Foods · 4 authors, 2 centres
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This study identified zoonotic and opportunistic bacteria, including Salmonella, Staphylococcus aureus, and Vibrio fluvialis, on the surface of frozen freshwater fish imported into Australia from two countries. The presence of mud, natural diet components, and vegetation was significantly associated with Pseudomonas contamination. These findings highlight potential food safety risks and suggest that current import testing may be insufficient to protect consumers.
**Background:** The global expansion of freshwater fish farming, often with inadequate biosecurity and antibiotic regulation, has led to bacterial contamination of fish products. Imported fish may carry zoonotic bacteria that pose risks to consumers through direct contact or consumption. Australia currently tests only for *Listeria monocytogenes* in high-risk imported fish. This study aimed to identify selected bacteria on imported freshwater fish from two high-risk countries (Country 20 and Country 22) and assess associations with supply chain breaches.
**Methods:** A total of 60 whole fish (Species A, Country 22) and 41 fillets (Species B, Country 20) were obtained from retail outlets. Fish were thawed in original packaging, and the outer surface of each fish and the bag water were swabbed for bacterial culture. Selective media were used to isolate *E. coli*, *Pseudomonas* spp., *Salmonella* spp., *Staphylococcus aureus*, *Clostridium* spp., and *Vibrio* spp. Bacterial identification was performed using Gram stain, biochemical tests (e.g., Catalase, Oxidase, Coagulase, Rapid ID™ NF Plus), and the *Salmonella* Latex Test. Antimicrobial susceptibility was tested via Kirby-Bauer disk diffusion for cefovecin, cefoxitin, ciprofloxacin, fosfomycin, vancomycin, and for *V. fluvialis* also colistin and cephalothin. Associations between bacteria presence and supply chain breaches (mud, natural diet, vegetation) were analyzed using univariable logistic regression.
**Key Results:** From Species A (Country 22), 66 samples yielded *Pseudomonas* spp. (recovery rate [RR] 51.5%, mean 104.0 cfu/g), *Micrococcus* sp. (RR 48.5%, mean 50.0 cfu/g), *Comamonas testosteroni* (RR 40.9%, mean 61.7 cfu/g), *Rhizobium radiobacter* (RR 4.5%, mean 7.6 cfu/g), *Salmonella* sp. (RR 1.5%, 32 cfu/g), and *Staphylococcus aureus* (RR 1.5%, 27 cfu/g). From Species B (Country 20), 47 samples yielded *Vibrio fluvialis* (RR 21.3%, mean 13.9 cfu/g), *Salmonella* sp. (RR 12.7%, mean 41.6 cfu/g), *Micrococcus* sp. (RR 6.4%, mean 4.6 cfu/g), and *S. aureus* (RR 2.1%, 1.0 cfu/g). No *E. coli* or *Clostridium* were detected. Antimicrobial resistance patterns showed that *Pseudomonas* spp. were 100% resistant to cefovecin, cefoxitin, and vancomycin; *Salmonella* sp. from Species A was resistant to all tested antimicrobials; *S. aureus* from Species B was resistant to all tested antimicrobials; *V. fluvialis* was 100% resistant to colistin, cephalothin, and vancomycin. Univariable logistic regression revealed significant associations: presence of mud (OR 7.59, CI 2.95–20.73, p<0.001), natural diet (OR 8.47, CI 3.37–22.94, p<0.001), and vegetation (OR 13.11, CI 3.10–90.27, p<0.001) were each associated with increased odds of *Pseudomonas* spp. recovery. *Salmonella* was less likely in Species A than Species B (OR 0.10, CI 0.005–0.61, p=0.010), while *Micrococcus* was more likely in Species A (OR 11.85, CI 3.76–52.80, p<0.001).
**Clinical Implications:** The presence of zoonotic bacteria such as *Salmonella*, *S. aureus*, and *V. fluvialis* on imported fish surfaces poses a risk of cross-contamination in kitchens and potential foodborne illness, especially in immunocompromised individuals. The high recovery rates and antimicrobial resistance patterns, including multidrug resistance, underscore the need for enhanced surveillance and possibly revised import testing protocols beyond *L. monocytogenes*. The significant association between supply chain breaches (mud, natural diet, vegetation) and bacterial contamination suggests that enforcing existing import cleanliness standards could reduce microbial hazards. Assistance to exporting countries to improve processing hygiene may also be beneficial.