Applied and Environmental Microbiology, June 2008, p. 3323-3327, Vol. 74, No. 11
0099-2240/08/$08.00+0 doi:10.1128/AEM.00060-08
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

United States Department of Agriculture, Agricultural Research Service, Dover, Delaware 19901,1 Pomona College, Claremont, California 91711,2 Haskin Shellfish Research Laboratory, Rutgers University, Port Norris, New Jersey 083493
Received 8 January 2008/ Accepted 23 March 2008
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Another marine bacterium, Photobacterium damselae (formerly known as Vibrio damselae), has been associated with morbidity and deaths due to necrotizing fasciitis from wound infections (2, 10, 33, 38, 39). Photobacterium damselae is also a significant pathogen in wild and farm-raised fish (13), and strains are biochemically similar to Shewanella spp. in that they are gram negative, oxidase positive, indole negative, and nonfermentative to most sugars and produce H2S. Photobacterium is currently a genus within the Vibrionaceae family.
In this paper, we report the isolation of Shewanella spp. and P. damselae subsp. damselae from oysters and seawater as part of a 2-year survey for Vibrionaceae in the Delaware Bay. There are little data on the presence or distribution of Shewanella spp. in the Delaware Bay, and this bay contains commercial shellfish harvesting areas. Because presumptive Shewanella species were being isolated and some species are human pathogens, we sought to evaluate the efficacy of conventional biochemical methods for their detection and differentiation and to confirm, by biochemical and sequence analyses, the identity of some of the isolates. Isolates of P. damselae subsp. damselae were also evaluated since they were occasionally identified by biochemical testing as presumptive S. putrefaciens isolates.
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Oysters and seawater.
Oysters and overlying seawater samples were collected during monthly sampling cruises from May 2004 to June 2006 to commercial oyster seedbeds along the New Jersey coast as part of a survey for total Vibrionaceae in the Delaware Bay. Oysters were harvested by dredge while seawater was obtained with a 2.2-liter-capacity Niskin-type sampler (Wildlife Supply Co., Buffalo, NY). Samples were transported at ambient temperatures to the Haskins Shellfish Research Laboratory, Port Norris, NJ, where testing was initiated the same day (generally within 6 h of collection). Each month for approximately 2 years, oysters and seawater were collected from six sites, and samples from each site were analyzed for a total of 138 oyster samples and 138 seawater samples.
Cultural and preliminary biochemical analyses.
Each oyster sample was a composite of 50 g of shucked meats from four to six oysters and was homogenized in 450 ml of 0.1% peptone (Becton Dickinson and Co., Sparks, MD) water. One hundred microliters of seawater, oyster homogenates, and serial 10-fold dilutions (in 0.1% peptone water) of the water and homogenates were separately spread plated on tryptic soy agar (Becton Dickinson and Co.) containing 0.5% additional NaCl (1% total) (TSA-N) and incubated at 37°C for 24 h. Up to 120 colonies/month were randomly picked for biochemical identifications from among the countable dilution plates.
Biochemical testing was performed using an API 20E system (bioMérieux Industries, Hazelwood, MO) and a BactiDrop oxidase test kit (Remel, Lenexa, KS). The nonselective nature of TSA-N allowed the rapid growth of many marine bacteria and potential human pathogens, including Shewanella spp., from both seawater and shellfish. Shewanella and Photobacterium spp. could be identified only if they were among the predominant organisms, since biochemical testing was performed only on colonies picked from the countable plates; lower-dilution plates were typically overgrown.
Biochemical and nucleotide sequence analyses of selected isolates.
Ten oyster and seawater isolates identified by API 20E as S. putrefaciens, 15 isolates that were nonfermentative but could not be identified by the API system, and the six stock Shewanella spp. were subjected to further biochemical analyses. The analyses were conducted at 37°C, unless otherwise noted, and included the suite of API-20E tests: growth on tryptic soy agar containing 1% and 6% total NaCl; the production of H2S on TSI (Becton Dickinson and Co.); hemolysis on plates of tryptic soy agar containing 5% sheep's blood (bioMérieux); growth on TSA-N at 25, 37, and 42°C; and the presence of phosphoglucose isomerase with a lysyl aminopeptidase (PGI-LysAP) activity. Lower-temperature incubations (25°C) were needed for four stock cultures that would not grow at 37°C, specifically, S. algidipiscicola, S. baltica, S. frigidimarina, and S. morhuae. The PGI-LysAP activity is characteristic of bacteria in the Vibrionaceae family and was determined by a colony overlay procedure for peptidases (29, 30). Shewanella algae is reported to grow at elevated NaCl concentrations (up to 6% NaCl) and at high temperatures (up to 42°C), but S. putrefaciens is not (14, 18).
