Survival, growth, and biogenic amine production of Enterococcus faecium FC12 in response to extracts and essential oils of Rubus fruticosus and Juniperus oxycedrus

Introduction

Enterococci are lactic acid bacteria (LAB) usually found as food contaminants in fermented products such as cheeses and fermented sausages. Due to their antibiotic resistance, the presence of virulence factors, and the ability to produce biogenic amines (BAs), the determination of these bacteria is crucial to assure food quality and safety. BAs production and consequent accumulation in foods can cause toxicological effects on human health. Plant phenolic compounds are promising alternatives to chemical preservatives and reflect consumers' demand for "green" solutions. In this study, the antimicrobial effect of blackberry (Rubus fruticosus) leaves and prickly juniper (Juniperus oxycedrus) needles, both as phenolic extracts (PE) and essential oils (EO), were evaluated against Enterococcus faecium FC12, a known tyramine-producing strain.

Methods

The growth kinetics in the presence of sub-lethal concentrations of such plant derivatives were modeled (Gompertz equation) and BA production was monitored over time by HPLC. Moreover, flow cytometry (FCM) was used to study the effects of EOs and PEs on cell viability.

Results

The EOs showed a higher antimicrobial effect (especially R. fruticosus added at 0.75 mg/ml), determining an initial decrease of culturable cells followed by a recovery, even if with lower growth rates and final cell loads. Different rates of BA formation were observed, with tyramine concentrations ranging from 120 to 160 mg/l after 96 h of incubation, and 2-phenylethylamine was produced in lower amounts, usually after reaching the peak of tyramine. FCM confirmed the higher efficacy of R. fruticosus EO that induced cell membrane injury in 93% of the total population. However, complete recovery occurred in the following incubation, demonstrating transient damage.

Discussion

Although further research is required to better investigate this recovery and to assess the suitability of this approach in a real food system, the present study showed the potential antimicrobial activity of plant derivatives, especially R. fruticosus EO, against the tyramine-producing E. faecium FC12.

Tissue and Growth Specific Chemical Profiles and Antimicrobial Activity of Libanotis buchtormensis (Fisch.) DC Extracts

Libanotis buchtormensis (Fisch.) DC. is a traditional Chinese medicinal herb used for the treatment of rheumatism, articular pain, and symptoms of the common cold. To evaluate the possible medicinal value of the aerial parts of L. buchtormensis, the chemical composition and antibacterial activity were assessed on a tissue and growth specific basis. Results based on high performance liquid chromatography (HPLC) showed that the samples were divided into two main clusters, one associated with roots and the other associated with the aerial parts. Analysis of the bioactive chemical contents revealed that coumarin was the most abundant compound in leaves (0.53 to 0.82%), while osthole and isoimperatorin exhibited relatively high concentrations in roots. Two-year growth samples, which correspond with the harvest period of L. buchtormensis, contained the highest amounts of total coumarin, osthole, and isoimperatorin. Broth microdilution tests showed that Gram-positive bacterial strains ( Staphylococcus aureus, S. epidermidis and Streptococcus agalactiae) were more sensitive to L. buchtormensis extracts than the Gram-negative bacterial strain ( Escherichia coli). Staphylococcus aureus and S. epidermidis were more sensitive to the extracts, exhibiting significant correlation with osthole and isoimperatorin contents. Comparative analysis of the chemical profiles and antibacterial activity revealed significant differences between the root and the aerial parts, which implied that the clinical application of the aerial parts needed further study. In addition, the aerial parts could serve as a material source for osthole, having comparable content to that of the root.