Gram's stain distinguishes between bacteria with walls containing little peptidoglycan and lipopolysaccharide (Gram-negative) from bacteria with walls containing lots of peptidoglycan and no lipopolysaccharide (Gram-positive).
Salmonella typhi is a Gram-negative bacillus belonging to the family Enterobacteriaceae. Salmonella typhi cells can grow with or without oxygen. They use flagellae for motility. They do not form spores. An estimated 107 different strains of Salmonella typhi have been isolated. Salmonella typhi causes typhoid. It is dependent on humans for transmission since it arises from the contamination of food or water with human waste. S. typhi is a strong, virulent pathogen containing an endotoxin and Vi antigen. It excretes the protein, invasin, which allows non-phagocytic cells to take in the bacteria. Symptoms of infection with S. typhi include fever, severe headache, nausea, spleen enlargement, and more. Infected individuals treated with antibiotics have a high survival rate.Those left untreated have a mortality rate of approximately 12-30%.