Histamine-Producing Bacteria and Histamine Induction in Retail Sardine and Mackerel from Fish Markets in Egypt.

This study examined the occurrence of histamine-producing bacteria (HPB) and histamine induction in retail sardine and mackerel in Egypt; and whether the fish vendors play a role in the transmission of HPB. Fish were collected from the fish markets, additionally; hand swab samples were taken from the fish vendors. All samples were cultured on modified Niven's medium (MNM); the positive colonies were subcultured on Violet Red Bile Glucose (VRBG) agar, followed by biochemical identification and histidine decarboxylase ()-gene-PCR of the VRBG-positive isolates. The -gene-positive fish and human isolates were subjected to partial -gene-sequencing and phylogenetic analysis. Production of histamine in the fish muscles was measured by high-performance liquid chromatography. A higher percentage of sardine showed the presence of MNM-positive bacteria (84%) than mackerel (53%). was the dominant family; the most frequent species were , , and . Higher proportion of the isolates were positive as compared with the other species. Only 32% sardine and 17% mackerel of the MNM-positive isolates carried the gene. Fish muscles that contain -positive bacteria exhibit higher levels of histamine (median 86; IQR 80-1112 mg/kg) than those with -negative bacteria (48; 75-223 mg/kg). The level of histamine was significantly higher in sardine (109; 104-1094 mg/kg) than in mackerel (40; 49-106 mg/kg). The 20 fish vendor samples were MNM positive, 2 of them were -gene positive. The close genetic relatedness between the human and fish strains isolated from the same markets suggests a possible bidirectional transmission of the HPB. This warns for the presence of HPB carrying gene in retail sardine and mackerel, which is associated with a relatively high level of histamine. Regular inspection of the fish markets is required, including accurate determination of HPB by using a combination of the MNM culture, gene PCR, and measurement of histamine level.