Growth Rates of Vibrio parahaemolyticus Sequence Type 36 Strains in Live Oysters and in Culture Medium

The pathogenic marine bacterium Vibrio parahaemolyticus can cause seafood-related gastroenteritis via the consumption of raw or undercooked seafood. Infections originating from relatively cool waters in the northeast United States are typically rare, but recently, this region has shown an increase in infections attributed to the ecological introduction of pathogenic sequence type 36 (ST36) strains, which are endemic to the cool waters of the Pacific Northwest. A 2005 risk assessment performed by the Food and Drug Administration (FDA) modeled the postharvest growth of V. parahaemolyticus in oysters as a function of air temperature and the length of time the oysters remained unrefrigerated. This model, while useful, has raised questions about strain growth differences in oyster tissue and whether invasive pathogenic strains exhibit different growth rates than nonclinical strains, particularly at lower temperatures. To investigate this question, live eastern oysters were injected with ST36 clinical strains and non-ST36 nonclinical strains, and growth rates were measured using the most probable number (MPN) enumeration. The presence of V. parahaemolyticus was confirmed using PCR by targeting the thermolabile hemolysin gene (tlh), thermostable direct hemolysin (tdh), tdh-related hemolysin (trh), and a pathogenesis-related protein (prp). The growth rates of the ST36 strains were compared to the FDA model and several other data sets of V. parahaemolyticus growth in naturally inoculated oysters harvested from the Chesapeake Bay. Our data indicate that the growth rates from most studies fall within the mean of the FDA model, but with slightly higher growth at lower temperatures for ST36 strains injected into live oysters. These data suggest that further investigations of ST36 growth capability in oysters at temperatures previously thought unsuitably low for Vibrio growth are warranted. IMPORTANCE Vibrio parahaemolyticus is the leading cause of seafood-related gastroenteritis in the United States, with an estimated 45,000 cases per year. Most individuals who suffer from vibriosis consume raw or undercooked seafood, including oysters. While gastroenteritis vibriosis is usually self-limiting and treatable, V. parahaemolyticus infections are a stressor on the growing aquaculture industry. Much effort has been placed on modeling the growth of Vibrio cells in oysters in order to aid oyster growers in designing harvesting best practices and ultimately, to protect the consumer. However, ecological invasions of nonnative bacterial strains make modeling their growth complicated, as these strains are not accounted for in current models. The National Shellfish Sanitation Program (NSSP) considers 10°C (50°F) a temperature too low to enable Vibrio growth, where 15°C is considered a cutoff temperature for optimal Vibrio growth, with temperatures approaching 20°C supporting higher growth rates. However, invasive strains may be native to cooler waters. This research aimed to understand strain growth in live oysters by measuring growth rates when oysters containing ST36 strains, which may be endemic to the U.S. Pacific Northwest, were exposed to multiple temperatures postharvest. Our results will be used to aid future model development and harvesting best practices for the aquaculture industry.

Vibrio parahaemolyticus type IV pili mediate interactions with diatom-derived chitin and point to an unexplored mechanism of environmental persistence

Vibrio parahaemolyticus is a naturally occurring bacterium common in coastal waters where it concentrates in shellfish through filter feeding. The bacterium is a human pathogen and the leading cause of seafood-borne gastroenteritis. Presently there is little information regarding mechanisms of environmental persistence of V.parahaemolyticus or an accurate early warning system for outbreak prediction. Vibrios have been shown to adhere to several substrates in the environment, including chitin, one of the most abundant polymers in the ocean. Diatoms are abundant in estuarine waters and some species produce chitin as a component of the silica cell wall or as extracellular fibrils. We examined the role of specific surface structures on the bacterium, the type IV pilins PilA and MshA, in adherence to diatom-derived chitin. Biofilm formation and adherence of V.parahaemolyticus to chitin is mediated by the ability of the bacterium to express functional type IV pili. The amount of adherence to diatom-derived chitin is controlled by increased chitin production that occurs in later stages of diatom growth. The data presented here suggest late-stage diatom blooms may harbour high concentrations of V.parahaemolyticus and could serve as the foundation for a more accurate early warning system for outbreaks of this human pathogen.

