Antimicrobial activity of rhamnolipids against Listeria monocytogenes and their synergistic interaction with nisin

The rhamnolipids (RL) are biodegradable biosurfactants which have low toxicity and surface active properties that can be useful for food processing industries. The objective of this study was to evaluate the antimicrobial potential of rhamnolipids against Listeria monocytogenes. Susceptibility tests were performed by the minimal inhibitory concentration (MIC) using the micro-broth dilution technique. The MIC values varied from 78.1 μg/mL to 2500 μg/mL with the 2500 μg/mL being the predominant value. Among the 32 tested cultures, 90.6% were susceptible to RL. Results showed that the rhamnolipid activity was primarily bacteriostatic. The interaction of rhamnolipid with nisin was also investigated. The combined effect of nisin and RL was evaluated against two wild-type isolates of L. monocytogenes, L12 sensitive to RL (MIC 156.2 μg/mL) and L17 less sensitive to RL (2500 μg/mL). The FIC indexes for the isolates were 0.18 and 0.078 for L12 and L17 respectively, indicating a strong synergistic effect. The survival curve of isolates L12 and L17 showed that the combination between nisin and RL was bactericidal at lower concentration than for the individual antimicrobials. For the L12 isolate 78.1 μg/mL of RL and 160 IU/mL of nisin eliminated the population after 30 min of incubation. The combination of 156.2 μg/mL of RL and 320 IU/mL of nisin reduced completely L17 population after 2 h of incubation. Rhamnolipids showed antimicrobial activity against L. monocytogenes and presented a synergistic effect when combined with nisin.