182 DIFCO MANUAL 



BACTO 



SIMMONS CITRATE AGAR (B91) 



DEHYDRATED 



Magnesium Sulfate 0.2 g. 



Monoammonium Phosphate 1 g. 



Dipotassium Phosphate 1 g. 



Sodium Citrate 2 g. 



Sodium Chloride 5 g. 



Bacto-Agar 15 g. 



Bacto-Brom Thymol Blue 0.08 g. 



Bacto-Simmons Citrate Agar is a medium capable of differentiating between 

 fecal coli and members of the aerogenes group on the basis of citrate utilization. 

 It may also be used to differentiate certain members of the Salmonella group. 

 Bacto-Simmons Citrate Agar is prepared according to the formula of Simmons.^ 



The differential between fecal coli and aerogenes as described by Koser- is 

 based upon the inability of fecal coli to develop in a medium containing inorganic 

 ammonium salts as the only source of nitrogen with citrate as the sole source 

 of carbon, whereas strains of aerogenes grow quite unrestrictedly. Likewise, 

 Salmonella typhosa, S. paratyphi and Shigella can be differentiated from S. 

 5chottm,uelleri, S. enteritidis and S. typhimurium, the latter group being able to 

 utilize citrate in such a medium while the former cannot. 



This principle was first employed in a liquid medium by Koser. The liquid 

 medium had the disadvantage of appearing turbid when large inocula were 

 used even when no growth ensued. This observation led Simmons^ to devise a 

 solid medium which obviates the disadvantage of turbidity as a criterion of 

 growth. 



Simmons' Medium is essentially Koser's Medium to which brom thymol blue 

 and 1.5 per cent agar have been added. It is a valuable aid in differentiating 

 certain members of the Salmonella group from S. paratyphi, S. typhosa and 

 Shigella. The medium is usually prepared as agar slopes, inoculated by stab and 

 streak, and incubated at 37°G. Some workers, however, prefer to use the medium 

 in petri dishes. On such a solid medium, growth is indicated very clearly by 

 colony formation and is usually accompanied by a color change of the indicator 

 due to acid or alkali production. Strains of aerogenes grow luxuriantly upon Sim- 

 mons Citrate Agar, producing alkali and changing the medium from its initial 

 green to deep blue in 24-48 hours. Fecal coli either do not grow at all upon this 

 medium or grow so sparsely that no change in reaction is apparent. S. schott- 

 muelleri, S. enteritidis and S. typhimurium, which develop on the medium, pro- 

 duce alkali as indicated by a deep prussian blue color. 



Simmons also recommended its use for isolating and identifying certain fungi 

 and fungi imperfecti. 



To rehydrate the medium, suspend 24.2 grams of Bacto-Simmons Citrate Agar 

 in 1000 ml. of cold distilled water and heat to boiling to dissolve the medium 

 completely. Distribute in tubes or flasks and sterilize by autoclaving for 15 minutes 

 at 15 pounds pressure (121° C). Allow the tubed medium to cool in a slanting 

 position. The final reaction of the medium will be pH 6.8. 



One pound of Bacto-Simmons Citrate Agar will make 18.7 liters of medium. 



1 J. Infectious Diseases, 39:209:1926. ^ J. Bact., 8:493:1923. 



