# A Review on Food Safety: The Case of _Citrobacter_ sp., Fish and Fish Products
## 3 Fish
### Microbiology of Fish  
In living or recently caught fish, the microorganisms are located on the skin, gills, and intestines. The bacterial population is variable, with a normal range of \(10^{2}\)-\(10^{7}\) CFU/cm\({}^{2}\) on the surface of the skin and gills; meanwhile in the intestines, they range between \(10^{3}\) and \(10^{9}\) CFU/g [3; 29], and in fresh muscle, they range from \(10^{2}\) to \(10^{6}\) CFU/g [30]. Fish and other aquatic organisms have a microbial population that is dependent on that present in the aquatic environment where they live [29; 31].  
Bacteria on the skin and the gastrointestinal content of freshly caught, living fish do not invade the muscular tissue and internal organs, being considered sterile, due to the action of their natural defenses [29; 31]. Although [PERSON] et al. [29] reported that fish rearing in treated domestic sewage presented bacteria in muscle, where the number of bacteria causing their appearance in fish muscle has been termed the \"threshold concentration\",  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Hazard Type** & **Disease Causal Agent** & **Example** \\ \hline \hline \multirow{8}{*}{**Biological**} & _Vibrio_ spp., _Salmonella_ spp., _Shigella_ spp., _Plesiomas_ stigelloides, _Edwardsiella tarda_, _Listeria monocytogenes_, _Staphylococcus aureus_, _Escherichia coli_, _Clostridium_ sp., _Bacillus cereus_, _Campylobacter jejuni_, _Aeromonas_ spp., _Citrobacter freundii_, and _Aerobacter traakii_, _Dicocarpta_,  
concluding that for aspects of public health, fish can be raised in treated wastewater if the bacteriological quality of the water is compatible with the \"threshold concentration\" levels of the fish grown in ponds. When the fish dies, the bacteria present can penetrate inside, giving rise to spoilage, meaning that if the conditions and practices in capture and processing procedures are inadequate, contamination can be generated, accelerating spoilage and a risk to the health of consumers [31; 32].  
Health risks can be caused by different pathogenic bacteria, which can be classified as follows: (1) autochthonous, which are those whose natural habitat is water (_C. botulinum, Listeria spp., Vibrio spp., Plesiomonas shigelloides, Edwardsiella tarda, Pseudomonas_ spp., and _Aeromonas_ spp., among others) and are widely distributed in the aquatic environments of different places in the world, where temperature and salinity have a selective effect. They are also considered part of the natural microbiota of the fish [2; 4; 33]; and (2) non-native, which includes bacteria present in water with fecal contamination and/or associated with inadequate hygiene practices and conditions during capture, transportation, processing, and handling phases, identifying various enterobacteria, such as _E. coli, Shigella spp., Salmonella spp., Enterobacter cloacae, Citrobacter_ sp., _Serratia_ sp., _P. vulgaris, Listeria monocytogenes, S. agalactica, Staphylococcus epidermidis_, and _Staphylococcus aureus_, among others [2; 4; 30; 31; 34].  
Therefore, the microbial load of freshly caught, living fish bacteria considerably influences their quality, safety, and useful life [4; 10; 30]. Factors such as food, season of the year, geographical area, age and species of the fish, capture system, conditions, and hygienic practices in the capture, harvesting, handling, conservation, storage, and transportation of fish qualitatively and quantitatively influence the spoilage and health risk for consumers of fish [4; 10; 30; 31; 33].