Isolation, Phenotypic Characterization, and Functional Potential of Indigenous Lactic Acid Bacteria from Fresh Duck Eggs

Abstract

Duck eggs are a highly nutritious source of animal protein; however, they are particularly susceptible to microbial contamination. Indigenous lactic acid bacteria (LAB) capable of colonizing fresh egg matrices may possess adaptive traits, making them effective natural biopreservation agents. The present study aimed to isolate, phenotypically characterize, and evaluate the antimicrobial activity of LAB derived from different compartments of fresh duck eggs, specifically the inner eggshell, egg membrane, and egg contents. Duck eggs were obtained from two intensive farms (I1 and I2) and two extensive farms (E1 and E2) located in Sidrap Regency, South Sulawesi, Indonesia, during 2024. The LAB isolates were obtained using a selective culture method employing De Man, Rogosa and Sharpe agar enriched with 1% CaCO₂. Twenty fresh duck eggs (five per location) were sampled, yielding 35 isolates, of which seven were confirmed as LAB based on morphological and biochemical analyses. Characterization included Gram, endospore, and acid-fast staining, catalase testing, and carbohydrate fermentation profiling. Antimicrobial activity against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) was evaluated using agar well diffusion method at the original (acidic) and neutral pH levels to identify bacteriocin-like inhibitory substances (BLIS). Seven selected LAB isolates were obtained, including five from extensive farms (E1S1, E1S3, E1M1, E2S2, E2M2) and two from intensive farms (I1M2, I2M5). All isolates demonstrated complete glucose fermentation and a homofermentative pattern. Isolates E2S2, E2M2, and I1M2 fermented glucose, lactose, fructose, and mannitol, whereas E1S1, E1S3, E1M1, and I2M5 fermented glucose exclusively. Based on phenotypic data, the isolates were preliminarily classified as Lactobacillus and Pediococcus species. Antimicrobial activity testing demonstrated significant inhibitory potential, with maximum inhibition zones of 27.5 mm against E. coli and 17.0 mm against S. aureus. Although inhibition zone diameters decreased at neutral pH, the persistence of a clear zone against E. coli indicated BLIS production. The disappearance of inhibitory activity against S. aureus at neutral pH indicated that organic acids caused the pathogen's inhibition. The current findings indicated that fresh duck eggs contain a high abundance of homofermentative LAB, which could serve as effective natural biopreservation agents to improve food safety and prolong shelf life.

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