1948a Cytochemical mechanisms of penicillin action. VI. The influence of cobalt on the optimal bacteriostatic concentration of penicillin

In a previous paper we described the gross appearance of penicillin assay plates after chemical treatments which reveal sharp boundaries around the zones of inhibition (Dufrenoy and Pratt, 1947). Most of the tests that were described are effective following exposure of the test organisms to penicillin for periods as short as 2 to 3 hours, i.e., before zones of inhibition are discernible on untreated plates. Several of the tests may find application as rapid cylinder plate assay methods (Pratt and Dufrenoy, 1947). The sharp delineation of the inhibition zones was interpreted as an expression of a threshold effect; positive reactions for-SH groups and for OH=C-C==OH groups were obtained outside the zones of inhibition, but not inside. The evidence suggested an increase.of rH within the zones and that this increase was correlated with inhibition of dehy-drogenase systems. Since there is strong evidence in the literature that dehy-drogenase activity depends on the structural integrity of the microorganism (Guggenheim, 1945), it seemed of interest to study the changes in response to various reagents in different parts of the cells of test organisms following exposur