Which microorganism will grow only on culture media supplemented with either cysteine or cystine?

• A. Actinobacillus lignieresii
• B. Bartonella bacilliformis
• C. Francisella tularensis
• D. Kingella kingae

Answer: (C)

Growth factor needs determine what media will support a microorganism’s growth. Francisella tularensis is a classic fastidious organism that cannot synthesize enough cysteine on its own and therefore must be provided with an external supply of cysteine or cystine. Because of this strict requirement, it will grow only on media that include cysteine or cystine, making that supplement essential for recovering this bacterium in culture.

In contrast, the other organisms listed do not rely on cysteine supplementation as a strict growth requirement. They can be cultured on standard or enriched media under appropriate conditions, so they are not limited to cysteine-containing media for growth.