Which organism is commonly lactose fermenter on MacConkey agar?

• A. Staphylococcus aureus
• B. Streptococcus pneumoniae
• C. Pseudomonas aeruginosa
• D. Escherichia coli

Answer: (D)

MacConkey agar distinguishes lactose fermenters from nonfermenters among Gram-negative bacteria by a pH indicator that turns pink when acid is produced from lactose. Escherichia coli is a classic lactose fermenter; it rapidly metabolizes lactose, acidifying the medium and producing pink/red colonies (often with surrounding precipitated bile). In contrast, organisms like Pseudomonas aeruginosa do not ferment lactose and tend to produce colorless colonies, and Gram-positive bacteria such as Staphylococcus aureus or Streptococcus pneumoniae are inhibited by the bile salts/crystal violet in MacConkey and typically do not grow well there. Hence, the organism most characteristically seen as a lactose fermenter on MacConkey agar is Escherichia coli.