RESISTANCE TO HEAT, AND TO VARIOUS CHEMICAL SUBSTANCES 659 



In faeces stored at 0° C. colifonn organisms may be demonstrated for a year or 

 more, the coli type being often gradually supplanted by the intermediate and 

 aerogenes types (Parr 1938). 



There are certain chemical substances which exert a definitely selective 

 bactericidal or inhibitory action. 



The typhoid bacillus is less resistant to the lethal action of mineral acids than is the 

 colon bacillus. Winslow and Lochridge (1906) showed that the bactericidal eflFect was due 

 to the action of the dissociated hydrogen ions ; and found that the concentration required 

 to bring about a 99 per cent, reduction in the viable organisms in a bacterial suspension was 

 2-94 per mihion in the case of Salm. typhi, and 7-49 per million in the case of Bad. coli. 



Malachite green, in suitable concentration, kills Bact. coli or inhibits its growth without 

 exerting the same effect on Salm. typhi (Loeflfler 1903, 1906, Lentz and Tietz 1903, 1905). 

 There are other green dyes that have a" similar selective action ; and more recent studies 

 (see Browning, Gilmour and Mackie 1913, Krumwiede and Pratt 1914) have shown that 

 brilliant green gives the best differential results. To this dye the bacilli of the para- 

 typhoid group are most resistant, the typhoid bacillus is somewhat less resistant, while 

 the dysentery bacilli, and still more the members of the Bad. coli group, are very susceptible. 



Caffeine (Roth 1903, Hoffman and Ficker 1904) and hthium chloride (Gray 1931, Havens 

 and Mayfield 1933) are other substances that inhibit the growth of Bad. coli in concentra- 

 tions that have no effect on the typhoid bacillus ; while cholesterol (see Manfredi 1917) 

 appears to inhibit the growth of typhoid or paratyphoid bacilli in concentrations that 

 permit the growth of Bad. coli. 



•Sodium desoxycholate in the presence of sodium citrate inhibits the growth of coh- 

 form bacilli, while having little effect on organisms of the Salmonella and Shigella groups 

 (Leifson 1935, Hynes 1942). Potassium tellurite, in suitable concentration, in the presence 

 of iron alum, is said to inhibit coUform and salmonellae, but not the Flexner dysentery 

 bacilli (Wilson and Blair 1941). Selenium salts were found by Haendel (see Guth 1916, 

 Leifson 1936) to inhibit coliform more than typhoid baciUi ; and tetrathionate was 

 found by MuUer (1923) (see also Knox, Gell and Pollock 1943) to have much the same 

 effect. 



Differences of this kind have not, however, been employed for the purposes of identi- 

 fication or classification. They have, on the other hand, been extensively exploited in 

 devising selective, or " enrichment," media for the isolation of the pathogenic species 

 from faeces or water. They are considered from this point of view in Chapters 69 and 

 92. 



Biochemical Activities. — From the first isolation of Bact. coli by Escherich, 

 fermentation tests were found to provide the readiest method of distinguishing one 

 species of Bacterium from another. It was soon found, for instance, that Bact. 

 coli actively fermented lactose, while Salm. typhi did not (Chantemesse and 

 Widal 1887, Smith 1890) ; and the production of acid and gas from glucose by 

 Bact. coli, but of acid alone by Salm. typhi, was pointed out by Chantemesse 

 and Widal in 1891. The addition of a suitable indicator to the test media, to 

 register acidity (Wurtz 1892), and the introduction of the simple fermentation tube 

 as a test for gas production (Smith 1890, 1893, Durham 1898) greatly increased the 

 facility with which a large series of comparative qualitative tests could be carried 

 out. To dextrose and lactose other test substances have, from time to time, been 

 added, such as the hexoses, fructose, laevulose and galactose ; the disaccharides, 

 maltose and saccharose ; the trisaccharide, rafiinose ; polysaccharides, such as 

 dextrin, starch and inulin ; the pentoses, arabinose and xylose ; the methyl-pentose, 

 rhamnose ; the hexahydric alcohols, dulcitol and mannitol ; the glucoside, salicin ; 

 and the cyclohexanehexol, inositol. The reaction in litmus milk, the presence or 



