DEHYDRATED CULTURE MEDIA 179 



A Lead Semisolid Agar was developed by Friewer and Shaughnessy^ as a 

 screening medium, in the preliminary classification of the Salmonella-Shigella 

 group. The determination of motility, considered by some workers to be most 

 helpful as an early step in the classification of the group, is achieved by the use 

 of a semisolid agar. This Lead Semisolid Agar also contained lactose and brom 

 thymol blue in addition to lead acetate. It was possible, therefore, with this 

 single medium to determine motility, the production of acid or acid and gas from 

 lactose and the production of hydrogen sulfide. These characteristics, i.e., motility, 

 lactose fermentation and hydrogen sulfide production, are essential criteria in 

 practically all methods of classification of members of the enteric group. 



Using this medium under practical conditions, colonies may be picked from 

 any primary plating medium such as Bacto-S S Agar, Bacto-Bismuth Sulfite Agar 

 or Bacto-MacConkey Agar, or inoculation may be made from a broth or agar 

 subculture from these media to the Friewer Shaughnessy Medium. 



If there is any question as to the purity of the suspected colony, it should be 

 inoculated into a broth or peptone tube and streaked on a plating medium such 

 as Bacto-MacConkey Agar or Bacto-E.M.B. Agar, to establish its purity, before 

 proceeding with the screening test. A stab inoculation, using a straight wire, and 

 extending down the center of the medium to approximately one-half its depth, is 

 most satisfactory. 



After 12-24 hours incubation at 37°C. the tubes are read for the following 

 information : 



1. Motility. A motile culture is characterized by a diffusion of the growth 

 from the line of inoculation. Some cultures, such as certain members of the 

 Proteus and Salmonella groups, are actively motile, producing complete diffusion 

 throughout the medium. Other cultures, as for example most strains of 

 Salmonella typhosa, are less actively motile, and growth extends out from the 

 line of inoculation without giving the appearance of dilTuse turbidity. This 

 difference in the degree of motility is an aid in the choice of subsequent tests, for 

 the cultures with moderate diffusion from the line of inoculation plus a hydrogen 

 sulfide reaction can be selected for special tests for S. typhosa. 



Shigella organisms are non-motile and grow only along the line of inoculation. 

 Strictly aerobic rods and cocci grow only on the surface of the medium and are 

 readily differentiated from the non-motile facultative anaerobes such as members 

 of the Shigella group. 



2. Hydrogen Sulfide. The production of a brown or black color is an indica- 

 tion that hydrogen sulfide has been produced. Motility accentuates the diffusion 

 of the hydrogen sulfide, whereas in a non-motile culture the brown or black 

 coloration is confined to the line of inoculation. 



3. Lactose Fermentation. When lactose is fermented, the medium in the 

 growth area develops a yellow color. Formation of gas is evidenced by gas bubbles 

 in the medium or at the surface. The intensity of the lead sulfide reaction is not 

 great enough to mask the color change. 



The following key, suggested by Friewer and Shaughnessy,^ may be used as a 

 guide for the identification of the pathogenic Gram-negative bacilli. Serological 

 reactions are used for final confirmation. 



J Fermentation of Lactose. 



A. Acid and Gas— DISCARD. 



B. Acid fermentation, motile culture — DISCARD. 



C. Acid fermentation, non-motile culture (see non-motile cultures, III, 



below). 



II Surface Growth Reactors. (No growth along line of stab, growth only at 

 surface). 



