376 MILK 



CHROMOGENIC BACTERIA IN MILK 



It happens not infrequently that milk assumes a peculiar color 

 after it has stood for some time. The change in color is due in 

 most instances to the multiplication of pigment-forming bacteria, 

 which, it is true, are almost universally present in milk, but ordi- 

 narily do not grow in sufficiently great measure to impart their 

 peculiar pigment to the milk. Thus milk sometimes appears 

 blue, red, yellow, chocolate brown, and other colors. 



The bacteria producing these pigments are not related to 

 each other in such manner as to form a distinct group. The 

 chief property in common is their ability to produce pigment; 

 otherwise there is no marked resemblance. The pigments are 

 quite different when produced by bacteria of various groups. 

 For example, a yellow sarcina produces a yellow sediment in 

 milk; Bacillus prodigiosus produces a red pigment; B. cyanogenes, 

 a blue pigment. These pigments are not always in evidence even 

 when the bacteria are present, as certain conditions are required 

 for the formation of pigments. Acid is sometimes inhibitory to 

 pigment formation, as, for example, in the case of the fluorescent 

 pigment of Bacillus fluorescens. The acid which develops in 

 milk soon after milking prevents the formation of the fluorescent 

 pigment, even though the bacilli may be present in fairly large 

 numbers. The blue pigment produced by Bacillus cyanogenes, 

 on the other hand, becomes more intense when acid develops, 

 although in this instance the result is probably due to increased 

 growth rather than to the formation of a pigment of greater 

 intensity. A sort of metabiosis between the lactic acid bacteria 

 and Bacillus cyanogenes is responsible for greater growth of the 

 latter in presence of acid than without the acid. 



The presence of free oxygen is necessary for the formation of 

 many pigments, so that these appear near the surface of the milk 

 rather than in deep layers. There are, however, exceptions to 

 this rule. Bacillus erythrogenes, for example, forms pigment in 

 the absence of free oxygen. A stab-culture of this organism de- 

 velops pigment in absence of free oxygen, but it grows well also 

 under aerobic conditions. 



Temperature is an important factor in pigment formation. 

 At 37 C. many chromogenic bacteria do not produce pigment, 

 while at a lower temperature it may be formed in abundance. 

 Light has an inhibitory effect on pigment formation, but in an 

 opaque fluid like milk light is unable to penetrate to an appre- 

 ciable depth, and pigment bacteria are not materially influenced. 



Finally, it should be remembered that bacteria have a tendency 

 to rise with the cream, so that the cream layer frequently shows 



