Evaluation of histamine productivity of Tetragenococcus halophilus isolated from salted mackerel (saba-shiokara)

Abstract

The consumption of salted mackerel shiokara contaminated with halophilic bacteria that have histidine decarboxylase enzymes can lead to histamine poisining in humans. In previous research, Tetragenococcus halophilus strains were found to be the dominant salt-tolerant bacteria in mackerel shiokara samples, and two such strains were isolated and identified as T. halophilus A116 and A124 using 16S rDNA and the API 50 CHL test. In the work described here, in order to understand the regulation of histamine production, the hdcA gene was investigated. Only the A124 strain was found to contain the hdcA gene. The hdcA gene cluster consisted of the partial genes dapE, tnpA, and hdcRS, as well as the full-length genes hdcP, hdcA, and hdcB. This gene cluster was located in the chromosomal DNA of the A124 strain. Transcription of the hdcA gene was determined under various pH conditions using qRT-PCR. pH (3.0–7.0) did not have an effect on hdcA expression in T. halophilus A124. In contrast, the growth of strain A124 was very limited at pH <5.5, as evidenced by low histamine accumulation in the medium at this pH. Possible control mechanisms in histamine production are the amount of T. halophilus cells carrying the hdcA gene as well as the optimum pH for HdcA protein activity.