This study evaluated the influence of parameters relevant for cheese making on histamine formation by Streptococcus thermophilus. Strains possessing a histidine decarboxylase (hdcA) gene represented 6% of the dairy isolates screened. The most histaminogenic, S. thermophilus PRI60, exhibited in skim milk a high basal level of expression of hdcA, upregulation in the presence of free histidine and salt, and repression after thermization. HdcA activity persisted in cell extracts, indicating that histamine might accumulate after cell lysis in cheese.

In the present study, 20 enterococci belonging to the species Enterococcus faecalis (12 strains), Enterococcus faecium (4), Enterococcus durans (2), Enterococcus hirae (1) and Enterococcus mundtii (1) and originating from a total production chain of swine meat commodities were analysed to investigate the diversity of their tetracycline resistance gene tet(M). PCR-RFLP and sequence analysis showed that the tet(M) gene of most strains can be correlated with the Tn916 transposon. Conversely, tet(M) of six E. faecalis and the E. hirae strain, all isolated from pig faecal samples, may be associated with previously undescribed members of the Tn916-1545 transposon family. In vitro filter conjugation trials showed the ability of 50% of the enterococcal strains, including E. mundtii, to transfer the tet(M) gene (and the associated Tn916 and new transposons) to E. faecalis or Listeria innocua recipient strains. tet(M) gene transfer to L. innocua recipient was also directly observed in meat food products. Collectively, these sequence and conjugation data indicate that various transposons can be responsible of the spread of tetracycline resistance in enterococci and validate the opinion that Enterococcus species are important sources of antibiotic resistance genes for potentially pathogenic bacteria occurring in the food chain.

In this study, a tyrosine decarboxylase gene (tdcA) was identified in 1 among 83 Streptococcus thermophilus strains tested. Its sequence, nearly identical to that of a tdcA of Lactobacillus curvatus, indicated a horizontal gene transfer event. Transcription in milk and the formation of critical levels of tyramine were observed in the presence of tyrosine.Biogenic amine (BA) formation in fermented food has recently been included by the European Food Safety Authority (EFSA) among the safety concerns requiring evaluation. Among these compounds, tyramine is the most common in ripened cheeses and fermented sausages, where it usually accumulates during ripening and can be present in toxicologically relevant levels (Ͼ100 mg kg Ϫ1 ) (4). The symptoms of tyramine poisoning, also known as "cheese reaction," are hypertension due to peripheral vasoconstriction caused by the release of noradrenaline, pupil dilation, and increased respiration frequency and blood sugar level (22).Tyramine formation is the result of bacterial decarboxylation of tyrosine that is carried out by several microorganisms, including strains of lactic acid bacteria (LAB) belonging to different genera and species (7, 23). Among LAB, Streptococcus thermophilus is of major importance for the dairy industry, where it is commonly used in starter cultures for the manufacture of yogurt, fermented milks, and many varieties of cheese. Recently, a few strains of dairy origin assigned to the species S. thermophilus have been described for their tyraminogenic activity by both physiological assays (15) and consensus PCR on tyrosine decarboxylase genes (tdcA genes) (3). However, the identity of the genetic determinant involved was not unveiled.The aims of this study were (i) to screen the tyramine production of S. thermophilus in synthetic medium, (ii) to identify the gene responsible for this activity, (iii) to quantify its expression by quantitative real-time reverse transcription-PCR (RT-qPCR), and (iv) to monitor the levels of tyramine production in the presence of physicochemical parameters common to a cheese-making process in order to obtain indications on how to limit the formation of this BA.Screening of S. thermophilus strains for tyramine production. Eighty-three S. thermophilus dairy isolates, identified and genotypically characterized in previous studies (12, 20), were investigated. All strains were subcultured in LM17 medium, i.e., M17 medium (Fluka, Italy) supplemented with 0.5% (wt/vol) lactose, at 37°C for 24 h. Initially, the strains were screened for tyramine production according to the method of Bover-Cid and Holzapfel (2). Only the strain S. thermophilus 1TT45 (16S rRNA gene sequence has EMBL/GenBank/DDBJ accession no. FR725449), isolated from Taleggio cheese, gave a positive reaction in plates. Consensus tdcA-targeted PCR with the degenerate primers DEC5/DEC3 (23) on genomic DNA extracted with the method of Rossi et al. (21) from 2-ml cultures generated an amplicon of the expected size of close to 336 bp only from this strain (data not shown). T...

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Tetracycline-resistant Streptococcus thermophilus isolates from soft cheeses harbored the genes tet(S), tet(M), and tet(L). Molecular analysis of these genes revealed their expression, localization on plasmids or Tn916-Tn1545 family transposons, and their similarity with published sequences. The study highlights the importance of an accurate safety assessment of using S. thermophilus as a starter culture.