Abstract Interest in the microbiota of traditional fermented dairy products, such as yoghurt, continues due to the need for new lactic acid bacteria (LAB) strains that can complement or replace currently-in-use starters. In this work, five traditional yoghurts and one milk sample (for making yoghurt) from different areas of the Khorasan-e-Razavi region, Iran, were subjected to microbial characterization. Among the 102 isolates recovered, which included 29 from milk, the thermophilic LAB species Streptococcus thermophilus (34) and Lactobacillus delbrueckii (36)(with similar numbers of the subsp. bulgaricus and lactis) were the majority bacteria in all samples. Wide intraspecies phenotypic and genetic diversity was encountered among the isolates. The safety and technological features of 29 strains belonging to the species S. thermophilus, L. delbrueckii and Lactobacillus helveticus, all candidates for use in yoghurt-making, were examined. In culture, two S. thermophilus strains produced the biogenic amine histamine from histidine (its precursor amino acid), and five L. delbrueckii strains showed atypical resistance to tetracycline (MIC ≥ 8 μg mL−1). The remaining strains coagulated milk, producing a coagulum of good appearance and pleasant acidic aroma. None of them produced appreciable amounts of exopolysaccharides in milk under the incubation conditions. After fermentation of milk with individual and mixed strains, major yoghurt aroma volatile compounds (i.e. acetaldehyde, diacetyl and acetic acid) were detected. Also as being used in the design of commercial starters, the thermophilic LAB strains characterized in this study might be used to help improve the safety of traditional yoghurts, while maintaining their original qualitative and sensorial characteristics.

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