Streak-stab technique & MacConkey agar

Streak-stab technique

Often when inoculating a BAP to observe hemoloysis patterns, investigators will also stab several times through the agar using an inoculating loop. This stab allows for the detection of streptolysin O, a specific hemolysin produced by Streptococcus pyogenes. This hemolysin is inactivated by O₂ and is only seen subsurface (in an anaerobic environment) around the stab mark. Note the oval-shaped areas of clearing around the stab marks in the picture below; these are caused by streptolysin O.

MacConkey agar

This medium is both selective and differential. The selective ingredients are the bile salts and the dye, crystal violet which inhibit the growth of Gram-positive bacteria. The differential ingredient is lactose. Fermentation of this sugar results in an acidic pH and causes the pH indicator, neutral red, to turn a bright pinky-red color. Thus organisms capable of lactose fermentation such as Escherichia coli, form bright pinky-red colonies (plate pictured on the left here). MacConkey agar is commonly used to differentiate between the Enterobacteriaceae.

Organism on left is positive for lactose fermentation and that on the right is negative.