The Quantitative Assessment Of Antimicrobial Agents: Subinhibitory Concentrations Of Eucalyptus Globulus Essential Oil Against Staphylococcus Aureus

We have applied techniques that enable us to measure quantitatively the action of an antimicrobial agent upon the five characteristic growth parameters of a microbial culture. The methods were used to study the action of Eucalyptus globulus ( Myrtaceae) essential oil upon a suspension of Staphylococcus aureus in a standard growth medium. We showed that the Eucalyptus oil produced a cultivable population reduction during both the adaptation phase and the exponential growth phase with the same specific reduction rate. This reduction rate was determined as an explicit function of the oil concentration. The oil does not affect the adaptation time. Our results suggest that the oil inactivates the bacteria via a "target" that is accessible to the oil while the internal cell transformation prepares the cell for its exponential growth period, but later, as the cells transform themselves into the stationary phase, this target ceases to be "accessible" to the oil. This process is reversible; the change from "accessible" to "inaccessible" occurs when the bacteria are transforming into the stationary phase.