40. Which of the following is the example of Gram-negative bacteria?
- Lactobacillus
- Escherichia coli
- Bacillus subtilis
- Staphylococcus aureus
Escherichia coli is the classic example of a Gram-negative bacterium among the options. This classification stems from its thin peptidoglycan layer and outer lipopolysaccharide membrane, which cause it to appear pink under Gram staining.
Gram Staining Basics
Gram staining differentiates bacteria by cell wall structure: Gram-positive retain crystal violet (purple) due to thick peptidoglycan; Gram-negative lose it and take safranin counterstain (pink/red).
Gram-negative feature a thin peptidoglycan layer sandwiched between inner cytoplasmic and outer membranes rich in lipopolysaccharides (LPS/endotoxin).
This impacts antibiotic sensitivity, virulence, and lab ID.Correct Answer: Escherichia coli
Escherichia coli (E. coli) is rod-shaped (bacillus), facultative anaerobic, and hallmark Gram-negative with outer LPS membrane—key model organism in Enterobacteriaceae.
It causes UTIs, food poisoning; stains pink; sensitive to some antibiotics due to outer barrier.Option Breakdown
Option Gram Status Key Traits & Reason Correct? Lactobacillus Positive Lactic acid bacteria; thick peptidoglycan; Gram+ rods/cocci in yogurt, gut. No Escherichia coli Negative Thin wall + LPS outer membrane; pink stain; enteric pathogen/model. Yes Bacillus subtilis Positive Spore-forming rods; thick wall; soil/environmental Gram+. No Staphylococcus aureus Positive Cocci clusters; thick peptidoglycan; causes skin infections, MRSA. No Clinical Relevance
Gram-negatives like E. coli drive hospital infections (e.g., sepsis) due to LPS endotoxins and resistance.
All listed alternatives are Gram-positive with no outer membrane.
