BACTERIOLOGICAL EXAMINATION OF EGGS. 19 



Dilutions of 1 to 100, 1 to 1,000, 1 to 10,000, and any desired num- 

 ber with multiples of 10 may be made by placing 1 cc of the next 

 lower dilution in another flask containing 9 cc of sterile water and 

 shaking each dilution at least 25 tunes to insure an even mixture. 

 Liquid substances are diluted in the same manner as solid samples 

 after making the first dilution of 1 to 10. 



PLATING CULTURES. 



At least four dilutions should be made from each sample for every 

 kind of media employed. The strength of the dilutions will neces- 

 sarily vary according to the experience and material examined. The 

 following dilutions are given as examples: 



Water: Dilution. 



Unpolluted ................................... 1 to j-.vov 



Polluted ....................................... ' T V to To; V<n> 



Milk: 



Certified ....................................... ^ to 



Market ........................................ TrrVff to 



Eggs: 



Fresh in shell .................................. 1 to 



Frozen in bulk ................................. TorW to 



Desiccated .................................... T<nWo to 



Oysters: 



In shell (individual) ........................... 1 to 



In shell (composite) ............................ ^ to 



Shucked ...................................... to 



Petri plates are prepared by placing 1 cc of each dilution desired in 

 a corresponding number of sterile plates. Liquefied culture media 

 kept at about 43 C. are then added to each plate containing a dilu- 

 tion, the whole mixture being gently rotated and shaken in such a 

 manner as to produce an even distribution of the organisms present. 

 Plates thus prepared should be solidified at once. 



FERMENTATION TESTS. 



One cubic centimeter quantities from at least four dilutions, pref- 

 erably more, should be placed in suitable tubes containing fermentable 

 substances. The ordinary Smith fermentation tube or inverted 

 tubes within larger ones may be used for this purpose. The medium 

 employed may be sterilized ox bile containing 1 per cent of peptone 

 and 1 per cent of lactose, or plain bouillon, to which 1 per cent of 

 dextrose or other sugar has been added. 



ANAEROBIC CULTURES. 



Anaerobic organisms may be grown in Smith fermentation tubes; 

 shake dextrose in vacuum or under conditions whereby free oxygen 

 has been removed or replaced w r ith hydrogen or some other gas suit- 

 able for the growth of such bacteria. 



