Seafood Spoilage.....

Seafood is one of the most highly perishable food products because of the chemical effects of atmospheric oxygen and the growth of spoilage microorganisms.....

 

Spoilage of seafood can be caused by enzymes, dehydration, oxidation, contamination and physical damage. Sulphurous, ammoniacal, or fishy odours are some of the main organoleptic changes taking place during spoilage development.

The major cause of seafood spoilage is microbial growth and metabolic activity which result in the formation of amines, sulphides, alcohols, aldehydes, ketones, and organic acids with unpleasant and unacceptable off-flavours .

However, only a fraction of the initial microbiota of seafood known as specific spoilage organisms (SSOs), which is favoured by storage conditions (e.g., atmosphere, temperature), prevails over the rest of the microbiota, reaching high populations and producing corresponding metabolites (biochemical spoilage indices) .

Quorum sensing (QS), which involves the production, release and community-wide detection of extracellular signaling molecules called autoinducers, is a cell-to-cell communication process enabling microorganisms to collectively alter behavior patterns upon changes in cell density and species composition in surrounding community.

When a threshold concentration of the signaling molecule is reached, the group detects and responds to it with a populationwide alteration in gene expression. Therefore, QS-controlled processes, such as bioluminescence, the secretion of virulence factors, biofilm formation and the production of public goods, require the collective action of the group to be effective.

The most commonly studied autoinducers of QS signals include N-acyl-L-homoserine lactones (AHLs) in Gram-negative bacteria, oligopeptide in Gram-positive bacteria, and autoinducer-2 (AI-2) used in both Gram-negative and Grampositive bacteria.

Beyond these classes, recently, a range of cyclic dipeptides (diketopiperazines, DKPs) produced by multiple Gram-negative bacteria were reported to modulate supposedly AHLs-specific sensor system.

Therefore, DKPs have been suggested to represent a new class of naturally occurring QS signals, and to potentially play a role in both intra- and interspecies QS regulation.

The physiological and clinical aspects of QS have attracted considerable attention and been studied at the molecular level. However, there is a lack of knowledge on the role of QS in food spoilage. As cell-to-cell communication occurs in diverse bacterial species, QS likely plays a role in the microbial ecology of foods .
Main Spoilage Microorganisms in Different Seafood and Seafood Products

The recent establishment of the SSO concept has contributed significantly to our understanding of seafood spoilage.

The growth of different SSOs depends on several parameters: food product, type of preservation, temperature, atmosphere, and salt content, among others. During storage, the microflora changes owing to different abilities of the microorganisms to tolerate the preservation conditions .
 

Fresh Seafood Stored in Ice or under MAP/VP

In newly caught marine seafood from temperate waters, microflora is formed mainly by aerobic rods-shapes, anaerobic facultative and psychrotrophic Gram-negative bacteria, whose growth is possible at 0 and optimal at around 25. The majority belongs to the Gammaproteobacteria: Pseudomonas, Shewanella, Acinetobacter, Aeromonas, Vibrio, Moraxella, Psychrobacter, Photobacterium, etc. The same bacterial genus can be found in tropical marine seafood, but Cram-positive bacteria, Enterobacteriaceae and Vibrionaceae are often dominant . Generally, Pseudomonas spp., S. putrefaciens, S. baltica or Aeromonas spp. were common dominant spoilage bacteria in iced sea salmon ; gutted sea bass ; chilled fresh Mediterranean swordfish ; tropical prawns ; large yellow croaker . Pseudoalteromonas and Vibrio were dominant microorganisms in shucked oysters during iced-storage , as spoilage proceeded, enterococci, lactobacilli, and yeasts dominated at the later stages , the spoilage patterns of Mollusca shellfish differ in most species of seafood as they contain high levels of carbohydrate in the form of glycogen . Modified Atmosphere Packaging (MAP) and Vacuum-Packaging (VP), along with refrigeration, have become increasingly popular preservation techniques. Dominant strains isolated from spoiled squid were identified as Photobacterium phosphoreum . Bacteria grew faster under aerobic conditions, while the increase of CO2 and O2 reduction in MAP inhibited the bacterial growth and changed the microbial spoilage by suppressing mostly the Gram negatives and favoring the Gram positives . P. phosphoreum and L. piscium were identified as the main bacterial groups in MAP/VP raw salmon . The main SSO of modified atmosphere packaged Norway lobster is P. phosphoreum, since P. phosphoreum is known to withstand high CO2 concentrations . Lactic Acid Bacteria (LAB) and Brochothrix thermosphacta were co-dominant with Pseudomonas and H2S producing bacteria in gutted sea bass stored at 2 under MAP . Carnobacterium. maltaromaticum was the organism that showed the highest resistance to CO2 and to the lack of O2 among the organisms responsible for spoilage in mackerel fillets packed under modified atmospheres.

