Which amino acids are essential for the growth of Francisella tularensis?

Francisella tularensis, the causative agent of tularemia, is an intracellular bacterium that has specific nutritional requirements for growth, which includes an essential need for certain amino acids. Cysteine and cystine are particularly important for this organism because they serve as sources of sulfur and play key roles in cellular metabolism and the synthesis of important biomolecules, including proteins and coenzymes.

Cysteine is crucial for the formation of disulfide bonds in proteins, influencing protein structure and function, while cystine, a dimer of cysteine, is involved in maintaining redox balance within the cell. The inability of F. tularensis to synthesize certain amino acids means it must acquire them from its environment, particularly when grown in laboratory settings.

Other options like leucine, arginine, lysine, histidine, and tryptophan do not have the same critical roles in the growth of Francisella tularensis compared to cysteine and cystine, making them non-essential for its growth in the same sense that cysteine is. Thus, it's the dependence on cysteine and cystine that makes this pair the correct answer regarding essential amino acids for the growth of this particular bacterium.