Safety assessment of Streptococcus thermophilus by PCR-based assays

Streptococcus thermophilus is a thermophilic lactic acid bacterium (LAB) widely used as commercial and natural starter in the manufacture of many dairy products. Strains belonging to this species are considered to be harmless on the basis of long lasting experience of safe use. However, the acquisition of risk-related genes, such as genes encoding antibiotic resistance (AR) and other unwanted metabolic activities, through events of horizontal gene transfer, has been poorly investigated in this dairy LAB. Objective: to analyze a collection of 83 strains isolated from natural whey cultures and retail cheeses for the presence of six genes involved in resistance to tetracycline [tet(K), tet(L), tet(M), tet(O), tet(S), and tet(W)]. In addition, the spread of the genes hdcA and tdcA coding for the amino acid decarboxylases involved in the formation of the most hazardous biogenic amines (BAs), histamine and tyramine, was investigated by means of recently developed PCR assays. Conclusions. Data obtained demonstrated that the safety of S. thermophilus cannot be asserted only on the basis of taxonomic identification and absence of acquired antibiotic resistance, but that other safety-related features, i.e. incapability to produce BAs, have to be carefully evaluated. The use of screening methods for risk-related genetic traits based on PCR with primers constructed on consensus sequences should be envisaged as a routine procedure when selecting for new starter cultures. Lastly, the rapid tests developed for S. thermophilus must be extended and adapted to other starter LAB species to detect transfer events of acquired hazardous genes to undesirable species present in the same dairy environment.