Bacterial Multiplication 149 



For those who have difficulty following diagrams, the complete 

 steps involved in preparing the above plates will be put into words. 



1. Shake the raw milk sample in the prescribed manner. 



2. Pipette 1.0 ml, of the sample into 99 ml. of sterile water. 

 (Bottle #1.) This will give a 1:100 dilution of the milk. 



3. Shake this water dilution in the standard way. 



4. Transfer 1.0 ml. of the diluted milk into a petri dish labeled 

 1:100. 



5. From this same water dilution bottle pipette 0.1 ml. into a 

 second petri dish labeled 1 : 1000. 



6. Take 1.0 ml. from dilution bottle ^1 and transfer it into dilu- 

 tion bottle #2 which contains 99 ml. of sterile water. This 

 second bottle gives a 1 : 10,000 dilution of the original raw milk. 



7. Shake bottle #2 in the prescribed manner. 



8. Pipette 1.0 ml. from bottle #2 into a petri dish labeled 

 1:10,000. 



9. From this same bottle #2 transfer 0.1 ml. into a petri dish, 

 labeled 1:100,000. 



10. Into the 1:100 plate pour one tube of melted and cooled (to 

 between 45-50° C.) agar. Mix the agar and diluted milk by 

 gently rotating the petri dish on a flat surface such as a table 

 top. The mixing should be thorough but not so vigorous that 

 the agar slops over the side of the petri dish. 



11. Pour one tube of melted and cooled agar into the 1:1000 plate 

 as you did with the 1:100 plate above. Continue this pro- 

 cedure with each plate prepared. Do not allow more than 20 

 minutes to elapse between the time the milk sample is origi- 

 nally diluted and the time that the agar is poured into the petri 

 dish. 



12. Set the plates aside until the medium solidifies, then incubate 

 them in an inverted position to prevent water of condensation 

 from forming on the lids and dropping back onto the agar 

 surface. At the conclusion of the prescribed incubation period, 

 the plates are examined. Count only the plate having be- 

 tween thirty and three-hundred colonies, and multiply the 



