Hurdle Technology Approach to Control Listeria monocytogenes Using Rhamnolipid Biosurfactant.

This study evaluates the combination of mild heat with a natural surfactant for the inactivation of L. monocytogenes Scott A in low-water-activity (aw) model systems. Glycerol or NaCl was used to reduce the aw to 0.92, and different concentrations of rhamnolipid (RL) biosurfactant were added before heat treatment (60 °C, 5 min). Using glycerol, RL treatment (50-250 µg/mL) reduced bacterial population by less than 0.2 log and heat treatment up to 1.5 log, while the combination of both hurdles reached around 5.0 log reduction. In the NaCl medium, RL treatment displayed higher inactivation than in the glycerol medium at the same aw level and a larger synergistic lethal effect when combined with heat, achieving ≥ 6.0 log reduction at 10-250 µg/mL RL concentrations. The growth inhibition activity of RL was enhanced by the presence of the monovalent salts NaCl and KCl, reducing MIC values from >2500 µg/mL (without salt) to 39 µg/mL (with 7.5% salt). The enhanced antimicrobial activity of RL promoted by the presence of salts was shown to be pH-dependent and more effective under neutral conditions. Overall, results demonstrate that RL can be exploited to design novel strategies based on hurdle approaches aiming to control L. monocytogenes.

Bactericidal activity and biofilm inhibition of F18 bioactive glass against Staphylococcus aureus.

Antimicrobial treatment failure has been increasing at alarming rates. In this context, the bactericidal properties of biocompatible antimicrobial agents have been widely studied. F18 is a recently developed bioactive glass that presents a much wider working range when compared to other bioactive glasses, a feature that allows it to be used for coating metallic implants, sintering scaffolds or manufacturing fibers for wound healing applications. The aim of this study was to investigate the in vitro bactericidal and anti-biofilm activity of F18 glass as a powder and as a coating on steel samples, and to explore the effects of its dissolution products at concentrations from 3 mg/mL to 50 mg/mL against the Staphylococcus aureus and methicillin-resistant Staphylococcus aureus (MRSA) biofilms. Furthermore, we intend to verify whether changes in the medium pH could influence the bactericidal activity of F18. The results indicated that F18 presented bactericidal activity in preformed S. aureus and MRSA biofilms, reducing more than 6 logs of the viable cells that remained in contact with 50 mg/mL for 24 h. Moreover, an anti-biofilm activity was observed after 12 h of direct contact, with a drop of more than 6 logs of the viable bacterial population. Neutralization of the F18 solution pH decreased its bactericidal efficacy. These results indicate that the F18 glass could be considered as an alternative material for controlling and treating infections by S. aureus.