Comparison of differential plating media and two chromatography techniquesfor the detection of histamine production in bacteria

The bacterial enzyme histidine decarboxylase (Hdc) catalyses the conversion of histidine into histamine. This amine is essential for the biosynthesis of iron chelators (siderophores) and is an important cause of food poisonin g after consumption of fish contaminated with histamine-producing bacteria. In this work we compared different methods for detecting histamine secrete d by different bacterial strains. The presence of histamine in the culture supernatant of Vibrio anguillarum, which produces Hdc and secretes the hist amine-containing siderophore anguibactin, was detected by thin-layer chroma tography. Similar results were obtained using the culture supernatant of th e Acinetobacter baumannii 19606 prototype strain that secretes the histamin e-containing siderophore acinetobactin. Conversely, histamine was not detec ted in the culture supernatant of an isogenic V. anguillarum Hdc mutant and the A. baumannii 8399 strain that secretes a catechol siderophore differen t from anguibactin and acinetobactin. These results were confirmed by capil lary gas chromatography/mass spectrometry. However, all these strains teste d positive for histamine secretion when cultured on differential plating me dia containing histidine and a pH indicator, which were specifically design ed for the detection of histamine-producing bacteria. The pH increase of th e medium surrounding the bacterial colonies was however drastically reduced when the histidine-containing medium was supplemented with peptone, beef e xtract, and glucose. The histidine-containing culture supernatants of the A . baumannii and V. anguillarum strains showed an increase of about two unit s of pH, turned purple upon the addition of cresol red, and contained high amounts of ammonia. Escherichia coli strains, which are Hdc negative and do not use histidine as a carbon, nitrogen, and energy source, gave negative results with the differential solid medium and produced only moderate amoun ts of ammonia when cultured in the presence of excess histidine. This study demonstrates that, although more laborious and requiring some expensive eq uipment, thin-layer and gas chromatography/mass spectrometry are more accur ate than differential media for detecting bacterial histamine secretion. Th e results obtained with these analytical methods are not affected by byprod ucts such as ammonia, which are generated during the degradation of histidi ne and produce false positive results with the differential plating media. (C) 1999 Elsevier Science B.V. All rights reserved.