A gram-negative, fastidious bacillus that is difficult to stain by the Gram stain is

The correct answer is Legionella pneumophila, which is a gram-negative bacillus known for being fastidious and challenging to stain effectively with the Gram stain. This bacterium requires specific growth conditions, such as enriched media and an increased humidity level, as it does not grow well on standard culture media.

Legionella pneumophila is also notorious for its weak reaction to Gram staining, often appearing as Gram-negative rods or even being missed entirely in some preparations due to its thin cell wall structure and the presence of intracellular lipopolysaccharides. The Gram stain method might not adequately reflect its morphological characteristics.

In the context of the other organisms mentioned, while Proteus vulgaris, Brucella abortus, and Edwardsiella tarda are also Gram-negative bacteria, they do not exhibit the same difficulties with staining and growth as Legionella pneumophila. Proteus vulgaris and Edwardsiella tarda grow well on standard media and have more consistent reactions to Gram staining. Brucella abortus, although fastidious, is often highlighted for its unique growth requirements and pathogenic nature but does not share the same staining challenges as Legionella species. Hence, Legionella pneumophila stands out due to its specific characteristics.