The minimum inhibitory concentrations (MIC) of 60 terpenoids against Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus and Candida albicans have been determined. Hierarchical cluster analysis was used to group the compounds into five groups according to their activity patterns against the four microorganisms. K‐Means cluster analysis was then used to confirm these groupings and to show the differences in the activity patterns of the groups. Ten molecular properties of the terpenoids, either calculated via molecular modelling or determined by direct measurement, were then used as variables in a forward stepwise discriminant analysis to identify which variables discriminated between groups. Low water solubility of Group IV compounds, mainly hydrocarbons and acetates, was found to be associated with their relative inactivity. The remaining groups, all containing oxygenated terpenoids, showed characteristic but distinct activity patterns towards the four test organisms. Hydrogen bonding parameters were found to be associated with antimicrobial activity in all cases. Activity against Gram‐negative E. coli and P. aeruginosa was associated with a combination of a hydrogen bonding and size parameters. This was not found to be the case for the Gram‐positive S. aureus or the yeast C. albicans. Copyright © 1999 John Wiley & Sons, Ltd.

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Four pairs of oxygenated terpenes, with closely related chemical structures but considerably different minimum inhibitory concentration values (1) against P. aemginosa or E. coli, showed differences in rate of cells killed over 2 h. Addition of polymyxin B nonapeptide (PMBN) as an outer membrane permeabilising agent was found to significantly increase the initial rates and overall numbers of cells killed for all compounds. The hydrocarbon limonene and the ester geranyl acetate, normally inactive, were also investigated. Both compounds had little killing effect when added alone to the cells but did show an enhanced killing capacity upon the addition of PBMN.

The known antimicrobially active oxygenated terpenes, carvone, terpinen-4-ol, α-terpineol, carveol and myrtenol were found to cause signifi cant K + leakage from E. coli cells. In contrast, the relatively inactive oxygenated terpenes, 1,8-cineole and p-menth-6-ene-2,8-diol, did not cause signifi cant K + leakage. Slight structural differences between active terpenes were found to affect the rate of K + leakage from E. coli cells. The membrane permeabilizing effect of the active terpenes is considered a signifi cant factor in their antimicrobial activity.