Every AMA has a specific effect on selected microorganisms. Depending on the drug pair as well as the organism involved, either synergism (supra-additive effect), additive action, indifference or antagonism may be observed when two AMAs belonging to different classes are used together Every combination is unique; the same drugs may be synergistic for one organism but antagonistic for another. However, general guidelines are: (a) Two bacteriostatic agents are often additive, rarely synergistic,i.e. combination of tetracyclines, chloramphenicol, erythromycin, etc. (b) Two bactericidal drugs are frequently additive and sometime synergistic if the organism is sensitive to both, e.g.: * Penicillin/ ampicillin + streptomycin/ gentamicin or vancomycin + gentamicin for enterococcal SABE. Penicillins by acting on the cell wall may enhance the penetration of the aminoglycoside into the bacterium (c) Combination of a bactericidal with a bacteriostatic drug may be synergistic or antagonistic depending on the organism ESSENTIALS OF MEDICAL PHARMACOLOGY K.D.TRIPATHI SIXTH EDITION PAGE NO:677