SUMMARY: By using growth and division-inhibition at 16 hr as indices of activity, eight penicillins, differing in the side chain, were tested against an species, and compared with benzylpenicillin. DL-α-Aminobenzylpenicillin, DL-3-chloro-α-aminobenzylpenicillin, D-α-aminobenzylpenicillin, and methicillin were more toxic for growth than was benzylpenicillin. 6-Aminopenicillanic acid and cloxacillin were about equal to benzylpenicillin in toxicity; triphenylmethylpenicillin and 2-ethoxy-1-naphthylpenicillin were much less toxic. All the compounds inhibited division of the bacteria but, at 50% of normal growth, methicillin, cloxacillin and 2-ethoxy-1-naphthylpenicillin resulted in significantly longer organisms than the others. Pantoyl lactone, when present in the medium from the time of inoculation, in all cases decreased the length of the organisms, increased growth (with a highly toxic concentration of penicillin), and decreased the accumulation of keto acids and ultraviolet-absorbing materials in the medium. It was concluded that the side chain of these penicillins is not essential for the inhibition of growth and division of this sp.; quantitative differences in activity associated with the side chain appear to result from the influence of the side chain on factors such as penetration, or strength of binding at the active site.


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