Microbiocidal activity of selected weak organic acids

Research Doctorate - Doctor of Philosophy (PhD)Organic acids occur naturally in plants and in animals and can be produced by microbial fermentation. In plants they play a major role in their defence mechanism, e.g. against viruses. Organic acids are generally weak acids, as they do not dissociate completely in water. Ascorbic, acetic, citric, formic, lactic, malic, uric acids are members of the weak organic acid (WOA) family and they have diverse applications in several industries such as food, oil, agriculture, and health. Several empirical and historical data supports their antimicrobial effect; however, no disinfectant registered to be effective as a hospital grade disinfectant in Australia (at the completion of this project) consists solely of weak organic acid (WOAs). WOAs have been used in combination with other antimicrobial agents such as ethanol or benzalkonium chloride and probably have synergistic effects with these agents. Antibacterial disinfectants may eliminate planktonic cells, but biofilm penetration is limited by factors such as surface adsorption and absorption by the biofilm matrix thus limiting the availability of the active component of the disinfectant. The other limiting factor in the application of disinfectants is toxicity. The toxicity of WOAs is relatively low in comparison with the other disinfectants and because of that rinsing off after application is not required. Studying the efficacy of WOAs against multi antibiotic resistant bacteria can provide useful information about their antimicrobial efficacy. In future, such a disinfectant may be used against pathogenic multi antibiotic resistant bacteria such as methicillin resistant Staphylococcus aureus (MRSA), vancomycin resistant Enterococcus spp. (VRE), multi antibiotic resistant Acinetobacter baumanii (MRAB), or extended spectrum beta lactamase enzyme producing Gram negatives, such as Klebsiella pneumoniae. These bacteria seldom develop total resistance to organic acids but increased acid tolerance has been observed. This project investigated the efficacy of selected WOAs and their combination against enveloped (Herpes simplex type 1) and non-enveloped (Human adenovirus type 4) viruses, against intracellular bacteria (Rickettsia honei) and concluded that concentrations as low as 6 mg/ml (1.5 mg/ml against intracellular bacteria) can be effective to eliminate viruses or intracellular bacteria. Antibacterial efficacy against planktonic multi antibiotic resistant bacteria such as multi (methicillin) resistant Staphylococcus aureus, vancomycin resistant Enterococcus spp., multi resistant Acinetobacter baumanii and extended spectrum beta lactamase positive Klebsiella pneumoniae was studied using a time-kill assay and an assay developed in-house. Results showed that four WOAs in combination (ascorbic, citric, lactic, and malic) at 10% (w/v or v/v) concentration was an effective disinfectant. Studying the efficacy of the above combination against biofilm embedded multi antibiotic resistant bacteria showed that these acids not only penetrate into biofilms, eliminating the bacteria but they are also able to destroy the principal structure of biofilms, the extracellular polymeric substance. Activity against yeast (Candida albicans) and a fungus (Aspergillus niger) was moderate, and required at least 200 mg/ml. The development of acid tolerance was also investigated using microarray assay and data obtained were indicative, but not conclusive, of gene expression changes responsible for the production of acid shock proteins. An in situ study of a disinfectant containing all four WOAs at 10% (w/v or v/v) against VRE at Barwon Health, Geelong, Australia, showed that this formulation could eliminate bacteria from inanimate surfaces in a real life situation. Independent testing by an external laboratory confirmed that the proposed disinfectant (10% mixed WOAs) could be registered as a hospital grade disinfectant and based on the low toxicity of the components it could be a low risk alternative to other commercially available disinfectants, in the future