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)

Some bacteria are extremely picky about what their growth media provide, and Francisella tularensis is a classic example. It will only form colonies on media that supply cysteine or cystine because it cannot grow well without this amino acid. In the lab, F. tularensis is routinely cultured on cysteine-enriched media (for example, cysteine heart infusion agar or other cysteine-supplemented media). Without this nutrient boost, it won’t grow, which makes its identification rely on these specific culture requirements. The other organisms have different growth needs: they do not require cysteine specifically and can grow on standard enriched media or on media supplemented with other factors (for instance, Kingella kingae needs X and V factors, not cysteine; Bartonella and Actinobacillus species have their own distinct requirements). So the reliance on cysteine or cystine to support growth points to Francisella tularensis.