BackgroundThe Streptococcus bovis/Streptococcus equinus complex (SBSEC) comprises seven (sub)species classified as human and animal commensals, emerging opportunistic pathogens and food fermentative organisms. Changing taxonomy, shared habitats, natural competence and evidence for horizontal gene transfer pose difficulties for determining their phylogeny, epidemiology and virulence mechanisms. Thus, novel phylogenetic and functional classifications are required. An SBSEC overarching multi locus sequence type (MLST) scheme targeting 10 housekeeping genes was developed, validated and combined with host-related properties of adhesion to extracellular matrix proteins (ECM), activation of the immune responses via NF-KB and survival in simulated gastric juice (SGJ).ResultsCommensal and pathogenic SBSEC strains (n = 74) of human, animal and food origin from Europe, Asia, America and Africa were used in the MLST scheme yielding 66 sequence types and 10 clonal complexes differentiated into distinct habitat-associated and mixed lineages. Adhesion to ECMs collagen I and mucin type II was a common characteristic (23 % of strains) followed by adhesion to fibronectin and fibrinogen (19.7 %). High adhesion abilities were found for East African dairy and human blood isolate branches whereas commensal fecal SBSEC displayed low adhesion. NF-KB activation was observed for a limited number of dairy and blood isolates suggesting the potential of some pathogenic strains for reduced immune activation. Strains from dairy MLST clades displayed the highest relative survival to SGJ independently of dairy adaptation markers lacS/lacZ.ConclusionCombining phylogenetic and functional analyses via SBSEC MLST enabled the clear delineation of strain clades to unravel the complexity of this bacterial group. High adhesion values shared between certain dairy and blood strains as well as the behavior of NF-KB activation are concerning for specific lineages. They highlighted the health risk among shared lineages and establish the basis to elucidate (zoonotic-) transmission, host specificity, virulence mechanisms and enhanced risk assessment as pathobionts in an overarching One Health approach.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-016-0735-2) contains supplementary material, which is available to authorized users.

This study reports the first Tn916-like element associated with a trimethoprim resistance gene, as well as the first fully characterized transposon conferring multidrug resistance in L. monocytogenes. This is of concern, as trimethoprim is administered to listeriosis patients with β-lactam allergy and as Tn6198 has a large potential for dissemination, indicated by both intra-species and inter-genus transfer.

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Fermented foods can cause human illness because of the unhealthy effect of biogenic amines (BAs) that accumulate by decarboxylation of free amino acids. Salami-type fermented sausages can contain BAs, but it is still unclear which bacteria and which environmental factors contribute to BA production. Therefore, 62 sausages purchased on the Swiss market were investigated on their decarboxylating bacterial strains and the content of the BAs cadaverine, histamine, putrescine and tyramine. Based on the size and number of employees of the meat plants, sausages were distinct into two groups: artisanally- and industrially-produced ones. All four BAs had higher concentrations in industrially-produced sausages compared to artisanally-produced ones. Tyramine was the major amine detected in 46 of 62 sausages, with a maximum amount of 785.22 mg/kg and enterococci, as well as coagulase-negative staphylococci, mainly the meat starter culture S. xylosus, could be identified as the main tyramine producers. Putrescine was found in 20 of 62 samples, with a maximum amount of 707.77 mg/kg. These two BAs showed a significant correlation (P = 0.0407) for their concentrations. Cadaverine and putrescine were detected in nine or eight samples respectively, and both were found in significantly higher levels (P = 0.019) and (P = 0.036) in industrial sausages. Based on the quantitative tyramine content, five groups of fermented sausages were identified. Group 1 included products with a very high tyramine level (> 700 mg/kg), group 2 with a high level (400 – 700 mg/kg), group 3 with a moderate level (200 – 400 mg/kg), group 4 with a low level (< 200 mg/kg) and group 5 with a tyramine level below the detection limit (0.05 mg/kg). Samples with a tyramine level higher than 200 mg/kg could be considered as products of less quality because consumption of such samples could be unhealthy for sensitive individual consumers.

Tyramine is a health-adverse biogenic amine, which can accumulate in fermented foods like cheese by decarboxylation of the free amino acid tyrosine by either starter cultures or resident microbes such as lactic acid bacteria including Enterococcus spp., respectively. Our study aimed to show the effect of sodium chloride concentrations on tyramine production as well as to characterise bacterial strains as anti-tyramine biocontrol agents in a 2 mL micro-cheese fermentation model. The effect of sodium chloride on tyramine production was assayed with tyramine producing strains from eight different species or subspecies. Generally, an increase in sodium chloride concentration enhanced tyramine production, e.g. from 0% to 1.5% of sodium chloride resulted in an increase of tyramine of 870% with a Staphylococcus xylosus strain. In the biocontrol screening among lactic acid bacteria, a Lactobacillus plantarum JA-1199 strain was screened that could consume in successful competition with other resident bacteria tyrosine in the micro-cheese model as a source of energy gain. Thereby tyramine accumulation was reduced between 4% to 99%. The results of this study disclose a feasible strategy for decreasing tyramine concentration and increasing the safety level of fermented food. It is an example of development and application of bacterial isolates as starter or protective cultures in food, a biocontrol topic, which Oreste Ghisalba – in his project evaluation function of SNF and later on CTI – was promoting with great emphasis in our ETH Food Biotechnology research group.