Nucleotide sequence analyses were performed for confirmation of species identity. For sequencing, genomic DNA was purified from overnight cultures of the isolates after growth in LB or marine broth 2216 media (Becton Dickinson and Co.) by using a DNeasy tissue kit (Qiagen, Valencia, CA). A fragment of the 16S rRNA gene was PCR amplified from each genomic preparation using forward primer 63F (5'-CAG GCC TAA CAC ATG CAA GTC-3') and reverse primer 1387R (5'-GGG CGG WGT GTA CAA GGC-3'), as described by Marchesi et al. (22). PCRs were performed in a total volume of 50 µl containing 1 U of Eppendorf HotMaster Taq DNA polymerase (Brinkmann Instruments Inc., Westbury, NY), 0.25 µg of each primer, approximately 100 ng of template DNA, 200 µM deoxynucleoside triphosphates, and the buffer supplied with the HotMaster Taq polymerase. Reaction mixtures were incubated in an Eppendorf Mastercycler gradient thermocycler with an initial denaturation for 2.0 min at 94°C, followed by 25 cycles of 20 s at 94°C, 20 s at 50°C, and 80 s at 65°C. The PCR mixtures were checked for amplification of the 16S rRNA fragment by visualization of the reaction mixture on a 1% agarose gel, and the remaining PCR solution was prepared for sequencing by purification using a QIAquick PCR purification kit (Qiagen). The products were sequenced in the forward and reverse directions using the 63F and 1387R primers, respectively, by SeqXcel Inc., San Diego, CA. Approximately 750 bp of unambiguous sequence was obtained in both directions, and identification was made by comparing the sequence to the GenBank database using the BLASTn program (1). All matches were at 100% identity unless otherwise stated.
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3 mm), convex, circular, smooth, and pale orange, pink, or reddish. Presumptive S. putrefaciens isolates did not ferment D-glucose, D-mannitol, inositol, D-sorbitol, L-rhamnose, D-sucrose, D-melibiose, amygdalin, or L-arabinose (Table 2). It should be noted that the API database was originally designed to identify clinical pathogens but has evolved to include some environmental bacteria as well. Shewanella putrefaciens is the only species of Shewanella listed in the database, even though S. algae may be a more common pathogen (14, 18, 27). |
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TABLE 1. Biochemical profiles and sequence identification of nonfermentative bacterial isolates from oysters and seawater from the Delaware Bay
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TABLE 2. Biochemical characteristics of bacterial isolates by API codes shown in Table 1
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3 x 104/g) and the second-highest counts were obtained in August 2005 (
2 x 104/g). Counts are listed as greater than or equal to a value because only a portion of the isolates on the TSA-N plates were picked for identification. The highest counts for seawater occurred in August 2004, with
2 x 103 presumptive S. putrefaciens counts/ml. As total Vibrionaceae counts diminished during the winter and early spring, Shewanella identifications were made more readily in lower-dilution plates from both oyster and seawater samples. Presumptive S. putrefaciens counts ranged from nondetectable to
4 x 102/g of oyster samples and
2 x 101/ml of seawater samples during late winter and early spring (March to June). Oyster and seawater isolates identified by API 20E as either S. putrefaciens or nonfermentative bacteria that were not listed in the API database but had biochemical profiles similar to that of S. putrefaciens were sequenced and identified as S. abalonesis, S. algae, S. baltica, S. hafniensis, and S. marisflavi with 100% sequence identity (Table 1). To our knowledge, this is the first time these species have been isolated from shellfish or seawater from the Delaware Bay. One nonfermentative isolate that was unidentifiable by API was S. putrefaciens with >99% sequence identity; however, none of nine presumptive S. putrefaciens isolates, as determined by API 20E, were identified by sequencing as S. putrefaciens (Table 1). Of these nine presumptive S. putrefaciens isolates, two were sequenced as P. damselae subsp. damselae; five were S. abalonesis, S. baltica, S. marisflavi (two isolates), and the human pathogen S. algae; and the remaining two were Listonella anguillarum, a significant fish pathogen. The inability to differentiate Shewanella spp. using the API system was reported previously when S. algae was misidentified as S. putrefaciens (9, 14). Concerning the stock cultures, five of the six were incorrectly identified by API as S. putrefaciens, while the S. putrefaciens stock culture was unidentifiable by API (Table 1). Seven other nonfermentative isolates that were not listed in the API database were sequenced and were identified as L. anguillarum (Table 1).
All isolates and stock cultures grew readily on TSA-N; however, only S. abalonesis, one of three biotypes of P. damselae subsp. damselae, one biotype of L. anguillarum, and the stock culture of S. algae were capable of growing in the presence of 6% NaCl (Table 1). Literature indicates that S. algae can be differentiated from S. putrefaciens by its ability to grow in 6% NaCl (14, 18); however, an oyster-associated S. algae isolate with 100% sequence identity to the 16S rRNA gene of S. algae could not be grown on agar containing 6% NaCl. This isolate did produce H2S on TSI slants and exhibited β-hemolysis, both of which are reportedly characteristics of S. algae (14) (Table 1). Although oyster-associated S. algae produced β-hemolysis, the stock culture produced only
-hemolysis. Enzymes that are responsible for β-hemolysis are well established as virulence factors in a variety of bacterial pathogens, including the enterococci (23), Escherichia coli (3), Listeria monocytogenes (28), and Vibrio parahaemolyticus (24), and may well serve as an indicator of virulence in Shewanella spp. One of three species of P. damselae subsp. damselae was also beta-hemolytic (Table 1). Listonella anguillarum and S. baltica, species perceived as nonhuman pathogens, also exhibited β-hemolysis and growth at 37°C (Table 1).
