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Applied and Environmental Microbiology, November 2004, p. 6401-6406, Vol. 70, No. 11
0099-2240/04/$08.00+0 DOI: 10.1128/AEM.70.11.6401-6406.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Department of Microbiology of the National School of Biological Sciences, Instituto Politécnico Nacional,1 Department of Biotechnology, Division of Biological and Health Sciences, Universidad Autónoma Metropolitana, Mexico City, Mexico2
Received 12 February 2004/ Accepted 25 June 2004
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Epidemiological studies have revealed an association between the Kanagawa phenomenon (KP) and gastroenteritis (23, 25). KP is a type of beta-hemolysis induced by the thermostable direct hemolysin (TDH) in Wagatsuma agar. Most (90%) of the strains isolated from clinical cases show this type of hemolysis, while only 1 to 2% of the strains of environmental origin are KP positive (20).
Several cases of gastroenteritis caused by hemolytic but KP-negative TDH-negative V. parahaemolyticus strains were reported in the 1980s (11), which led to identification of a new hemolysin known as TDH-related hemolysin (TRH) (12, 13, 30). TDH and TRH are encoded by the tdh and trh genes, respectively; these two genes both have 567-bp open reading frames, and they show 68.6% sequence similarity. These hemolysins are considered the main virulence factors of this microorganism (20).
In the present study we determined the prevalence of V. parahaemolyticus in seawater, oyster, and fish samples collected from the Pueblo Viejo Lagoon in Veracruz, an important estuary on the coastline of the Gulf of Mexico. The strains isolated were serotyped and screened for hemolytic activity and for the presence of the toxR, tdh, and trh genes.
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FIG. 1. Distribution of the 12 sampling sites in the Pueblo Viejo Lagoon, Veracruz. Site 1, El Bajo; site 2, Punto de Buda; site 3, Santa Clara; site 4, Ensenada; site 5, El Ciruelo; site 6, Cruz de Piedra; site 7, Isleta de Coralillo; site 8, Isleta Tomate; site 9, Medianía; site 10, Tamales; site 11, Punta de Mala Gana; site 12, Puente Hondo. Modified from reference 3.
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V. parahaemolyticus strain isolation and identification.
V. parahaemolyticus was isolated and identified as described in the Bacteriological Analytical Manual of the Food and Drug Administration (8). All commercial reagents used were obtained from Difco unless indicated otherwise. The oysters were shucked in aseptic conditions. The heads and tails of the fish were cut off, and the guts were removed. The oyster and fish samples were then homogenized in blenders, and 50 g of each homogenate was placed in 450 ml of alkaline peptone water (APW) to obtain a 101 dilution. Seawater samples (25 ml) were added to 225 ml of APW, and 102 and 103 APW dilutions were prepared in duplicate and incubated at 37 and 42°C for 6 to 24 h. Enrichment broth was streaked onto thiosulfate-citrate-bile salts-sucrose agar plates and incubated at 37°C for 18 to 24 h. Two or three suspect V. parahaemolyticus colonies (round, 1 to 2 mm in diameter, humid, shiny, sucrose negative) were selected. Halophilism tests were performed on NaCl-tryptone agar (T1N0, T1N3, T1N6, T1N8, and T1N10). Additional characterization tests included Gram staining, cytochrome oxidase activity tests, ornithine indole motility biochemical tests, triple sugar iron tests, lysine iron agar tests, arginine dehydrolase tests, urea tests, tests for glucose oxidation-fermentation in Hugh-Leifson broth, and tests for arabinose, lactose, mannitol, mannose, and sucrose fermentation.
Serotyping.
Serogroups were determined as described by Elliot et al. (8) by using commercial antisera (Denka Seiken, Co., Ltd., Tokyo Japan).
Hemolytic activity.
V. parahaemolyticus strains were seeded in Wagatsuma agar prepared as described by Elliot et al. (9); this agar contained 3 g of yeast extract, 10 g of peptone, 70 g of NaCl, 5 g of K2HPO4, 10 g of mannitol, 0.001 g of violet crystal, 15 g of agar, 1 liter of distilled water, and 50 ml of human anticoagulated blood. The bacteria were incubated at 37°C for 18 h. In order to characterize the TRH phenotype, strains were seeded in heart infusion broth and SPP broth (5 g of NaCl, 5 g of NaHPO4, and 2 g of glucose in 1 liter of distilled water; pH 7.6). Hemolytic activity was determined as described by Honda et al. (13) and Kishishita et al. (18). Positive and negative controls were included in all assays (Table 1).
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TABLE 1. Reference strains
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PCR.
The presence of the toxR gene was determined by using the following primers described by Kim et al. (17): 5'-GTCTTCTGACGCAATCGTTG-3' (forward) and 5'-ATACGAGTGGTTGCTGTCATG-3' (reverse), which produced a 368-bp amplicon. The PCR conditions were as follows: the reaction mixture (final volume, 25 µl) consisted of 3 µl of the solution containing DNA, 2.5 µl of 10x reaction buffer (Applied Biosystems), 6 µl of 25 mM MgCl2, 1 µl of Taq polymerase (5 U/µl), 4 µl of deoxynucleoside triphosphates (1 µM), 1 µl of each primer (0.0125 ng/µl), and 6.5 µl of distilled water. The reactions were performed with a Gene Amp PCR system 2700 thermocycler (Applied Biosystems) as follows: 1 min of initial denaturation at 94°C, followed by 20 cycles of denaturation at 94°C for 1 min, alignment at 63°C for 1.5 min, and extension at 72°C for 1.5 min and a final extension at 72°C for 7 min.
