32 CHANGES IN MILK CAUSED BY BACTERIA. 



Biological characteristics of B. lactis viscosus. The organism 

 has been isolated from water, which is considered its natural 

 habitat. Growth is fairly luxuriant at 15° C. (59" F.), best 

 at 25" C. (77° F.) and very slight at 37° C. (98° F.). How- 

 ever, 8° C. (46° F.) is not low enough to inhibit growth. The 

 organisms multiply rapidly enough in ice water to induce vis- 

 cosity in milk kept in cans in ice water. That is, this organ- 

 ism forms an exception to the rule that care in cooling milk 

 inhibits the multiplication of bacteria. At low temperatures 

 the ropy milk organisms have a comparatively clear field for 

 growth, free from serious competition with the lactic acid or- 

 ganisms. At higher temperatures, such is not the case. B. 

 lactis viscosus will grow only in the presence of air, which ac- 

 counts for the appearance of viscosity first on the top of milk. 

 This biological peculiarity is doubtless the reason that ropy 

 milk is never, or rarely, seen in bottled milk. The organism 

 does not form spores, and is killed in culture by exposure to 

 58° C. (136" F.) for ten minutes (16, 41). 



Sources of contamination of milk. The fact that .5. lactis vis- 

 cosus grows with great difficulty at blood heat, effectually dis- 

 poses of any suspicion that ropy milk is the result of disease 

 in the cow. Water must be regarded as the original source of 

 trouble, but it may by no means be the immediate oije. The 

 cow may, sometimes, carry the organisms on the hairy coat, 

 from water to the milk. Several observers have isolated ropy 

 milk organisms from stable air. When ropy milk is causing 

 trouble, the organisms are found with great frequency in sam- 

 ples taken from the floor dust of milk rooms. The practice of 

 cooling cans of milk nearly immersed in a vat of ice water af- 

 fords, in the milky water, favorable conditions for the multi- 

 plication of the organisms and for the contamination of the 

 milk by them. The spattering of water, incident to moving 

 cans or adding ice, gives opportunity for the contamination of 

 the milk. The corrugated milk cooler, containing water in 

 circulation, may have a leak, giving opportunity for direct 

 contamination from water. The practice of merely rinsing 

 utensils at night and using them in the morning without scald- 

 ing, gives another opportunity for bringing water into contact 



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