"Gram-negative" refers to a classification of bacteria based on their cell wall structure. These bacteria do not retain the crystal violet stain used in the Gram staining procedure, appearing pink or red under a microscope. This characteristic is due to their thin peptidoglycan layer and an outer membrane that contains lipopolysaccharides, which can make them more resistant to certain antibiotics compared to Gram-positive bacteria. Many pathogenic bacteria, such as Escherichia coli and Salmonella, are Gram-negative and can cause various infections in humans. Their unique structure not only helps them evade the immune system but also poses challenges in treatment, making it essential for healthcare professionals to understand their properties for effective management and prevention of infections.