SUMMARY 



23 



The reaction of the medium is to be 

 between pH = 6.2 and pH = 7.0. If 

 necessary to adjust the reaction, special 

 attention is to be given to the H-ion 

 concentration, making use of one of 

 the indicators, brom thymol blue or 

 brom cresol purple. 



Three dilutions are to be made in 

 plating: 1 to 100, 1 to 1,000 and 1 to 

 10,000 — unless the quality of the 

 milk is such that the highest or lowest 

 of these is known to be superfluous. 

 In no case are less than two plates to 

 be made from each sample. Each 

 sample, bottle and dilution bottle is to 

 be shaken 25 times with an up and 

 down motion of about one foot, in not 

 more than seven seconds. After dilu- 

 tion of the milk, the agar is to be 

 poured into the plates within 15 

 minutes. 



Incubation shall be at 37.5" C. for 

 48 hours. 



The plates used for counting are to 

 have, if possible, between 30 and 300 

 colonies each. If there are no plates 

 within the^e limits, the one having 

 nearest to 300 is to be counted. Count- 

 ing to is be done with a lens mag- 

 nifying lYi diameters. The exact 

 counts from each plate are to be 

 recorded, but not more than two 

 significant left hand digits are to be 

 used in making the final report. 



Results are to be expressed, not as 

 so many "bacteria per cc." of milk; 

 but as "colonies per cc." or better as 

 "ofiicial plate count" so much per cc. 

 The practice of publishing counts from 

 individual samples of milk as showing 

 the quality of a given milk supply is 

 not sanctioned, and it is required that 

 a series of samples be examined before 

 rendering judgment in regard to any 

 milk supply. 



Microscopic Count or Bacteria. 

 Milk to be taken in a capillary pipette 



discharging 0.01 cc. and dried over an 

 area of 1 sq. cm. on a microscopic 

 slide. It shall be stained in methylene 

 blue, after washing out the fat in 

 xylol and fixing in alcohol. The num- 

 ber of bacteria per cc. is to be estimated 

 by counting those within a given 'area 

 in a microscopic field, this area having 

 been carefully measured and its ratio 

 to a square centimeter determined, 

 at least 1/10,000 -part of a cc. of milk 

 is to be examined, and if the milk is 

 of high grade this must be done under 

 favorable conditions for accurate count- 

 ing. The ratio to be used in comparing 

 the "official plate counts" with the 

 counts of bacteria per cc. as shown 

 under the microscope, is provisionally 

 placed at 1 to 5. Preparations are to 

 be preserved a reasonable time after 

 the reports have been sent to the 

 person or persons interested. 



Sediment Test. Quart (or pint) 

 samples are to be filtered through 

 cotton disks, one inch in diameter. 

 Five degrees of dirtiness are to be 

 recognized, correspondingly respective- 

 ly to milk containing 0, 2.5, 5, 7, and 

 10 milligrams of dirt per quart (cor- 

 responding allowances to be made if 

 pint samples are used). 



Detection of Specific Pathogens 

 IN Milk. The only pathogenic or- 

 ganisms ordinarily sought in routine 

 analytical work are the streptococci 

 associated with udder inflammations. 

 They may be detected either on the 

 same slides as made for the microscopic 

 count of bacteria, or on slides smeared 

 with the sediment obtained by cen- 

 trifuging the milk. Proper precautions 

 are to be observed in distinguishing 

 between merely parasitic or saphro- 

 phytic streptococci and those causing 

 truly pathological conditions. Lab- 

 oratory findings are to be confirmed by 

 clinical examinations. 



