Prevalence, persistence and control of Salmonella and Listeria in shrimp and shrimp products: A review

Consumer method to control Salmonella and Listeria species in shrimp

The purpose of this study was to determine whether the current consumer method of boiling shrimp until floating and pink in color is adequate for destroying Listeria and Salmonella. Shrimp samples were submerged in bacterial suspensions of Listeria and Salmonella for 30 min and allowed to air dry for 1 h under a biosafety cabinet. Color parameters were then measured with a spectrophotometer programmed with the CIELAB system. Twenty-four shrimp samples were divided into groups (days 0, 1, or 2) and stored at 4°C. The samples were treated by placing them in boiling water (100°C) on days 0, 1, and 2. The shrimp were immediately removed from the boiling water once they floated to the surface, and color parameters were measured. Bacterial counts were determined, and the log CFU per gram was calculated. The effect of sodium tripolyphosphate on the color change of cooked shrimp also was determined. Initial bacterial counts on shrimp after air drying were 5.31 ± 0.14 log CFU/g for Salmonella Enteritidis, 5.24 ± 0.31 log CFU/g for Salmonella Infantis, 5.40 ± 0.16 log CFU/g for Salmonella Typhimurium, 3.91 + 0.11 log CFU/g for Listeria innocua, 4.45 ± 0.11 log CFU/g for Listeria monocytogenes (1/2a), and 3.70 ± 0.22 log CFU/g for Listeria welshimeri. On days 0, 1, and 2, all bacterial counts were reduced to nondetectable levels for shrimp samples that floated. The average time for shrimp to float was 96 ± 8 s. The bacterial counts remained at nondetectable levels (<10 log CFU/g) during refrigerated (4°C) storage of cooked shrimp for 2 days. The redness, yellowness, and lightness were significantly higher (P < 0.0001) for the cooked shrimp than for the uncooked shrimp on all days tested. The standard deviation for redness in the cooked shrimp was large, indicating a wide range of pink coloration on all days tested. The results suggest that boiling shrimp until they float will significantly reduce Listeria and Salmonella contamination, but color change is not a good indication of reduction of these pathogens because of the wide natural color variation.

Prevalence of Salmonella and Vibrio spp. in seafood products sold in Singapore

Foodborne hazards in seafood have only recently received increased attention in Singapore since the illness outbreak in 2009 that was associated with consumption of Indian rojak (a traditional salad of fruits, vegetables, and seafood). The microbiological quality of seafood must be evaluated for assurance of food safety. The aim of this study was to evaluate the microbiological quality and to determine the prevalence of Salmonella and Vibrio spp. in seafood sold in Singapore. A total of 116 samples (41 prawn, 44 shellfish, and 31 fishball samples) were collected from major supermarkets and wet markets in Singapore. The mesophilic and psychrotrophic bacterial counts for prawn, shellfish, and fishballs were 2 to 7 log CFU/g. One Salmonella Lexington strain was isolated from a thawed-frozen shellfish product and two Vibrio parahaemolyticus strains were isolated from commercial fishball and shrimp meat products. Thus, seafood sold in Singapore has the potential to be contaminated with Vibrio spp. and Salmonella, and proper handling at food service establishments is required to ensure food safety. Effective control measures also are needed to prevent cross-contamination during postharvest seafood processing.

Effects of immunostimulants targeting Ran GTPase on phagocytosis against virus infection in shrimp

The global shrimp aquaculture has been consistently beset by diseases that cause severe losses in production. To fight various harmful pathogens, the enhanced shrimp immunity by immunostimulants would play key roles against the invading pathogens. In aquaculture, however, the target proteins/genes which can be used for the screening of immunostimulants are very limited. Based on our previous study, in the present study, the shrimp Ran protein, which was required in shrimp antiviral phagocytosis, was used as the target protein to screen for immunostimulants. The GTPase activity assays showed that the IL-4 and lysophosphatidylcholine molecules could enhance the activity of Ran protein, suggesting that the two molecules might function in phagocytosis. When the IL-4 and lysophosphatidylcholine were respectively injected into shrimp, the results indicated that the two molecules enhanced the hemocytic phagocytosis against white spot syndrome virus (WSSV), suggesting that they improved the activity of phagocytosis through the activation of the Ran protein. It was evidenced that the enhancement of phagocytosis activity effectively inhibited the WSSV infection in shrimp, which further led to the decrease of mortalities of WSSV-infected shrimp. Therefore, our study presented a novel strategy for the screening of immunostimulants by using the key proteins in immune responses of aquatic organisms as the target proteins, which would be very helpful for the development of efficient approaches to prevent the aquatic organisms from pathogen infections.

