been steri- 

 lised by heating, or with boiling water or steam. If there be no 

 membrane, a swab can be readily extemporised by wrapping a 

 little wool or lint (non-antiseptic) round the end of a piece of wire, 

 knitting needle, hair-pin, penholder, or splinter of wood. The 

 wool may be sterilised by moistening with water and then holding 

 in a flame. Membrane or secretion may also be forwarded on 

 pledgets of wool, pieces of lint or calico, and even on paper, but 

 these are not so suitable. 



III. The Schick Test. This test is used to determine whether 

 immunity towards diphtheria exists or not (p. 329). The prin. 

 ciple of the test is that the intra-cutaneous injection of diphtheria 

 toxin gives rise to a local reaction at the site of inoculation in 

 unprotected subjects, while if specific antibodies exist, the toxin 

 is neutralised and no reaction follows. 



The diphtheria toxin used for the injection consists of a broth 

 culture of the diphtheria bacillus grown for six days at 37 C. It 

 is then treated with 0-5 per cent, carbolic acid, and the bacteria 

 are allowed to sediment in an ice-box for two to three days. The 

 supernatant fluid is filtered through a Berkefeld filter and stan- 

 dardised. A ripened toxin is usually used of an accurately 

 determined M.L.D. (minimum lethal dose) for a 250-grm. 

 guinea-pig. 



A primary dilution is made by diluting with normal saline, so 

 that each cubic centimetre of it will represent 10 M.L.D. The 

 bulk toxin keeps its strength well, the primary dilution also for 

 two weeks in the ice-box, but the final dilution must be made up 

 daily. The final dilution is made by diluting the primary dilution 

 one hundredfold, so that 1 c.c. of it will contain ^ M.L.D., 

 while 0-2 c.c. will contain ^ of a M.L.D. This amount, 0-2 c.c., 

 is the dose used. 



The injection must be intra (not sub-) cutaneous and be made 

 with a fine needle. 



A negative reaction is shown by the skin remaining normal. 

 With a positive reaction, a trace of redness appears slowly at the 



