The main differences among the individual aminoglycosides lie in their activities against specific organisms, particularly gram-negative rods. Gentamicin,tobramycin, and amikacin are important drugs for the treatment of serious infections caused by aerobic gram-negative bacteria, including Escherichia coli and Enterobacter, Klebsiella, Proteus, Providencia, Pseudomonas, and Serratia species. These aminoglycosides also have activity against strains of Haemophilus influenzae, Moraxella catarrhalis, and Shigella species, although they are not drugs of choice for infections caused by these organisms. In most cases, aminoglycosides are used in combination with a beta-lactam antibiotic. When used alone, aminoglycosides are not reliably effective in the treatment of infections caused by gram-positive cocci. Antibacterial synergy may occur when aminoglycosides are used in combination with cell wall synthesis inhibitors. Examples include their combined use with penicillins in the treatment of pseudomonal, listerial, and enterococcal infections.