R.J.W. Lambert

A study of the minimum inhibitory concentration and mode of action of oregano essential oil, thymol and carvacrol.

Aims: The minimum inhibitory concentration (MIC) of oregano essential oil (OEO) and two of its principle components, i.e. thymol and carvacrol, against Pseudomonas aeruginosa and Staphylococcus aureus was assessed by using an innovative technique. The mechanism of action of the above substances was also investigated.

The synergistic effect of EDTA/antimicrobial combinations on Pseudomonas aeruginosa

Aims:  To demonstrate that the nonlinear concentration‐dependent inhibition of Pseudomonas aeruginosa to EDTA can be used to successfully model and predict the potentiation of antimicrobials by EDTA.

A model for the efficacy of combined inhibitors

Aims: The method of the sum of the fractional inhibitory concentrations (∑FIC) is used ubiquitously in the investigation of antimicrobial combinations. The inherent assumption of this simple equation is that in a mixture all antimicrobials have identical dose responses. The aim of this work was to analyse the outcome of removing this assumption.

Membrane damage to bacteria caused by single and combined biocides

Aims: To examine the effect on the leakage of low molecular weight cytoplasmic constituents from Staphylococcus aureus using phenolics singly and in combination, and to see if the observations could be modelled using a non‐linear dose response.

Adaptive resistance to benzalkonium chloride, amikacin and tobramycin: the effect on susceptibility to other antimicrobials.

Aims: To produce strains of antimicrobial‐resistant Pseudomonas aeruginosa via adaptation to benzalkonium chloride, amikacin and tobramycin and to then examine the incidence, or otherwise, of cross‐resistance between antibiotics and between antibiotics and benzalkonium chloride.

Comparative analysis of antibiotic and antimicrobial biocide susceptibility data in clinical isolates of methicillin-sensitive Staphylococcus aureus, methicillin-resistant Staphylococcus aureus and Pseudomonas aeruginosa between 1989 and 2000.

Aims:  To analyse population minimum inhibitory concentrations (MICs) data from clinical strains of Staphylococcus aureus and Pseudomonas aeruginosa for changes over a 10‐year period and to look for correlations between the antimicrobials tested.

The relationships and susceptibilities of some industrial, laboratory and clinical isolates of Pseudomonas aeruginosa to some antibiotics and biocides.

Aims: To provide evidence to support or refute the hypothesis that cross‐resistance between antibiotics and biocides can occur.

A rapid method for assessing the suitability of quenching agents for individual biocides as well as combinations

Aims: To develop a novel, rapid method for testing the ability of quenching agents to neutralize disinfectants.

One explanation for the variability of the bacterial suspension test

Disinfection kinetic studies of sodium dodecyl sulphate, benzalkonium chloride and sodium hypochlorite against Staphylococcus aureus revealed that when a higher inoculum level of Staph. aureus than normal was used (approximately 1 log higher), the efficacy of disinfection was severely attenuated. Kinetic analysis using the Hom model for experiments carried out on tests using 3 × 108 organisms ml−1 were unable to account for the large increase in disinfection power observed when smaller inoculum levels were used. Since the inoculum was the same in every way except for the numbers used, the large variations in the log reduction/time curves could not be explained by a variation in the resistance of the population to the biocide, as identical log reduction–time curves should have resulted. The level of disinfection achieved for a given concentration of biocide was found to be approximately linearly related to the cell number ml−1 of test solution and not to the log number. The variation observed is believed to occur due to intrinsic self‐quenching of the biocide by the microbes during the course of the disinfection test. As the level of free biocide decreases, the rate of reaction decreases, giving the tails of the log reduction/time curves. Such intrinsic self‐quenching could explain the large variations known to occur in the legally required disinfection suspension tests.

A kinetic study of the effect of hydrogen peroxide and peracetic acid against Staphylococcus aureus and Pseudomonas aeruginosa using the Bioscreen disinfection method

Hydrogen peroxide and peracetic acid at pH 4 were examined against Staphylococcus aureus and Pseudomonas aeruginosa using the published ‘Bioscreen’ technique of biocide analysis. The data were examined using either classical Chick‐Watson (CW) log‐linear disinfection kinetics or the empirical, non‐linear time Hom model. In some cases, modelling the data with the classical CW method gave good linear correlations, in others, however, deviations from this model were observed. In such cases the Hom model proved an adequate descriptor of the data. The Bioscreen technique therefore gives data which can be analysed using the normal mechanistic and empirical models currently available.