bacterial pathogens in tuna

[shahbaz]

i did a microbiology test on two samples of tuna..one was fresh the other a couple of weeks old. The fresh sample tested positive for C.perfringens, Listeria and negative for Salmonella whilst the other sample was negative for all three pathogens. can someone explain why this was? is it because of the fish being fresh and bacteria having the suitable environment to live there? The tuna was also not in brine or oil...could that have also affected it?

[wildwood]

Yep.

[theprof]

I can give you some guidelines to answer this question, but first of all, you've not really provided all the information that would be needed to answer this question if it was included in an exam for example.

I'll assume that the "fresh" tuna as you refer to it was analysed within a day of capture. However, the "couple of weeks old" sample concerns me. You've not mentioned quite a few things about it. For example, how was it stored? What temperature was it stored at? Was it ever frozen? Were the fish eviscerated? Type of packaging eg modified atmosphere, vacuum packed? Were they whole fish or just meat? I'm not criticising, but all these factors are relevant as I'll try and show.

Fish meat per se is completely sterile. Bacteria are only found on skin, gills and the intestinal tract of fish species. Bacterial contamination of fish nearly always arises from contamination that occurs during gutting, contaminated ship equipment and surfaces, contaminated ice used in storage, contaminated storage containers, transfer of pathogens between animal/human vector and fish and partial decomposition arising from landing delays. Once landed, dockside equipment can also transfer bacteria to fish as well as unhygienic practices and human/fish vector transfer. Now to the bacteria.

(continued)

[theprof]

C perfringens is anaerobic and is indigenous to the aquatic environment. It does not grow at normal chilled temperatures and grows only moderately at temperatures below about 18 degrees C.

Listeria spp is virtually unknown in open water and cannot be isolated from fish caught in such water. Coastal waters and estuaries are the exception where the bacterium may be found in fish as a result of nearby agricultural activities. However L. monocytogenes is very common in processed fish products due to post processing contamination from the sources I discussed earlier. Listeria species are halotolerant but Nacl concentration does have predictable effects on growth rates. Freezing and heat treatment will eliminate Listeria growth. Listeria has been shown to grow very slowly in processed fish with high lipid content.

Salmonella spp are principally found in the intestinal tract of warm blooded animals including humans. However, there is also a reservoir of the bacterium to be found in molluscan shellfish. Salmonella is hardly ever found in marine waters with the exception of estuaries and coastal waters where contamination occurs as a result of human and animal activities. These include fertiliser run-offs as well as contamination from human excreta. Salmonella grows between about 5 degrees C and 46 degrees C. Post process contamination with Salmonella is prevalent throughout the world although Pasteurisation is an effective means of control in some foods.

I�ve given you enough information for you to answer your questions. Think about what I�ve said and factor in the storage conditions of the second sample.