Evaluating the Antimicrobial Potential of Eucalyptus baileyana F. Muell. and Eucalyptus major (Maiden) Blakely against the Fish Spoilage-causing Bacterium Shewanella putrefaciens

Introduction: Eucalyptus baileyana (Bailey’s stringy bark) and Eucalyptus major (Queensland grey gum) have been previously used as antimicrobials against a variety of ailments. This study evaluated the effectiveness of E. baileyana and E. major as inhibitory agents against Shewanella putrefaciens, a bacterium widely associated with fish spoilage. Methodology: E. baileyana and E. major extracts were prepared using the leaves of each plant with methanol or water as the extraction solvent. Growth inhibition and minimal inhibitory concentrations were determined against S. putrefaciens through disc diffusion assays. MIC values were subsequently quantified to evaluate the extracts efficacies as antibacterial agents. Finally, the toxicity of each extract was determined using the Artemia franciscana nauplii bioassay. Results: E. baileyana aqueous and methanolic leaf extracts inhibited the growth of S. putrefaciens in the disc diffusion assay, with MIC values of 1411 and 1221 μg/mL respectively. Similarly, E. major leaf extracts also showed growth inhibition of S. putrefaciens, with MIC values of 1686 μg/mL for the aqueous extract, and 1160 μg/mL for the methanolic extract. However, toxicity studies of the extracts revealed that all extracts were toxic and likely unsuitable for human consumption (LC50 values 455-1146 μg/mL) as determined by the Artemia franciscana bioassay. Conclusion: While the E. baileyana and E. major leaf extracts were effective in preventing microbial growth, given their relatively high levels of toxicity, they would not be suitable for use as a preservative in the prevention of fish spoilage. However, the antibacterial capacity of the extracts indicates that the extracts may show promise as a surface disinfectant, and this should be investigated further.