Which of the following is characterized as a "gram-negative bacilli"?

Pseudomonas aeruginosa is accurately characterized as a gram-negative bacilli. This bacterium is rod-shaped and is known for its resistance to many antibiotics, making it a significant pathogen in various healthcare-associated infections, particularly in immunocompromised patients. Its gram-negative classification is due to its cell wall structure, which includes a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides, giving it distinctive staining properties. This characteristic plays an essential role in its pathogenicity and efficacy in causing infections in different body systems, especially the lungs and urinary tract.

The other options represent different types of bacteria. Staphylococcus aureus is a gram-positive cocci, Streptococcus pneumoniae is also gram-positive cocci, and Clostridium botulinum is a gram-positive bacillus. Thus, Pseudomonas aeruginosa stands out as the only gram-negative bacilli among the options provided.