Incidence of histamine-forming bacteria and histamine content in scombroid fish species from retail markets in the Barcelona area.

Incidence and diversity of histidine decarboxylating bacteria were determined in samples of tunafish, bonito and mackerel purchased at different retail markets. Histamine-forming bacteria occurred in a low proportion and always accounted for less than 0.1% of the total bacterial load in the fish samples studied. Similarly, histamine content in fish samples also was low ( < 25 ppm) and all of them met current histamine standards established by the European Union. Histamine was found in 83.3% of the tested tunafish samples with an average of 8.9 ppm. In contrast, none of mackerel samples and only 2 out of 12 of bonito showed detectable amounts of histamine. Morganella morganii and Klebsiella oxytoca were the most active histamine formers under experimental conditions, and produced on average 2765 and 1415 ppm of histamine, respectively, after incubation at 37 degrees C for 18 h. Some new histamine formers, such as Plesiomonas shigelloides, Enterobacter intermedium, Serratia marcescens, Serratia plymuthica and Serratia fonticola, have been identified. Especially Plesiomonas shigelloides would have an important role within histidine decarboxylating bacteria because it was the sole histamine former isolated that has frequently been associated with the marine aquatic environment. However, only 8-340 ppm of histamine was formed by these strains in laboratory trials.