DEHYDRATED CULTURE MEDIA 131 



Primary Plating Media 



The media listed in this section are used generally in petri dishes for initial 

 isolation. In most cases the composition of the medium is such that it is selective 

 for a particular group of organisms. 



BACTO 

 MacGONKEY AGAR (B75) 



DEHYDRATED 



Bacto-Peptone 17 g. 



Proteose Peptone, Difco 3 g. 



Bacto-Lactose 10 g. 



Bacto-Bile Salts No. 3 1.5 g. 



Sodium Chloride 5 g. 



Bacto-Agar 13.5 g. 



Bacto-Neutral Red 0.03 g. 



Bacto-Grystal Violet 0.001 g. 



Bacto-MacConkey Agar is a differential plating medium recommended for use 

 in the detection and isolation of all types of dysentery, typhoid and paratyphoid 

 bacteria from stool specimens, urine and other materials harboring these organ- 

 isms. MacConkey Agar is suggested for direct plating of water samples for coli- 

 form counts in Appendix I of "Standard Methods for the Examination of Water 

 and Sewage."^ "Standard Methods for the Examination of Dairy Products"^ pre- 

 fers dehydrated Bacto-MacGonkey Agar for the isolation of pathogenic bacteria 

 from cheese. MacConkey Agar is also specified in "Diagnostic Procedures and 

 Reagents"^ of the American Public Health Association. 



Over a period of years, particularly in Great Britain, the Neutral Red Bile Salt 

 Agar of MacConkey* has been quite generally used for differentiating strains of 

 Salmonella typhosa from members of the coliform group. Bacto-MacConkey 

 Agar, as modified by the addition of 0.5 per cent sodium chloride, decreasing the 

 agar content to 1.35 per cent, and by altering the concentrations of bile salts and 

 neutral red, has the added advantage of supporting excellent growth of all 

 Shigella and Salmonella strains. It also gives a more clear-cut differential be- 

 tween these enteric pathogens and the coliform group, making it easier to read 

 than the original medium. Block and Ferguson^ investigating an outbreak of 

 Shiga dysentery found MacConkey Agar satisfactory in the isolation of this fas- 

 tidious strain. 



The fact that this medium promotes development of these organisms, and at 

 the same time differentiates them from lactose fermenting Gram-negative bacilli, 

 makes it an excellent substrate for the cultural detection of dysentery, typhoid and 

 other Salmonella organisms in stools and other infected material. Gram-positive 

 bacteria are inhibited. 



About 20 ml. of medium should be poured into previously sterilized petri 

 dishes to form a relatively thick layer. It is important that the surface of the 

 medium be quite dry when inoculated; this may be accomplished by allowing 

 the medium to solidify and to stand for about two hours with the covers of the 

 plates partially removed. 



The medium is inoculated by streaking or smearing with the material under 



