Purpose: The purpose of this study was to identify Photobacterium spp. capable of producing histamine at levels of human health concern (≥200 to 500 ppm).
Methods: Histamine production of 108 Photobacterium strains from 24 species was determined after incubation in 10 ml of TSB containing 1% histidine, at 20°C for 48 h with 200 rpm shaking. Histamine was determined using the modified AOAC fluorometric method (977.13). The presence of the histidine decarboxylase (hdc) gene was determined by real-time PCR. Selected Photobacterium spp. were shotgun sequenced by Ion Torrent Instrumentation to confirm the presence of the hdc gene.
Results: Four Photobacterium spp. had strains that were able to produce histamine concentrations of human health concern (≥200 to 500 ppm); Photobacterium angustum (1 of 12), Photobacterium aquimaris (2 of 3), Photobacterium kishitanii (20 of 20), and Photobacterium phosphoreum (3 of 19). All histamine-producing Photobacterium spp., except P. phosphoreum, were also positive by real-time PCR for the presence of the hdc gene. Shotgun sequencing did not identify the hdc gene in the three histamine-producing P. phosphoreum strains.
Significance: Results indicated that there may be variation in the hdc gene or other genes responsible for histamine production by P. phosphoreum strains. In addition, recognizing the histamine-producing species is important to characterizing conditions that can lead to toxic levels of histamine production in fish.