308 Bacteriologicai, Analyses 



Acidity 0.2 per cent. 



Add 1 c.c. of sterile one per cent tartaric acid solution to each 

 plate before pouring the medium over the dilution. 



Incubation. — ^Incubate agar, litmus agar and whey agar plates 

 at 35 degrees C. (95 degrees F.) for at least three days before 

 making counts. Incubate gelatin plates at 21 degrees C. (70 degrees 

 F.) for four to five days before making counts. 



Making Counts.— The colonies on the plates are counted most 

 conveniently by placing the plates over a standard counting plate. 

 In the absence of such a plate, place the petri dish upside down 

 on a dark surface and draw, with a blue crayon, radial lines, divid- 

 ing the field into segments. For plates containing not to exceed 

 100 colonies eight to sixteen segments are sufficient for easy 

 counting. 



Qualitative Determinations.— Numerical counts on the four 

 kinds of media recomlmended above usually furnish a fair general 

 idea of the types of bacteria present. 



For the detection of gas-producing species, nutrient bouillon 

 containing three per cent lactose and three per cent sucrose, re- 

 spectively, in fermentation tubes, or nutrient agar containing three 

 per cent lactose and three per cent sucrose, respectively, in test 

 tubes, are serviceable. 



Cans of sweetened condensed milk that show gaseous fermenta- 

 tion (swell heads) are usually due to certain species of yeast, which 

 thrive best in media containing sucrose. 



Cans of evaporated milk that show gaseous fermentation 

 (swell heads) are usually caused by anaerobic putrefactive bac- 

 teria, of which Plectridium foetidum is a most frequent repre- 

 sentative, see "Blown Evaporated Milk," page 226. This type of 

 micro-organisms requires strictly anaerobic cultural conditions. Un- 

 der lirriited laboratory facilities the anaerobic conditions are best 

 produced by the use of oxygen-absorbing chemicals, such as pyro- 

 gallol to which potassium hydroxide is added. Use dry commercial 

 pyrogallol and potassium hydroxide sticks, in proportion of 1 gram 

 pyrogallol to .7 gram potassium hydroxide, dissolved in about 2 c.c. 

 of water- 

 Place 50 grams of pyrogallol into the "bottom part of a large 

 size desiccator. Have the rim of the desiccator and the correspond- 



