Identify the Gram-negative genus: Which of the following bacterial groups includes Gram-negative organisms?

Introduction / Context:
An essential skill in medical and food microbiology is to quickly classify organisms by Gram reaction. Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane with lipopolysaccharide, which influences staining, antibiotic susceptibility, and pathogenesis.

Given Data / Assumptions:

• Candidates: Escherichia, Clostridia, Staphylococci.
• We are asked which is Gram-negative.
• Standard Gram stain characteristics apply.

Concept / Approach:

Escherichia (e.g., E. coli) belongs to the Enterobacterales and stains Gram-negative. Clostridia (e.g., Clostridium perfringens, C. botulinum) and Staphylococci (e.g., Staphylococcus aureus) are Gram-positive, possessing thick peptidoglycan and no outer membrane. Therefore only Escherichia fits the Gram-negative category among the options provided.

Step-by-Step Solution:

Verification / Alternative check:

Laboratory Gram stain and selective/differential media (e.g., MacConkey agar for Escherichia) corroborate Gram-negative identity. Biochemical profiles (lactose fermentation) further support classification.

Why Other Options Are Wrong:

• Clostridia: Gram-positive, obligate anaerobes, endospore formers.
• Staphylococci: Gram-positive cocci, catalase-positive, cluster-forming.
• All of these: false because two of the three are Gram-positive.

Common Pitfalls:

• Mistaking Gram-variable results (aging cultures) for true Gram reaction.
• Confusing “enteric” with “Gram-positive”; most enterics (including Escherichia) are Gram-negative.

Final Answer:

Escherichia