754 LACTOBACILLUS 



further distinguished from most of the other members of the group by its failure to 

 ferment lactose or to curdle milk. Cruickshank (1934) says that Doderlein's bacillus 

 ferments glycogen with the production of lactic acid in a few days, while strains of 

 acidophilus, odontolyticus, and hifidus take 7-10 days to ferment it ; bulgaricus does 

 not ferment it at all. It is very doubtful whether the reactions of individual species 

 and strains are constant ; according to some authors they are not (Mcintosh et al. 

 1924, Day and Gibbs 1928). Most, but not all, the members produce acid in milk, 

 often in sufficient quantity to precipitate the casein in the form of a loose clot, which 

 does not contract and express whey ; the litpaus is frequently decolorized, especially 

 in the lower part of the tube. The rate at which clotting is produced is of some slight 

 differential significance. White and Avery (1910) divided the acid-resistant 

 organisms obtained from milk into two types ; their Type A produced a large 

 quantity of lactic acid in milk — 2-7 to 3-7 per cent. ; their Type B produced a 

 smaller quantity — 1-2 to 1-6 per cent. The most active acid producer is L. bul- 

 garicus ; the least active L. hifidus ; L. acidophilus occupies an intermediate 

 position. 



Antigenic Structure, — The serological reactions of these organisms have been 

 incompletely studied, and so far no satisfactory classification of the group has been 

 possible by agglutination or absorption. Generally speaking, the members of a 

 single species show a considerable amount of heterogeneity (Kendall and Haner 

 19246, Lash and Kaplan 1926, Thomas 1928, Howitt 1930). Mcintosh and his 

 co-workers (Mcintosh et al. 1924), however, observed a marked group reaction 

 between members of the acidophilus and acidophilus-odontolyticus types ; and 

 Cruickshank (1925) and Weiss and Rettger (1934) found a close relationship between 

 L. hifidus and L. acidophilus. Thomas (1928) found that Doderlein's bacillus had 

 some relation to L. acidophilus, but none to L. bulgaricus. Working with oral 

 strains, many of which had been isolated several months previously, Harrison, 

 Zidek and Hemmens (1939) extracted a carbohydrate-containing substance from 

 lactobacilli, and by means of precipitating antisera prepared by the inoculation 

 of rabbits with heat-killed bacilli were able to divide the strains into four sero- 

 logical types and a heterogeneous group. 



Pathogenicity. — None of the members appears to be pathogenic to man or 

 animals. It is true that, in fermentative diarrhoea, acid-resisting bacteria may be 

 present in large numbers in the stools, but whether they are responsible for initiating 

 the diarrhoea, or whether they merely take advantage of the abnormal conditions 

 prevailing in the intestine to multiply abundantly, is not clear. Their numbers 

 increase in the intestine when lactose or dextrin are given in considerable quan- 

 tities in the diet (Eettger and Cheplin 1921, Cannon and McNease 1923) ; and 

 because, under these conditions, they tend to replace the proteolytic flora, their 

 administration along with these sugars has been advocated for therapeutic purposes. 



There is reason to believe that the oral lactobacilli play some part in the 

 development of dental caries. The opinion that this disease is due to the action 

 of acids formed in the mouth from retained food was put forward a long time 

 ago by Robertson (1835), Tomes (1873), and others. It was not, however, till 

 the appearance of Miller's (1889) monograph, in which it was maintained that 

 lactic acid resulting from the bacterial fermentation of starch and sugar was 

 primarily responsible for decalcification and solution of the enamel, that serious 

 suspicion was thrown on the acid-forming flora of the mouth. Though the strepto- 

 cocci have received attention from a number of workers, (see Snyder 1939), it 



