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SUMMARY: The synergistic effect of certain pairs of antibiotics might be due to action on 'persisters'; i.e. members of a bacterial population which survive exposure to single drugs, perhaps while in a state of reduced metabolic activity, although their offspring do not show enhanced resistance. Hence, antibiotics were tested in vitro under conditions minimizing bacterial multiplication and metabolism. At 37", neomycin, polymyxin, and streptomycin were more effective in the absence of nutrients than in nutrient broth ; oxytetracycline and bacitracin were equally effective in either environment; but penicillin had little effect in the absence of nutrients. At 4", bactericidal action varied with the test organism so that differences could not be attributed solely to the 'resting' state of the bacteria. Conditions not permitting bacterial multiplication prevented synergism and those antagonisms demonstrable in broth. Probably synergism depends upon rapid bactericidal action preventing the emergence of persisters rather than upon destruction of these relatively inactive forms. Most studies on the action of antimicrobial drugs in vitro have been made with multiplying bacteria, and the effect of combinations of antibiotics under con- ditions unfavourable to bacterial growth has not been investigated intensively. Reports on the effectiveness of antibiotics in environments which do not permit active bacterial multiplication seem contradictory, but some of the apparent disagreements are probably due to differences in the techniques used and in the bacteria tested. Thus far, synergism and antagonism among anti- biotics have been observed only under conditions which permit growth and multiplication (Jawetz & Gunnison, 1953). Information about the action of antibiotics on non-multiplying bacteria is particularly needed with regard to so-called 'persisters', a term applied by Bigger (1944) to those members of bacterial populations which survive the action of antimicrobial drugs, although their offspring are no more resistant to the drugs than the original parent culture. A possible explanation for this phenomenon is that persisters may be in a phase of relative metabolic inactivity. As expressed by Davis (1954), ' . . . these cells appear to be genotypically drug-sensitive but phenotypically drug-resistant '. In some chronic disease states, persisters may explain the failure of chemotherapeutic agents to eradicate infection. In certain chronic infections, synergistic drug combinations have resulted in cure, whereas single drugs, even though exhibiting antibacterial action, have failed. Synergism can

The Mode of Action of Antibiotic Synergism and Antagonism: the Effect in Vitro on Bacteria not Actively Multiplying