Date of Award


Document Type

Master Thesis

Degree Name

Masters of Science (Research)


Biological Sciences

First Advisor

Dr. Aidan Coffey


The overall objective of this thesis was to evaluate the antimicrobial effects of four novel broad-host-range antimicrobial agents. The first was a milk-derived lipoprotein developed by Westgate Biologicals with the support of Enterprise Ireland. This lipoprotein which was designated LactiSAL, was evaluated against a wide range of bacterial species associated with humans. Essentially, the LactiSAL was incorporated at a variety of concentrations into agar media, which were appropriate for cultivation of the different genera. Agars were examined for colony formation, after which it was possible to quantify the minimal inhibitory concentration for each genus. Subsequently, LactiSAL was incorporated at levels close to the minimal inhibitory concentration (MIC) into early log phase broth cultures for each genus. Optical density and bacterial titres were monitored throughout the growth cycle. The results indicated that the lipoprotein brought about lysis of bacterial cells, as evidenced by a reduction in optical density and a reduction in bacterial titre. This was confirmed by assays for leakage of the intracellular enzyme Lactate Dehydrogenase (LDfl). Concentrations in the range of 0.1 to 0.4% LactiSAL were sufficient to inhibit Grampositive bacteria, whereas levels of 2.0% or greater were necessary to inhibit Gram negatives. Various physical factors influencing its activity were recorded. The LactiSAL was also evaluated in the porcine gastrointestinal tract environment at levels dictated by Westgate Biologicals. The antimicrobial effect was significantly reduced in this environment and this is thought to have resulted from the degradative activity of digestive-tract enzymes. The second part of this thesis focussed on Cyclic dipeptide: Cyclo-(L-Leu-L-Pro); Cyclic dipeptide: Cyclo-(L-Phe-L-Pro) and Phenyllactic Acid. These three low molecular weight antimicrobial agents are naturally produced by Lactobacillus plantarum FSTL7. Initial antimicrobial assays were performed with the producer strains on neutralized MRS agar. These were followed up by studies with chemically synthesized pure forms of each of the three compounds. The approach for antimicrobial assays in this case used was broadly similar to that used for LactiSAL. As with lactiSAL, these agents were evaluated against a wide range of bacterial genera and its inhibitory levels were defined. The study confirmed that all four compounds have a broad inhibitory spectrum which includes Gram-positive and -negative bacteria and also fungi.

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