Effect of antimicrobial spice and herb extract combinations on Listeria monocytogenes, Staphylococcus aureus, and spoilage microflora growth on cooked ready-to-eat vacuum-packaged shrimp

Two spice and herb extract combinations from galangal (Alpinia galanga), rosemary (Rosmarinus officinalis), and lemon iron bark (Eucalyptus staigerana) were evaluated for their ability to inhibit the growth of inoculated Listeria monocytogenes and Staphylococcus aureus and naturally present spoilage microflora on cooked ready-to-eat shrimp stored for 16 days at 4 or 8 °C. A combination of galangal, rosemary, and lemon iron bark significantly reduced (P < 0.05) levels of aerobic bacteria and lactic acid bacteria at 4 °C on day 12 by 1.6 and 1.59 log CFU/g, respectively. By day 16, levels of these bacteria were equivalent to those of controls. The shrimp treated with this spice and herb extract combination had significantly lower (P < 0.05) lipid oxidation from day 4 to day 16. Similarly, a combination of galangal and rosemary extract significantly reduced (P < 0.05) levels of aerobic bacteria and lactic acid bacteria at 8 °C on day 8 by 2.82 and 2.61 log CFU/g, respectively. By days 12 and 16, levels of these bacteria were equivalent to those of controls. The shrimp treated with this spice and herb combination had significantly lower (P < 0.05) lipid oxidation on days 4 and 16. None of the spice and herb extract combinations had an effect on levels of L. monocytogenes or S. aureus or changed the color or pH of the shrimp during storage. The results of this study indicate that combinations of galangal, rosemary, and lemon iron bark extracts can be used to control the growth of spoilage microflora on ready-to-eat shrimp.

Enhancement of shrimp antiviral immune response through caspase-dependent apoptosis by small molecules

Epidemic diseases cost large amount of economic loss in the shrimp aquaculture. To control the epidemic diseases, it is a very efficient approach to enhance the shrimp immunity by immunostimulants. In aquaculture, however, the applications of the available immunostimulants are very limited due to the lack of information about the roles of these immunostimulants in animal immunity. In the present study, a caspase protein (PjCaspase), required in shrimp antiviral apoptosis, was used as the target protein to screen for small molecules which would enhance the shrimp immunity. Based on screening using the EGFP-PjCaspase fusion protein in insect cells, four small molecules could enhance the activity of PjCaspase protein. Among them, IL-2 and evodiamine were further evidenced to enhance the apoptotic activity of shrimp hemocytes in vivo, suggesting that the small molecules improved the activity of apoptosis through the activation of the PjCaspase protein. The results indicated that the enhancement of apoptotic activity effectively inhibited the white spot syndrome virus (WSSV) infection in shrimp, which further led to the decrease of mortalities of WSSV-infected shrimp. Therefore, our study, for the first time, presented that the strategy using the key proteins in immune responses of aquatic organisms as the target proteins was a very efficient approach for the screening of immunostimulants to prevent the aquatic organisms from pathogen infections.

Radiation inactivation of foodborne pathogens on frozen seafood products

Foodborne illness due to consumption of contaminated seafood is, unfortunately, a regular occurrence in the United States. Ionizing (gamma) radiation can effectively inactivate microorganisms and extend the shelf life of seafood. In this study, the ability of gamma irradiation to inactivate foodborne pathogens surface inoculated onto frozen seafood (scallops, lobster meat, blue crab, swordfish, octopus, and squid) was investigated. The radiation D(10)-values (the radiation dose needed to inactivate 1 log unit of a microorganism) for Listeria monocytogenes, Staphylococcus aureus, and Salmonella inoculated onto seafood samples that were then frozen and irradiated in the frozen state (-20°C) were 0.43 to 0.66, 0.48 to 0.71, and 0.47 to 0.70 kGy, respectively. In contrast, the radiation D(10)-value for the same pathogens suspended on frozen pork were 1.26, 0.98, and 1.18 kGy for L. monocytogenes, S. aureus, and Salmonella, respectively. The radiation dose needed to inactivate these foodborne pathogens on frozen seafood is significantly lower than that for frozen meat or frozen vegetables.

