28 STUDIES ON DAHI. III—TAXONOMY OF THE LACTIC ACID, ETC. [Vol. XXII, Part I
capacity of the species in distinguishing one from the other and considered that
sugar fermentations, morphology and cultural characteristics were of secondary im-
portance. His grouping of the organisms into homofermentative and heterofer-
mentative types and of the homofermentative lactobacilli into thermobacteria and
streptobacteria is based on such considerations. Although this scheme has been
generally accepted by later workers, the differentiation of closely allied species of
organisms has been a controversial topic.
One of the most classical controversies has been with regard to the identification
of L. bulgaricus and L. acidophilus. Several workers have attempted to differentiate
between the two organisms on the basis of sugar fermentations [Kulp and Rettger,
1924], inhibition by indol and phenol [Kulp, 1926] and the effect of surface tension
depressants [Albus and Holm, 1926 ; Kopeloff and Beerman, 1925]. These tests
were reported to be unreliable by Day and Gibbs [1928]. Kulp and Rettger [1924]
stated that the ability of L. acidophilus to survive in the intestines when the culture
was fed to laboratory animals was its most important distinguishing feature since
cultures of L. bulgaricus could not be implanted in the intestines. Curran, Rogers
and Whittier [1933] employed growth temperatures, colony appearance, fermentation
of raffinose and mannitol, optical rotation of the acid produced and tolerance to
phenol and indol, while Sherman and Hodge [1940] used salt tolerance, influence of
pH., growth temperature and fermentation of sugars as criteria for differentiating
between the two organisms as also for separating other species of lactobacilli. Lazi-
cova [1939] was of the opinion that both the organisms belonged to the same species
and any differences observed between the two were only due to strain variations.
On the other hand, L. acidophilus and L. casei were considered to be R and S forms
of a single type by Pederson [1947], who found that the strains of L. acidophilus,
originally described as forming round and filamentous colonies and producing inactive
acid [Curran, Rogers and Whittier, 1933], had changed into smooth forms producing
dextro-rotatory acid and showing other typical characteristics of L. casei. The
former is an intestinal organism while the latter is always associated with milk
and milk products and the gradual transformation in the characteristics of L. acid-
ophilus may be attributed to dissociation of the original R straius into R and S
forms and the subsequent preponderant development of the S forms during continued
maintenance in milk. The reversal of this process has not been reported by any
worker so far. Similar variations in characteristics due to dissociation of the organ-
isms have been observed [Okulitch, 1939 ; Barber and Frazier, 1945]. The distinctive
features which characterise L. casei, L. plantarum and heterofermentative lactobacilli
found in cheese have been described by Davis [1935] and Sherwood [1939]. The
former investigator, stressing the importance of ecological aspects in characterising
the organisms, suggested that they would influence the preference of the organisms
to grow in a particular medium or to ferment a particular sugar especially in the
case of freshly isolated strains since on prolonged cultivation in the laboratory,
the natural properties of the organisms are likely to undergo some variation. A
detailed description of various species of lactobacilli has also been given by Holland
[1920], Fred, Peterson and Anderson [1921 and 1922] and Pederson [1936 and 1938].
Recently, Tittsler, et al. [1947] has attempted to classify the lactobacilli on the basis
of their colony formation, optical activity of lactic acid produced, maximum acidity
produced, growth temperature and ability to produce gas.