Which one of the following bacteria is a gram-negative coccus?

Introduction:
Gram reaction and shape are foundational identifiers in bacteriology. Most cocci are gram-positive, but a few genera are gram-negative.

Given Data / Assumptions:

• We must pick the gram-negative coccus from the list.
• Options include common gram-positive anaerobic cocci and one gram-negative genus.

Concept / Approach:
Veillonella are small, anaerobic, gram-negative cocci typically found in the oral cavity and gut, unlike Peptococcus, Peptostreptococcus, Sporosarcina, and Staphylococcus, which are gram-positive.

Step-by-Step Solution:
Recall Gram characteristics for each genus. Identify the exception among predominantly gram-positive cocci. Veillonella fits as gram-negative cocci; others are gram-positive.

Verification / Alternative check:
Microbiology texts classify Veillonella as gram-negative anaerobic cocci.

Why Other Options Are Wrong:

• Peptococcus / Peptostreptococcus: Gram-positive anaerobic cocci.
• Sporosarcina / Staphylococcus: Gram-positive cocci (Sporosarcina is spore-forming).

Common Pitfalls:
Assuming all cocci are gram-positive; Veillonella and Neisseria are important exceptions (Neisseria not listed among the correct options here).

Final Answer:
Veillonella is the gram-negative coccus among the choices.