Discriminating multi-species populations in biofilms with peptide nucleic acid fluorescence in situ hybridization (PNA FISH)

BACKGROUND

Our current understanding of biofilms indicates that these structures are typically composed of many different microbial species. However, the lack of reliable techniques for the discrimination of each population has meant that studies focusing on multi-species biofilms are scarce and typically generate qualitative rather than quantitative data.

METHODOLOGY/PRINCIPAL FINDINGS

We employ peptide nucleic acid fluorescence in situ hybridization (PNA FISH) methods to quantify and visualize mixed biofilm populations. As a case study, we present the characterization of Salmonella enterica/Listeria monocytogenes/Escherichia coli single, dual and tri-species biofilms in seven different support materials. Ex-situ, we were able to monitor quantitatively the populations of ∼56 mixed species biofilms up to 48 h, regardless of the support material. In situ, a correct quantification remained more elusive, but a qualitative understanding of biofilm structure and composition is clearly possible by confocal laser scanning microscopy (CLSM) at least up to 192 h. Combining the data obtained from PNA FISH/CLSM with data from other established techniques and from calculated microbial parameters, we were able to develop a model for this tri-species biofilm. The higher growth rate and exopolymer production ability of E. coli probably led this microorganism to outcompete the other two [average cell numbers (cells/cm(2)) for 48 h biofilm: E. coli 2,1 × 10(8) (± 2,4 × 10(7)); L. monocytogenes 6,8 × 10(7) (± 9,4 × 10(6)); and S. enterica 1,4 × 10(6) (± 4,1 × 10(5))]. This overgrowth was confirmed by CSLM, with two well-defined layers being easily identified: the top one with E. coli, and the bottom one with mixed regions of L. monocytogenes and S. enterica.

SIGNIFICANCE

While PNA FISH has been described previously for the qualitative study of biofilm populations, the present investigation demonstrates that it can also be used for the accurate quantification and spatial distribution of species in polymicrobial communities. Thus, it facilitates the understanding of interspecies interactions and how these are affected by changes in the surrounding environment.

Lactic acid as a potential decontaminant of selected foodborne pathogenic bacteria in shrimp (Penaeus merguiensis de Man)

Fresh raw shrimps were dipped for 10, 20, and 30 min at room temperature (25°C ± 1°C) in lactic acid (LA; 1.5%, 3.0%, v/v) to evaluate their antipathogenic effects against Vibrio cholerae, Vibrio parahaemolyticus, Salmonella entreitidis, and Escherichia coli O157:H7 inoculated at a level of 10(5) CFU/g. Significant reductions in the population of all these pathogenic bacteria were recorded after dipping treatments, which were correlated to the corresponding LA concentrations and treatment time. With respect to the microbial quality, 3.0% LA treatment for 10 min was acceptable in reducing the pathogenic bacteria. Additionally, sensory evaluation results revealed a 10-min dip in 3.0% LA to be more acceptable organoleptically compared with 20 and 30 min of treatments. Results of the present study are envisaged to be useful for commercial applications for effective decontamination of shrimp.

Sources and risk factors for contamination, survival, persistence, and heat resistance of Salmonella in low-moisture foods

Sources and risk factors for contamination, survival, persistence, and heat resistance of Salmonella in low-moisture foods are reviewed. Processed products such as peanut butter, infant formula, chocolate, cereal products, and dried milk are characteristically low-water-activity foods and do not support growth of vegetative pathogens such as Salmonella. Significant food safety risk might occur when contamination takes place after a lethal processing step. Salmonella cross-contamination in low-moisture foods has been traced to factors such as poor sanitation practices, poor equipment design, and poor ingredient control. It is well recognized that Salmonella can survive for long periods in low-moisture food products. Although some die-off occurs in low-moisture foods during storage, the degree of reduction depends on factors such as storage temperature and product formulation. The heat resistance of Salmonella is affected by many factors, mostly by strain and serotypes tested, previous growth and storage conditions, the physical and chemical food composition, test media, and the media used to recover heat-damaged cells. Salmonella heat resistance generally increases with reducing moisture. Care must be taken when applying published D- and z-values to a specific food process. The product composition and heating medium and conditions should not be significantly different from the product and process parameters used by the processors.