The tdh and trh genes were amplified with the following primer sets: 5'-GGTACTAAATGGCTGACATC-3' (forward) and 5'-CCACTACCACTCTCATATGC-3' (reverse) for tdh; and 5'-GGCTCAAAATGGTTAAGCG-3' (forward) and 5'-CATTTCCGCTCTCATATGC-3' (reverse) for trh (26). These primer sets produced 251- and 250-bp amplicons, respectively. The reaction mixtures (final volume, 25 µl) contained 3 µl of the solution containing DNA, 2.5 µl of 10x reaction buffer (Applied Biosystems), 4 µl of 25 mM MgCl2, 1 µl of Taq polymerase (5 U/µl), 4 µl of deoxynucleoside triphosphates (1 µmol), 1 µl of each primer (0.025 ng/µl), and 8.5 µl of distilled water. The reactions were performed with a Gene Amp PCR system 2700 thermocycler (Applied Biosystems) as follows: 1 min of initial denaturation at 94°C, followed by 35 cycles of denaturation at 94°C for 1 min, alignment at 55°C for 1 min, and extension at 72°C for 1 min and a final extension at 72°C for 7 min.
Positive and negative DNA controls were included in all assays (Table 1). Amplified products were separated by electrophoresis in ethidium bromide-stained 2% agarose gels in Tris-borate-EDTA buffer at 120 V for 30 min. The gels were visualized with a UV transilluminator (Eagle Eye; Stratagene).
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FIG. 2. Number of samples (S) from the Pueblo Viejo Lagoon positive for V. parahaemolyticus over the study period. Jn, June; Ago, August; Sep, September; Nov, November; Feb, February; Mar, March; Ab, April; My, May.
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TABLE 2. Origins and serotypes of the V. parahaemolyticus strains isolated at different times
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TABLE 3. Serotyping of the 46 strains isolated
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FIG. 3. Amplification of the toxR gene by PCR. The primers amplified a 368-bp fragment. Lanes 1 and 6, molecular weight marker X147/HaeIII; lane 2, control strain V. parahaemolyticus ATCC 17802; lane 3, negative control strain Va-219; lanes 4 and 5, experimental strains.
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FIG. 4. Amplification of the tdh gene by PCR. The primers amplified a 251-bp fragment. Lane 1, molecular weight marker X174/HaeIII; lane 2, control strain V. parahaemolyticus WP1; lane 3, negative control strain Va-219; lanes 4 to 9, experimental strains.
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In the present study we demonstrated the presence of V. parahaemolyticus in 15% of the seawater, fish, and oyster samples analyzed. This study was the first research study to investigate isolation and the distribution of this pathogen in a Mexican coastal lagoon. There have been reports of V. parahaemolyticus isolation in Canada (16), France (10), Asia (2, 28), the United States (6), and Mexico (27). The factors affecting the incidence and distribution of V. parahaemolyticus in the environment include water temperature, salt and oxygen concentrations, interactions with the plankton, the presence of sediment, the organic matter in suspension, fish, and seafood, as well as the incorporation and tidal action of estuarine waters. The presence of V. parahaemolyticus seems to be constant in the Gulf of Mexico because the temperature does not drop below 11.6°C, unlike what occurs in Japan, Europe, Australia, and the United States, where isolation of this pathogen decreases during the winter months (14).
We found V. parahaemolyticus in 11 of the 12 sampling sites; the only site at which we did not find this pathogen was the Tamales site, which was located in the northeast part of the lagoon. Strains were serotyped for epidemiological purposes, and serogroup O3 was the most frequent serogroup (30%) (Table 2). Several researchers have pointed out that certain serogroups are constant in some geographical areas (1, 24). Serotypes O3:K6, O4:K68, and O1:KUT (K untypeable) have been related to the majority of the infections caused by this pathogen in Asia and the United States during the last 5 years (4, 5).
We observed positive tdh gene amplification in 4 of the 46 strains analyzed, which correlated with positive hemolytic activity in Wagatsuma agar (KP+). Two of these strains were isolated from fish, and two were isolated from water samples. Most environmental strains are known to be KP, and only 1 to 2% are KP+ (14, 20). This implies that there is a source of human fecal contamination in the estuarine waters of Pueblo Viejo Lagoon. The lagoon is surrounded by small rural populations, some of which use latrines that discharge feces into the lagoon. Furthermore, the water is very shallow (only 1.5 m deep during the rainy season), so fisherman, dogs, and other domestic animals can easily walk through it, which is an additional source of contamination.
Although it is known that 0 to 5% of the environmental strains produce TRH (10, 29), the trh gene and the TRH phenotype were not identified in any of the strains isolated here. The possibility of false-negative results is unlikely, as the positive control strain (ATCC 17802) (Table 1) amplified the 368-bp trh fragment in all assays performed. There have been few reports of the presence of the tdh and trh genes in V. parahaemolyticus strains of environmental origin; only 0 to 6% of the samples analyzed from the coasts of the United States (6, 7, 15), Europe (10), and Asia (2, 28) contained tdh-positive V. parahaemolyticus strains. The toxR gene fragment was amplified in all the strains isolated here, confirming that they were in fact V. parahaemolyticus, as this gene can be used to identify the species (17).
The data here presented are part of the first studies on the isolation, distribution, and detection of virulence factors of V. parahaemolyticus in Mexico. The presence of potentially pathogenic V. parahaemolyticus strains in the Pueblo Viejo Lagoon is matter of concern for sanitary authorities, because this organism has not been considered a health problem in spite of the information on infection outbreaks along the coastline of the Gulf of Mexico. This information may be important for preventing sanitary problems that might affect the health of the population.
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