| Summary: | ‘Hass’ avocado (<i>Persea americana</i> Mill) by-products (peel, and seed) are rich sources of potential antimicrobial agents. The current study used thiazolyl blue tetrazolium bromide (MTT) as a colour indicator to evaluate the antibacterial capacities of ‘Hass’ avocado by-product extracts from China, Chile, and New Zealand. MTT reagent can react with mitochondrial dehydrogenases in living bacterial cells to produce formazan. The number of colony-forming units (CFU) of <i>Staphylococcus aureus</i> were linearly correlated with formazan content. The current study used a formazan–CFU standard curve to evaluate the antibacterial properties of ‘Hass’ avocado by-product extracts by monitoring the CFU of <i>S. aureus</i> after 12 h treatment with the by-product extracts. The results showed that there was a significant (<i>p</i> < 0.05) difference in <i>S. aureus</i> survival if the ‘Hass’ avocado by-product extracts were recovered from mature unripe fruit or mature ripe fruit derived from the three countries. Chilean ripe seed extracts resulted in the lowest bacterial survival percentage (2% ± 0.98) after 12 h of treatment. Chilean ripe seed extracts had the highest <i>S. aureus</i> inhibition activity among all the extracts. In addition, there was a linear correlation between the total polyphenol content (TPC) and <i>S. aureus</i> (r = 0.546, <i>p</i> < 0.0010) bacterial survival percentage. Therefore, the current results lead us to consider that the main antioxidant polyphenol compounds of avocado by-product extracts are also the main compounds required to provide antibacterial activity inhibiting bacteria growth.
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