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    Identification of Streptococcus infantarius subsp. infantarius as the species primarily responsible for acid production in Izmir Brined Tulum Cheese from the Aegean Region of Turkiye
    (Elsevier, 2022) Guley, Ziba; Fallico, Vincenzo; Cabrera-Rubio, Raul; Cotter, Paul D.; Beresford, Tom
    Izmir Brined Tulum (IBT) Cheese is a traditional semi hard cheese produced in the Aegean region of Turkiye. Lactic acid bacteria (LAB) isolates from IBT cheese samples taken during manufacture and from mature IBT cheeses were investigated for their acid producing capability with the aim of detecting LAB strains responsible for acid production in IBT cheese. Forty two out of 216 isolates decreased the pH of milk to 5.0 or below in 18 h at 37 degrees C or 42 degrees C. 16S rRNA Sanger sequencing revealed the presence of LAB species that had not been detected in IBT cheese previously and, indeed, were identified for the first time as the primary acid producers. The majority of these acid producing isolates were identified as putative Streptococcus lutetiensis/Streptococcus infantarius subsp. infantarius (Sii). Further analysis by sequencing the groES/groEL genes of these isolates established that they were Sii. The remaining isolates from cheese samples taken during manufacture were identified as Streptococcus macedonicus, S. thermophilus, Lactococcus lactis subsp. lactis, Lactobacillus delbrueckii subsp. sunkii and L. delbrueckii subsp. indicus and, from mature cheeses, as Enterococcus faecalis and L. delbrueckii subsp. sunkii. Pulsed-field gel electrophoresis (PFGE) results revealed a large genetic diversity amongst the Sii isolates recovered from the IBT cheeses. It was also established that the Sii strains exhibited efficient and consistent acidification ability equivalent to S. thermophilus. Whole-genome sequencing (WGS) and comparative genome analysis of the representative Sii AYB210 strain provided further insights. More specifically, the genome of AYB210 differed from the previously sequenced African dairy isolate Sii CJ18 and the human isolate ATCC (R) BAA-102 T. Modifications in the lactose operon, which may be an indicator of dairy adaptation, were identified and a high number of CRISPR spacers and putative bacteriocin, virulence factor and antibiotic resistance genes were also detected.
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    Modification of fast-slow differential agar medium for selective isolation of potential starter lactic acid bacteria from cheese
    (Elsevier, 2022) Guley, Ziba; Fallico, Vincenzo; Beresford, Tom
    Starter Lactic Acid Bacteria (LAB) are responsible for converting lactose to lactic acid during cheese manufacturing and, as a result, play a critical role in defining the attributes of the final product. There is great interest in isolating novel starter LAB strains to provide alternatives to existing industry cultures or to help enhance the quality and safety of cheeses traditionally made without starter cultures addition [1]. The Fast-Slow Differential Agar (FSDA) medium was developed in 1984 and still remains the standard to rapidly differentiate fast and slow milk-coagulating lactic streptococci and thus avoid screening a large number of isolates for acid production capacity [2]. However, we found that FSDA was unable to selectively isolate fast acid-producing strains from young, traditional, starter-free Izmir Brined Tulum cheeses, due to the presence of a diverse microbiome including Non-Starter LAB and spoilage Gram-negative microbiota [1, 3]. Here, we describe a modified FSDA (mFSDA) with increased selectivity and recovery efficiency towards lactic streptococci, which was successfully used to rapidly isolate potential starters from Tulum cheeses [1] and could similarly outperform FSDA in raw milk cheeses and other varieties containing high levels of background microbiota. The main differences between FSDA and mFSDA media consist in the presence of nalidixic acid, ascorbic acid and yeast extract in mFSDA. These targeted additions provide mFSDA with a two-prong selectivity that (I) suppresses unwanted microbiota, and (II) increases the recovery efficiency of lactic streptocci adept to using milk nutrients. Specifically: Nalidixic acid is an antibiotic that primarily inhibit Gram-negative bacteria [4]. Ascorbic acid and yeast extract stimulate the growth of lactic streptococci [5] and were added to complement skim milk in creating an environment favoring the growth of lactose-positive, casein peptides-utilizing LAB. The pH indicator bromocresol purple enabled the chromogenic discrimination between LAB with different acid production capability (c) 2022 The Author(s). Published by Elsevier B.V.