Investigation of West African plant extracts as alternative, natural sources of antimicrobials against foodborne pathogens and spoilage organisms
Type of DegreeMaster's Thesis
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Consumers demand clean-label food products, necessitating the search for new, natural antimicrobials to meet demand while ensuring food safety. The purpose of this project was to evaluate the antimicrobial properties of crude methanolic and ethanolic extracts from plants [Alchornea cordifolia, Senna alata, Psidium guajava, Cryptolepis sanguinolenta, Solanum torvum, Piper guineense, and Aframomum melegueta] from West Africa against foodborne microorganisms [Listeria monocytogenes, Salmonella, Escherichia coli, Staphylococcus aureus, Enterococcus faecalis, Vibrio parahaemolyticus, Bacillus subtilis, Lactobacillus fermentum, and Saccharomyces cerevisiae] and determine which have the best potential for further development. A well diffusion assay was used to screen antimicrobial activity of extracts against strains of common foodborne pathogens and plant extracts with the highest antimicrobial activity [ethanolic extracts of P. guajava, C. sanguinolenta and S. alata] were selected and used to further evaluate their antimicrobial effects on the growth of various microorganisms over time. When screening the antimicrobial activity of the selected plant extracts on bacterial growth, C. sanguinolenta exhibited the strongest antimicrobial activity, decreasing levels of V. parahaemolyticus after 2 h, and S. aureus and L. fermentum after 4 h to below detectable limits of 1.00 log colony forming units (CFU)/mL. P. guajava required 2, 4 and 8 h to decrease V. parahaemolyticus, L. fermentum and S. aureus, respectively, to below detection limits. S. alata did not perform as well, requiring 24 h to reduce E. coli and Salmonella to below detection limits. Growth of B. subtilis was inhibited by all extracts, but levels were not decreased to below detectable limits. The minimum inhibitory concentrations (MIC) were then determined for P. guajava and C. sanguinolenta against E. coli ATCC 2196 and Salmonella Typhimurium and found to be 0.50 and 1.00% (w/v), respectively. Concentrations of 0.75% (w/v) P. guajava and 0.25% (w/v) C. sanguinolenta were then added to tryptic soy broth (TSB) inoculated with Salmonella or E. coli with an adjusted pH 6, 5, or 4.5. Adjusting pH had no effect on the antimicrobial activity of P. guajava against Salmonella, whereas, at a pH of 6 and 5, effectiveness of C. sanguinolenta was decreased, with cell counts of Salmonella rising over 2.00 log CFU/mL greater than samples with unaltered pH after 24 h. The inhibitory effect of both plant extracts against E. coli were decreased by the lowered pH; at an unaltered pH, it was inhibited to below the detection limits of 1.00 log CFU/mL after 24 h, however, at a pH of 6, cell numbers rose to over 8.00 log CFU/mL and 6.00 log CFU/mL when P. guajava and C. sanguinolenta were applied, respectively. Lastly, antimicrobial activity of P. guajava at 1.00% (w/v) and C. sanguinolenta at 0.50% (w/v) against E. coli and Salmonella were evaluated in milk at room temperature for 72 h. P. guajava exhibited bacteriostatic antimicrobial activity against E. coli and continued to keep levels of Salmonella less than that of the control from 8 - 72 h at 22 °C. The antimicrobial activity of C. sanguinolenta remained significantly less than the control from 8- 72 h at 22 °C against both microorganism, but was not as effective as P. guajava at the level tested. The extracts were overall less effective in reducing bacterial populations when tested in a model food system, rather than the ideal conditions in microbiological media. To summarize this research, extracts were more effective against Gram-negative bacteria when compared to Gram-positive bacteria and yeast. This study suggests that C. sanguinolenta and P. guajava may be useful as alternative antimicrobials against select pathogens causing foodborne illness; however, antimicrobial efficacy may be lowered when applied in a food system.