Gram-negative bacteria feature a thin peptidoglycan layer and an outer membrane containing lipopolysaccharides, causing them to stain pink in the Gram technique. Among the provided choices:
1. Lactobacillus: A Gram-positive bacterium, frequently found in probiotics.
2. Escherichia coli (E. coli): A Gram-negative bacterium, typically found in the gut of warm-blooded organisms.
3. Bacillus subtilis: A Gram-positive bacterium, broadly used in industrial processes.
4. Staphylococcus aureus: A Gram-positive bacterium, known to cause infections such as MRSA. Therefore, Escherichia coli is the identified Gram-negative bacterium.