Culture media

Clostridium Difficile Agar Base is recommended for selective isolation of Clostridium difficile from food and certain pathological specimens.

Blood Agar Base w/ Nalidixic Acid is recommended for differentiation of haemolytic activity of Streptococci.It is a highly nutritive medium. Microorganisms producing haemolysin give visible haemolytic zones on this medium. Blood Agar Base is modified with addition of Nalidixic acid which acts as an inhibitor for the accompanying flora and to support the growth of Staphylococci, haemolytic Streptococci and Enterococci when supplemented with 5% blood. Nalidixic acid blocks the DNA replication of susceptible bacteria and acts against many Gram-negative bacteria.

This medium detects urease activity by all rapidly urease-positive Proteus organisms and also by other members of Enterobacteriaceae that exhibits a delayed urease reaction. Prolonged incubation may cause alkaline reaction in the medium. A medium without urea serves as negative control to rule out false positive results. Also, all urea test media rely on the alkalinity formation and so they are not specific for determining the absolute rate of urease activity. The utilization of proteins may raise the pH to alkalinity due to protein hydrolysis and excess of amino acids liberation results in false positive reaction.

Originally proposed by Sulkin and Willett and modified by Hajna for identifying Enterobacteriaceae . This medium complies with the recommendation of APHA, for the examination of meat and food products , for the examination of milk and dairy products and for microbial limit test for confirming the presence of Salmonellae and in the identification of gram-negative bacilli. The reactions can be interpreted as follows: Alkaline slant / acid butt- only glucose fermented, Acid slant / acid butt- dextrose and sucrose fermented or dextrose and lactose fermented or all the three sugars, dextrose,lactose and sucrose fermented, Bubbles or cracks present-gas production Black precipitate present-H2S gas production

This medium is highly recommended by APHA for the selective isolation of V. cholerae and V. parahaemolyticus. Enrichment in Alkaline Peptone Water (M618), followed by isolation on TCBS Agar is routinely used for isolation of V.cholerae.

Streptococcus Selection Agar is recommended for selective isolation and enumeration of all types of Streptococci, including group A beta haemolytic strains. Sodium azide, sodium sulphite inhibits gram-negative rods and the crystal violet suppresses Staphylococci. Growth of coliforms, Proteus, Pseudomonas and Bacillus species is markedly suppressed in this medium.

These media are used for the differentiation between Enterobacteriaceae and the members of aerogenes group on the basis of citrate utilization as sole carbon source by IMViC tests.