Molecular characterization of histidine and tyrosine decarboxylating Enterococcus species isolated from some milk...

Fermented foods can cause adverse effects on human health because of the biogenic amines (BAs) accumulating through amino acid decarboxylation. This study investigated the presence of BAs including tyramine and histamine in 240 samples of some cheese and fermented milk samples using high-performance liquid chromatography. Another aim of this study is to isolate and identify Enterococcus spp. as the most important and frequent BA producer in the examined samples. The isolated Enterococcus spp. was investigated phenotypically for their capacity to produce amino acid decarboxylase enzyme using decarboxylase microplate assay, and genotypically through molecular detection of some genes encoding amino acid decarboxylation (tyrdc and hdc). Biogenic amines producing enterococci were then investigated for their antimicrobial resistance, biofilm production as well as their virulence determinants.

Tyramine and histamine could be detected in 86.7 and 87.9% of the investigated samples with 52.9% being contaminated with Enterococcus spp. Significant correlation between the incidence of Enterococci enterococci and BAs formed in the examined samples (P < 0.0001). tyrdc and hdc genes were detected in 85 and 5% of amino acid decarboxylating Enterococcus spp., respectively. A high percentage of Enterococcus isolates (57.5%) were multidrug-resistant and resistance against penicillin was widespread among isolates followed by tetracycline, vancomycin, erythromycin and linezolid. Also, 77.5% of the isolates were capable of forming biofilms and a highly significant correlation (P < 0.0001) was found between biofilm formation and multidrug resistance. The results showed that the rates of most virulence genes gelE, esp, ace, asa1, and cylA were 77.5. 47.5, 47.5, 35 and 7.5%, respectively, while the hyl gene was not detected in any isolates.

The study highlights the significant presence of BAs (TYM and HIS) in cheese and fermented milk samples, with a strong correlation between enterococci contamination and TYM production. The high prevalence of tyramine-producing Enterococcus species poses a notable public health concern especially with the high prevalence of multidrug-resistant, biofilm production and virulence in BAs producing Enterococcus spp. in dairy products, emphasizing the urgent need for improved antimicrobial stewardship among food producers and veterinarians to mitigate the risk of transferring resistant strains to humans.