LACTIC ACID FERMENTATION 195 



usually smooth, free from gas bubbles, has a pleasant acid 

 flavor and with little or no tendency to shrink and expel the 

 whey. Its optimum growth temperature, apparently, is 

 about blood heat, but it grows well (particularly in milk) at 

 room temperatures and slowly at the temperature of the ice 

 chest. Apparently there are strains of this organism which 

 can resist the ordinary temperatures used in pasteurization. 

 This is evidenced by the fact that milk which has been pas- 

 teurized in commercial plants usually retains sufficient num- 

 bers of these organisms in the living condition so that it 

 sours. 



The Grenus Lactobacillus. The bacteria of this genus 

 are all Gram-positive, nonspore-producing rods developing 

 commonly in milk and various fermented foodstuffs. A 

 few species have been reported from the alimentary tract 

 of man and animals. The group is sometimes known as the 

 group of high acid bacteria because certain of the species 

 will grow in a medium having a higher hydrogen ion con- 

 centration than will permit the growth of most bacteria, 

 even of the Streptococci. Some of the species are relatively 



a bed 



FIG. 50. LACTIC ACID BACTERIA. A. Streptococcus lacticus. B. Lac- 

 tobacillus bulgaricus. G. Lactobacillus lactus acidi. D. Bacterium 

 aero genes. 



short rods. Most of them, however, are decidedly elongated, 

 sometimes almost filamentous. The shortest rod forms 

 apparently intergrade with the Streptococci; it is accord- 

 ingly sometimes difficult to differentiate between Strepto- 

 coccus lacticus and the Lactobacillus lactis acidi. 



One of the best known species belonging to this group is 

 the Lactobacillus bulgaricus. The organism was first 



