THE RELATIVE PROPORTION OF BACTERIA IN TOP MILK 
(CREAM LAYER) AND BOTTOM MILK (SKIM MILK), AND 
ITS BEARING ON INFANT FEEDING.'* 
By John F. Anderson, 
Passed Assistant Hurgeon and Assistant Director Hygienic Lahoratory, PuUic 
Health and Marine-Hospital Service, Washington, D. C. 
In the course of a study of tubercle bacilli in market milk ^ and 
of a later study of the best procedure for their detection in milk it 
was noticed that Avhen guinea pigs were inoculated Avdth the cream a 
very much higher percentage died from acute infections than when 
the sediment was used. The inference was natural that the cream 
contained more bacteria than the bottom milk or sediment. A few 
j^reliminary examinations having shown this supposition to be 
correct, a study Avas begun as to the number of bacteria in the whole 
milk, the bottom milk or sediment, and the cream, both that collected 
by gravity and by centrifugation. 
The relatiA^e number of bacteria in the top milk and in the bottom 
milk is a subject of very great importance in the modification of milk 
for infant feeding. All the workers on pediatrics and infant feeding 
give formulae for the modification of milk based upon the use of 
jA'arious amounts of top milk. My studies show that top milk, such 
as is advised for use in the above formulae, contains from 10 to 500 
times as many bacteria per cubic centimeter as the mixed milk. This 
preponderance of bacteria in top milk may account for the fact that 
sometimes children do not thrive on modified milk when made from 
top milk, but improve when the whole milk is used for modification. 
The various bacteria causing acute infections, as well as tubercle 
bacilli, are more numerous in the top milk than in the bottom milk. 
In many cases this difference is more than a hundredfold and, as 
infection must depend to some extent on the number of bacteria intro- 
®This is a summary of a paper read before the American Public Health 
Association in Winnipeg, Canada, August 31, 1908. 
^ Anderson, John F. : “ The frequency of tubercle bacilli in the market milk 
of the city of Washington, D. C.” Bui. No. 41, Hyg. Lab., U. S. Pub. Health 
and Mar. Hosp. Serv., Wash., 1908, p. 163. ~ 
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