# A Review on Food Safety: The Case of _Citrobacter_ sp., Fish and Fish Products
## 4 _Citrobacter_ sp.
### Citrobacter, Histamine, and Food Diseases by Fish Consumption  
Fish, besides being a nutritious source of food, are susceptible to deterioration and endogenous or exogenous contamination by bacteria, particularly those that produce biogenic amines [23]. Biogenic amines are biologically active low-molecular-weight, organic, non-protein nitrogenous compounds present in numerous foods, such as cheese, wine, meat, vegetables, and fish [53; 54]. Protein hydrolysis is carried out by the action of endoenzymes that release amino acids that, in turn, serve as a substrate for enzymes (amino acid decarboxylase) of microbial origin, resulting in the generation of biogenic amines (Figure 1) [39; 54; 55]. Histamine-producing microorganisms are related to the natural microbiota of fish, but they can also come from contamination in post-capture stages, such as processing, conservation, storage, or distribution systems [26; 39; 53; 56]. Histamine-producing microorganisms in fish are varied, including species of the genera _Streptococcus_ spp., _Clostridium_ spp., _Tetragonococcus_ spp., _Bacillus_ spp., _Acinetobacter_ spp., and _Pseudomonas_ spp. Some enterobacteria considered indicators of food hygiene are _Citrobacter_ spp., _Serratia_ spp., _Hafnia_ spp., _Vibrio_ spp., _Escherichia_ spp., _Edwarsiella_ spp., _Klepsiella_ spp., _Salmonella_ spp., _Shigella_ spp., _Photobacterium_ spp., _Enterobacter_ spp., _Plesiomonas_ spp., _Proteus_ spp., and _Morganella_, with the species _M. morganii_ considered the main producer [26; 53; 57; 58; 59; 60; 61; 62; 63]. The amino acids that undergo decarboxylation reactions generate various biogenic amines, where histidine produces histamine, tryptophan produces tryptamine, lysine produces cadaverine, and where arginine gives rise to putrescine, spermidine, spermine and tyrosine gives rise to tyramine. These compounds have the particularity of presenting ammoniacal odors and of putfefaction, which are indicators of fish deterioration, and have good correlation with values of total volatile bases, trimethylamines, hypoxanthine, and pH [32; 55]. In fresh or recently caught fish, the content of biogenic amines is low or negligible and histamine levels are less than 0.01 mg/100g [65]. In fresh sardines, histamine is present at levels of less than 5 mg/100 g, cadaverine is present at levels of less than 15 mg/100 g, and putrescine is present at levels of less than 1 mg/100 g, but they increase as deterioration progresses [66; 55]. It has been pointed out that the generation of biogenic amines results from the availability of free amino acids and microorganisms that produce decarboxylate enzymes, in addition to other factors such as temperature, salts, presence of fermentable carbohydrates, oxygen concentration, redox potential, and pH [32]. The consumption of foods with high levels of biogenic amines has been related to negative effects on the health of consumers, with gastrointestinal, neurological, and hemodynamic disorders with different symptoms, such as malaise, nausea, respiratory disorders, hot flashes, sweating, palpitations, migraines, headaches, head, itchy eyes, hyper and hypotension, stomach and intestinal problems, and pseudo-allergic reactions [54; 56]. The presence of biogenic amines especially affects consumers with cardiovascular, respiratory,  
Figure 1: Degradation of proteins and amino acids in postmortem fish and formation of biogenic amines [55; 64]. and gastrointestinal disorders; alcoholism problems; and vitamin B12 deficiency, among others [32]. Among the different biogenic amines, histamine is mainly related to cases of poisoning from fish consumption due to its high levels [54; 66]. Histamine has a vasoactive and psychoactive action, and it is a mediator of allergic diseases, so the consumption of foods with histamine can present the same symptoms of allergic processes [54].