Six colicinogenic strains, producing colicine A, B, D, K, S4, and V, showed a variation in the reproducibility of the potency of their crude extracts, although a standard technique was used. The importance of this variation on typing was shown by the effect of different colicine potencies on the patterns of inhibition of the Abbott and Shannon indicators. Using a diffusion technique, 3,004 assays were performed and it was found that the patterns of inhibition depended mainly on the strength of the colicine.
A further source of error in typing by production patterns was the sensitivity of the indicator strain. Two sets of indicators from different sources were compared, and though there was agreement in most strains, there were differences which varied from slightly more resistant to totally resistant.
It is suggested that the potency of the colicine produced by the colicinogenic strain should be known before typing is performed in order that standard comparisons may be achieved. The precise sensitivity of the indicators should also be confirmed periodically.
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