Laboratory Outline for General Bacteriology 



Pour into the plates melted agar cooled to 40 C. and mix well 

 with the water. 



When hard place plates and fermentation tubes in 37 C. 

 incubator. 



Incubate two days. They are then ready to be studied. Meas- 

 ure the percentage of gas by the gasometer chart on page 41. 



Count the numbers of bacteria on the plates using the ruled 

 glass counting plate and a hand lens. Make an average of the four 

 plates before stating the number of bacteria per cubic centimeter. 

 To make this average multiply the .1 cc. plate count by 10, the 

 .2 cc. by 5, and the .5 cc. by 2 before adding to the 1 cc. plate count. 

 Divide by four. 



Gas percentages are not averaged. Results are stated in num- 

 ber of tubes showing more than 10 ( /( of gas. 



BACTERIA IN MILK 



Samples are taken from well mixed milk into sterile bottles 

 and tested as soon as possible. Samples that cannot be tested at 

 once should be kept cold below 40 F. Standard agar is used 

 for plating. 



Milk cannot be added directly to the plates as is done in plat- 

 ing ordinary water samples because it usually contains so many 

 bacteria that the colonies would not have room to develop properly 

 or be readily counted. Also the milk would cloud the agar so the 

 colonies could not be easily seen. By diluting the milk with ster- 

 ile water clear plates having between 25 and 300 colonies can 

 usually be obtained. Plates having numbers of colonies outside 

 these limits do not give as reliable results. Four plates are made 

 using at least two different dilutions. For miscellaneous milk 

 samples the character of which is not known dilutions may be 

 made ranging from 1-100 to 1-10000. When something is known 

 as to the quality of the milk, dilutions are made so that the highest 

 one will give about 100 colonies to the plate. For a sample of milk 

 which may contain 20,000 bacteria per cc. dilutions of 200 and 100 

 would be best for giving the desired numbers of colonies on the 

 plates. The method of plating is as follows: Shake the milk 

 sample vigorously 25 times. With a sterile pipette transfer 1 cc. 

 to dilution bottle containing 199 cc. of sterile water. As this 

 pipette is to be used once later it is laid aside so that the lower 

 half will touch nothing. Shake dilution 25 times and with another 

 sterile pipette transfer 1 cc. to each of two plates. This gives the 

 200 dilution. Lay aside this pipette also so it will not become con- 



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