Seafood spoilers: Streptococcal bacteria in vacuum-packed seafood product isolated and characterised

Microbial spoilage

 

The food industry uses refrigeration and vacuum packaging to preserve fresh and processed foods and extend their shelf lives but the process is not foolproof. Some pathogenic or spoilage bacteria still manage to grow, even if it is at a reduced rate. Seafood is no exception to the perils of bacteria and various genera have been isolated and identified from chilled fish and shellfish.

One strain found recently in seafood is Streptococcus parauberis. It is best known for causing mastitis in dairy cattle but also induces streptococcosis in fish, an infectious disease which has a high mortality rate for farmed fish. The bacterium has also been detected in meat and seafood products. Although it is not harmful to humans, it can still grow at refrigerated temperatures, so is a threat to the integrity of stored seafood.

A team of Spanish scientists has now isolated S. parauberis from seafood for the first time and carried out a full characterisation, enabling its rapid identification in future. Jorge Barros-Velazquez and colleagues from the University of Santiago de Compostela, the Institute for Marine Research (IIM-CSIC), Vigo, and the University Complutense of Madrid used a variety of methods in their study.

Biomarker panel extracted from mass spectra

A spoiled surimi-based seafood product that had been vacuum packed and refrigerated was acquired from a local company. It was darker than unspoiled samples and emitted an unpleasant smell when it was opened. The food consisted primarily of minced fish supplemented with egg white, wheat starch, salt, water, sunflower oil and flavour enhancers.

Two distinct microbial isolates were recovered from agar gels and they were each subjected to phenotyping, genotyping and proteomics examination. Both isolates were Gram-positive species and they were found to be identical. Their 16S sRNA sequences were compared to reference strains in the GenBank and a phylogenetic tree placed them with other S. parauberis strains, separated from the closely related S. uberis strains.

Further characterisation was carried out by analysis by matrix-assisted laser desorption/ionisation mass spectrometry (MALDI MS). The soluble proteins were extracted from the intact bacterial cells of each isolate and analysed intact, without any prior enzymatic breakdown.

The protein mass spectra of both isolates were very similar, with about 30 identical peak masses that varied minimally in relative intensities. They were also remarkably similar to the spectra of two other S. parauberis reference strains but could be distinguished visually from other potential seafood bacteria such as Clostridium perfringens, Carnobacterium maltaromaticum and Stenotrophomonas maltophilis.

In fact, the mass spectral profiles were regarded as unique when compared with those of 90 bacterial species that are associated with seafood safety and quality. This distinctiveness was illustrated by the identification of five peak masses in the range m/z 2200-6000 that were exclusive to this species. This subset of peaks could provide a panel of biomarkers for the rapid and reliable identification of S. parauberis in seafood spoilage studies.

This is the first report of the isolation of S. parauberis in seafood, and in spoiled, refrigerated, vacuum-packed seafood in particular, suggesting that it should be considered as a potential spoilage bacterium in future food studies. However, it might not occur with all types of foods. Its survival could depend on the presence of carbohydrates in the food, in this case wheat starch, which is consistent with reported occurrences in other processed foods.