Lightly preserved seafood

Lightly preserved seafood are uncooked or mildly cooked products with low level preservatives which can influence their aw, pH, including brined/pickled/marinated seafood, cooked and peeled shrimp and shucked shellfish stored in MAP/VP or in brine, cold-smoked fish, etc. As a result, aerobic Gramnegative bacteria are inhibited, which allows the growth of other organisms more resistant to reduced aw.

Psychrobacter spp. and Pseudoalteromonas spp. were the dominant microbiota of cooked brown shrimp and enhanced spoilage by breaking down lipids and hydrolysing amino acids and proteins . The major spoilage bacterial isolates from spoiled cooked and whole tropical shrimp stored under MAP were C. maltaromaticum and S. baltica . LAB and Brochothrix spp. were dominant bacteria in the latter storage period of the VP-packed cold-smoked salmon, whereas Brochothrix spp. rather than LAB were responsible for spoilage . Differently, Joffraud et al. identified L. sakei and S. liquefaciens-like as the most spoiling bacteria. Besides, psychrotrophic marine vibrio and Photobacterium spp. were reported to be dominant microflora . The different spoilage microorganism’s profiles of cold-smoked salmon may result from the different treatments and environment.

In conclusion, the microflora changes owing to different abilities of the microorganisms to tolerate the storage conditions. Pseudomonas spp. and a few other Gram-negative psychrotrophic organisms will dominate seafoods stored aerobically at chill temperatures. CO2 packing or vacuum packing will inhibit the respiratory pseudomonads and cause a shift in the microflora to P. phosphoreum, LAB, Enterobacteriaceae and sometimes B. thermosphacta. Increasing the preservation by a decrease in pH, an increase in the NaCl concentration and by adding low level preservatives eliminates the Gram-negative microflora, LAB is the remaining organisms in semi-preserved fish products.
QS Regulated Seafood Spoilage

The physiological and clinical aspects of QS have attracted considerable attention and been studied at the molecular level. However, there is a lack of knowledge on the role of QS in food spoilage, especially in seafood. As cell-to-cell communication exists in diverse bacterial species, QS likely plays a role in the microbial ecology of foods. In the past few years, the possible role of QS in food spoilage has been explored, including siderophore synthesis, metabolic activities and biofilm formation, predominantly.

The Siderophores synthesis

All aerobic and facultative anaerobic bacteria require iron for growth and only LAB do not depend on supplementation of this mineral. In fish muscle, the environment is iron-limited despite of the rich nutrient and high affinity chelators, the so-called siderophores are produced to scavenge ironduring bacterial growth. Although fish tissue allowed the siderophore production by most Pseudomonas and S. putrefaciens isolates from fish, S. putrefaciens was inhibited by Pseudomonas sp. particularly when iron was limited. Later, the biosynthesis of siderophore in Pseudomonas aeruginosa was firstly reported to be controlled by QS system, lasR mutants showed a reproducible 2-fold decrease in production of the catecholate-hydroxamate siderophore pyoverdine during grown under iron-limited conditions. Similarly, lasI mutants defective in the biosynthesis of the autoinducer PAI-1 also had a 2-fold decrease in pyoverdine production which could be largely restored upon addition of exogenous PAI-I.

It was reported that exogenous AHL was required for the stimulated biosynthesis of heterologous siderophore in marine-isolated bacteria, and stimulated growth by exogenous siderophores and AHLs was also observed in other non-siderophore- producing bacteria . Rasch et al. have reported that bacterial spoilage of bean sprouts was influenced by QS, the AHL-negative mutant of Enterobacteriaceae was impaired in siderophore activities and spoilage potential, for the first time demonstrating that iron chelation in Enterobacteriaceae was regulated by AHL . These reports offer a new perspective for exploring seafood spoilage mediated by intra and inter-species cell-cell communication, although little study has focused on the regulation of QS on the siderophore-associated spoilage in seafood.