Inhibition of Listeria monocytogenes in Cold-process (Smoked) Salmon by Sodium Nitrite and Packaging Method

The behavior of Listeria monocytogenes in relation to sodium nitrite (NaNO₂) in combination with sodium chloride (NaCl) was evaluated in cold-process (smoked) salmon during storage at 5 or 10°C in either oxygen-permeable film or vacuum-sealed impermeable film. Salmon slices containing either 3 or 5% water-phase NaCl, with or without 190-200 ppm of NaNO₂, were inoculated with 10 or 327 CFU/g (150 or 4.9 × 10³ CFU/15-g sample) of strain Scott A. The inhibitory contribution of NaNO₂ was relative to inoculum size, storage time and temperature, packaging method, and concentration of NaCl. There was less growth of L. monocytogenes in vacuum-packaged samples as compared to those packaged in oxygen-permeable film. The most inhibition was achieved in vacuum-packaged products stored at 5°C, where NaNO₂ in combination with 5% water-phase NaCl prevented any increase in a 10 CFU/g-inoculum during 34 d storage. At 10°C, inhibition was initially enhanced by NaNO₂, but by 32 d L. monocytogenes populations had increased from a 10 CFU/g-inoculum to the range of 10⁶ CFU/g in vacuum-packaged products and 10⁸ CFU/g in permeable-film packaged products, regardless of NaNO₂ or NaCl concentration. Growth of naturally occurring aerobic microorganisms was also inhibited by NaNO₂ but to a lesser degree than L. monocytogenes .

Feasibility of a Heat-Pasteurization Process for the Inactivation of Nonproteolytic Clostridium botulinum types B and E in Vacuum-Packaged, Hot-Process (Smoked) Fish

This study demonstrates the feasibility of a heat-pasteurization process for certain vacuum-packaged hot-smoked fishery products for inactivation of the spores of the nonproteolytic Group II Clostridium botulinum types B, E, and F. This process permits the use of lower concentrations of salt and other inhibitors without jeopardizing safety and quality of the products during prolonged refrigerated storage. The pasteurization treatment was developed based upon the inactivation of nonproteolytic types B or E in hot-process (smoked) salmon. Smoke was not applied to the samples inoculated with types B and E because of its possible inhibitory effects. After processing in the smokehouse, each sample was cooled to 34°F (1.1°C), injected with 10⁶ spores, vacuum-packaged, and then heat-pasteurized in a water bath held at a constant temperature. A total of 85, 65, and 55 min in the 185°F (85°C), 192°F (88.9°C), and 198°F (92.2°C) baths, respectively, prevented toxin production by type E during 21 d of incubation at 25°C. Longer times, 175, 85, and 65 min, respectively, were required to prevent toxin production by nonproteolytic type B. Toxin production by type E during 120 d of storage at 10°C was prevented by a 45-minute treatment in the 198°F (92.2°C) bath. When heat-pasteurized samples were transferred into TPGY broth and incubated anaerobically for 150 d at 25°C, outgrowth and toxin production by type E was prevented by a 55-minute process at 198°F (92.2°C) and type B was prevented by a 65-minute process. This process does not, however, inactivate the more heat-resistant proteolytic strains of C. botulinum Group I or other spore-formers. The packages and master cartons of these pasteurized products therefore should follow the existing recommendations for smoked fishery products and be labeled "Keep refrigerated - Store below 38°F (3.3°C)."

Inhibition of Clostridium botulinum Type E Toxin Formation by Potassium Chloride and Sodium Chloride in Hot-Process (Smoked) Whitefish ( Coregonus clupeaformis )

Whitefish steaks were brined in NaCl, KCl, or equimolar NaCl:KCl to contain similar chloride ion concentration and inoculated intramuscularly with 10 or 100 spores of Clostridium botulinum type E per gram of fish. Steaks were then heated in a simulated (i.e., without smoke) hot-smoke process to internal temperatures of 62.8° to 76.7°C (145°-170°F) for the final 30 min of a 2- to 3-h process, packaged under vacuum in oxygen-impermeable film, and stored at abuse temperature of 25°C. During 7 d of storage, toxin production was inhibited in steaks containing more than 0.66 ionic strength NaCl, 0.64 KCl, or 0.71 equimolar NaCl:KCl. The results indicate that it is feasible to substitute KCl for NaCl in hot-process smoked fish for inhibition of outgrowth and toxin production by Clostridium botulinum type E.

Inhibition of Clostridium botulinum Types A and E Toxin Production by Liquid Smoke and NaCl in Hot-Process Smoke-Flavored Fish

Liquid smoke in combination with NaCl was an effective inhibitor of outgrowth and toxin production by Clostridium botulinum types A and E spores in hot-processed whitefish, chub and carp stored at an abuse temperature of 25°C for 7 or 14 d. Surface-inoculated type E produced toxin in control samples containing 3.7% water-phase NaCl, but not in liquid smoke-treated samples having less than 2.0% water-phase NaCl. Liquid smoke was less effective when type E spores were injected intramuscularly. Liquid smoke lowered the concentration of NaCl required to inhibit toxin production by surface-inoculated type A from 4.6 to 2.8% in samples stored 7 d. Liquid smoke enhanced the ability of NaCl to prevent toxin production, but should not be considered a substitute for NaCl or refrigerated storage (below 3.3°C).