Histamine forming behaviour of bacterial isolates from aged cheese.

This study was performed to investigate the presence of histamine forming bacteria in commercially available cheeses as well as to evaluate the histamine forming potential using in vitro models. Five long-time-ripened cheeses made from different milk types were analysed for histamine producing bacterial isolates. The ability of the isolates to produce histamine was tested by incubation at 37 °C for five days in a restricted media with a pH indicator. Changes in the amino-compound profile were investigated using ultra performance liquid chromatography. Only eight of 106 isolates were able to produce compounds raising the pH and seven of those were confirmed to be histamine producers. Despite the fact that all isolates were obtained from the same vintage Danish Gouda cheese, made from raw cow milk, the amino-compound profile as well as the response to different environmental conditions diverged between the isolates. Rep-PCR and 16S rRNA gene sequencing was used to further characterize the isolates. Pediococcus pentosaceus was for the first time reported to be a histamine producer in cheese. The presence of the histidine decarboxylase gene (hdcA) was confirmed by PCR amplification of the histidine decarboxylase gene in four of the isolates. The results indicate that evaluating the presence and concentration of histamine is not only a relevant parameter to evaluate quality and safety, but is also an important tool to classify histamine producers in cheese.