The esters of 4‐hydroxybenzoic acid and related compounds. Part I. Methods for the study of their fungistatic properties

Abstract This introductory paper describes the method and criteria that have been developed for a critical evaluation of die fungistatic action of chemically related compounds together with some related aspects arising from the application of these methods. Inhibition is determined as reduction in growth rate and particular attention has been giver, to dosage/response curves as providing a sound basis for comparisons between substances. Three moulds have been used : Aspergillus niger, Byssochlamys fulva and Penicillium roqueforti. In the absence of inhibitor, growth/time curves were linear for all three organisms once the colony had attained a diameter of 10 mm. A. miger and P. roqucforti behaved similarly in the presence of inhibitor but B. fulva showed an accelerating growth that persisted and often resulted in the final diameter exceeding that of the control. In the first two organisms growth rate (mm. per 24 hours) in the linear phase has been used; for B. fulva the rate was determined as soon as possible after the colony had reached 10 mm. and before it exceeded 20 mm. The dosage/response relationship for A. niger has the form: y = ax b , where y = inhibition, x = concentration of inhibitor; a and b are constants, the latter less than I. From this relationship inhibitory substances can be described in terms of b (slope of log. dose/log. response curve) and log I 30 (—log. concentration giving 50% inhibition). B. fulva can be interpreted similarly although complicated by acceleration in the growth/time curve. It is better to use log I 67 in place of log I 50 . Experimental values for P. roqueforti can be interpreted as the resultant of an inhibitory action (logarithmic as before) and a stimulation rising to an early limiting value. In this case determination of b is not practicable and inhibition is best expressed as log I 50 obtained graphically from experimental points. Log I 25 permits comparison at another part of the curve. Constants b and log I 50 determined for A. niger can vary significantly between experiments although the cause of this variation is not apparent. Log I 50 is related to some factor that affects the control rate. The methyl ester causes striking morphological changes in the hyphal tips although concentrations of higher esters causing comparable inhibition fail to bring about such changes.