Histamine contents and histamine-forming bacteria in natto products in Taiwan

Thirty-six natto products imported from Japan and three domestic natto products sold in the supermarkets in southern Taiwan were purchased and tested to determine the occurrence of histamine and histamine-forming bacteria. The levels of pH and aerobic plate count (APC) in all samples ranged from 4.9 to 7.3 and 7.8 to 11.2 log CFU/g, respectively. None of these samples contained total coliform and E. coli. Although the average content for each of the nine biogenic amines in all samples was less than 5 mg/100 g, seven of them (17.9%) had histamine content greater than 5 mg/100 g, allowable limit suggested by the US Food and Drug Administration for scombroid fish and/or product. Four histamine-producing bacterial strains capable of producing 13.4 ppm to 17.5 ppm of histamine in trypticase soy broth (TSB) supplemented with 1.0% l-histidine (TSBH) were identified as Bacillus subtilis (two strains) and Staphylococcus pasteuri (two strains) by 16S rDNA sequencing with PCR amplification. To our knowledge, this is the first report to demonstrate the occurrence of histamine-forming bacteria in natto products.