Oyster and seawater isolates all grew at 25 and 37°C; however, the stock cultures, with the exception of S. algae and S. putrefaciens, grew only at 25°C (Table 1). It is generally reported that S. baltica, a cold-water species, will not grow at 37°C (40); however, three strains from the Delaware Bay (two of which had sequence identity to the 16S rRNA gene of S. baltica) grew readily at 37°C, suggesting the adaptation of S. baltica to warmer climates. Alternatively, stock cultures could behave atypically due to repeated laboratory passage. The fact that one seawater isolate with sequence identity to S. baltica was also beta-hemolytic at 37°C suggests a potential human pathogenic strain. In addition to the anticipated growth of S. algae at 42°C, one of the two biotypes of S. abalonesis and one of the three biotypes of P. damselae subsp. damselae were capable of growth at this temperature (Table 1).
The seawater and oyster isolates and the stock cultures listed in Table 1 produced brightly fluorescent foci when colonies were overlaid with membranes during the colony overlay procedure for peptidases. This indicates that these cultures contain PGI-LysAP activity, which has been observed for all Vibrionaceae and Aeromonidaceae family members tested to date (29). Such activity was not previously detected in pathogens or nonpathogens from other families of bacteria (29), further suggesting the relatedness of Shewanella spp. to members of the Vibrionaceae family (21). Photobacterium damselae also produced bright fluorescent foci, as anticipated, since it is classified as Vibrionaceae.
Although API 20E misidentified isolates as S. putrefaciens, the isolates were related to each other biochemically as oxidase-positive, nonfermentative bacteria within the genera Photobacterium and Listonella (both Vibrionaceae) and Shewanella (also related by sequence to the Vibrionaceae). Unfortunately, there are no simple assays to detect S. algae or S. putrefaciens from among the many nonfermentative isolates present in the marine environment, and routine sequence confirmation of isolates is impractical. The lack of discrimination by the API-20E system for Shewanella-like bacteria is understandable since the isolates show little reactivity in biochemical testing, due in part to their nonfermentative nature (Table 2). The inability to differentiate Shewanella spp. from each other, the misidentification of species as S. putrefaciens, and the inability to identify even a stock culture of S. putrefaciens as S. putrefaciens highlight a need for improved identification schemes to identify pathogenic and nonpathogenic strains of S. algae, S. putrefaciens, and P. damselae subsp. damselae.
Quantification and analysis of the Shewanella and Photobacterium isolates were not an objective of our initial Vibrio study, and our data are very limited; however, the identification of beta-hemolytic S. algae, S. baltica, and P. damselae subsp. damselae isolates is of interest because of their potential pathogenicity. In fact, there is a general lack of knowledge about the presence and proliferation of these species in the Delaware Bay and in bays and tributaries worldwide. The infectious doses for these species are unknown, and summertime levels may be sufficiently high to elicit infection, particularly in immunocompromised individuals. The inability to properly identify Shewanella and Photobacterium species complicates disease diagnosis and may mask the true incidence of infection. When our isolates were subjected to the API-20E test, most were misidentified as S. putrefaciens or they remained unidentified because they were not listed in the API database, further demonstrating the need for improved differentiation methods. Our supplemental biochemical and growth temperature studies showed variability in reactions, depending on individual strain characteristics.
Previously, Shewanella algae was found at higher levels in seawater and shellfish during the warmer summer months (11, 14) and was associated with a higher incidence of illness during the summer (14). The beta-hemolytic strains of S. algae and P. damselae subsp. damselae showed both tolerance to and growth at temperatures as high as 42°C, thus demonstrating their permissiveness to high temperatures. It is not known whether Shewanella or Photobacterium species from the Delaware Bay have caused illnesses through ingestion of contaminated oysters, clams, or other seafood; by cuts or abrasions acquired in the marine environment; or by swimming or other recreational activities. However, their detection suggests the need for closer scrutiny of their levels and of any association they may have with disease.
In summary, this paper identified potentially pathogenic species of Shewanella and Photobacterium for the first time in the Delaware Bay. Species were identified by sequencing of their 16S rRNA genes, and this, at the present time, is the method of choice for their identification, since conventional biochemical methods failed to reveal their identity. We further identified hemolytic activity in some isolates, suggesting a pathogenic potential for these bacteria. High numbers of Shewanella and Photobacterium isolates in oysters and seawater during the summer months suggest that monitoring the levels of pathogenic species and strains should be continued in the Delaware Bay and expanded to other geographic locations.
This work was supported by a USDA Agricultural Research Service (ARS) specific cooperative agreement (no. 58-1935-4-421) (to D.B.), by a David L. Hirsch III and Susan H. Hirsch Research Initiation Grant (to E.J.C.), and through intramural funds from the USDA ARS Microbial Food Safety Research Unit (to G.P.R.).
Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.
Published ahead of print on 31 March 2008. ![]()
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