RESISTANCE TO HEAT AND TO VARIOUS CHEMICAL SUBSTANCES 705 



aud a more effuse serrated edge ; or they may assume the vine-leaf form, which 

 used to be regarded as characteristic of Salm. typhi. Dwarf colony forms are 

 sometimes met with. They were first described by Jacobsen in 1910, and have 

 since been reported on by several workers (Mellon and Jost 1926, W. J. Wilson 

 1938, Morris, Sellers, and Brown 1941, Morris, Barnes, and Sellers 1943). On 

 ordinary agar the colonies after 24 hours' incubation are only about 0-2-0-3 mm. 

 in diameter. According, however, to the original observations of Jacobsen (1910), 

 which have since been confirmed many times, colonies of more normal size are 

 formed on media containing assimilable sulphur compounds. 



As has already been noted in the previous section, certain species, notably 

 Salm. 'paratyphi B, give rise under favourable conditions to a mucoid growth. 

 Sometimes the colonies are mucoid after 24 hours' incubation ; they are about 

 twice the size of normal colonies and resemble large drops of mucilage (Fletcher 

 1918). More often the mucoid appearance is developed as a secondary phenomenon 

 after prolonged incubation. Thus, if an agar plate is inoculated in three or four 



Fig. 148. — Salm. tyj)hi. Fig. 149. — Salm. typhi-murium. 



Surface colonies on agar, 24 hours, Surface colonies on agar, 24 hours, 



at 37° C. (X 8). at 37° C. (x 8). 



places with the point of a needle, and after one day's incubation at 37° C. is left 

 at room temperature for a few days, large colonies are formed characterized by 

 a depressed centre surrounded by a luxuriant mucoid wall. The " mucoid wall 

 test ", or Schleimwall-Versuch, described originally by Miiller (1910), is of some 

 differential value, being common with most freshly isolated cZ-tartrate-negative 

 strains of Salm. paratyphi B and generally negative with Salm. typhi-murium 

 (Kauffmann 1941). The mucoid material contains a polysaccharide (Birch- 

 Hirschfeld 1935), which appears to be antigenically homogeneous no matter by 

 what species of Salmonella it is formed. 



Resistance to Heat and to various Chemical Substances. — Most members of 

 this group are killed by exposure to a temperature of 55° C. for about 1 hour, 

 or of 60° C. for 15-20 minutes. Many observations have been made on the resist- 

 ance of salmonellae to different chemical reagents, chiefly in an endeavour to 

 prepare selective media on which the growth of coliform bacilli would be inhibited. 

 Malachite green, in suitable concentration, kills Bact. coli or inhibits its growth 

 without exerting the same effect on Salm. typhi (Loeffler 1903, 1906, Lentz and 

 Tietz 1903, 1906). There are other green dyes that have a similar selective action ; 



