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Aeromonas Species in Foods 575 Journalof Food Protection, Vol. 65, No. 3, 2002, Pages 575– 582 Copyright Q,InternationalAssociation for FoodProtection Review Aeromonas Species inFoods JAMIEH. ISONHOOD AND MARYANNE DRAKE * Departmentof Food Science, NorthCarolina State University, Raleigh,North Carolina 27695, USA MS01-318: Received 28August 2001/ Accepted 16October 2001 ABSTRACT Downloaded from http://meridian.allenpress.com/jfp/article-pdf/65/3/575/1674790/0362-028x-65_3_575.pdf by guest on 01 October 2021 Aeromonas specieshave been recognized as potential or emerging foodborne pathogens for more than 20 years. Aero- monadsare estuarine bacteria and are ubiquitous in freshwater ,shand shell sh, meats, and fresh vegetables. Actual sourced foodborneoutbreaks are few, but epidemiological evidence suggests that the bacterium can cause self-limiting diarrhea, with childrenbeing the most susceptible population. Most aeromonads are psychrotrophic and can grow in foods during cold storage.Aeromonads are not resistant to foodprocessing regimes and are readily killed by heattreatment. A hostof virulence factorsare present, but the exact role of each in human disease has not been fully elucidated. Thebacterial genus Aeromonas isnow of cially clas- cently,pectinolytic activity has been reported in a newly sied within the family Aeromonadaceae andconsists of identied strain (80). Most Aeromonas spp.can grow at a 14different con rmed species (51) (anasterisk indicates pHof 5.5 to 9.0. Colonies formed after 24 h onnutrient thata specieshas been isolated from clinicalspecimens): agarare 1 to3 mm indiameter ,convex,smooth, whitish, A.allosaccharophila,A.hydrophila, * A.bestiarum, * A. andtranslucent. Colonies turn a lightbeige color after pro- caviae,* A.encheleia, A. eucrinophila,A. jandaei, * A. po- longedincubation (55). pofi, A.media, * A.salmonicida,A. schubertii, * A. sobria, Similar to Vibrio species,aeromonads have been re- A. veronii,* and A. trota.*Thegenus was formerlyclassi- portedto havea viablebut nonculturable state (92). Certain ed within Vibrionaceae, andmisidenti cation of Aero- strainsdisplay a rapiddecline following 12 to 18 h of monas spp. as Vibrio spp.still poses a problemin clinical growthin media containing glucose. This phenomenon is settings (2, 8). Originally,four Aeromonas specieswere referred toas the suicide phenomenon (70). The optimal identi ed: A.hydrophila,A. sobria,A. caviae, and the sh growthtemperature is generally believed to be 28 8C (58, pathogen A.salmonicida(82). Subsequentwork with these 82), buta widegrowth range and variability in the optimum fourspecies indicated distinct biochemical groupings (des- temperaturehave been observed (54,57, 58, 65, 80). Many ignatedphenospecies) and distinct DNA hybridization strainscan grow at ,58C,making aeromonads particularly groupings(designated genospecies, sometimes referred to signicant in refrigerated foods (24,27, 77, 78). A. hydro- ashybridization groups [HGs]), leadingto variability in phila hasbeen reported to have an optimal growth temper- speciesdesignation and classi cation (1,8, 13, 18, 38, 54, ature of 288C,althoughit cangrow at temperatures ranging 58). Thesegram-negative facultative organisms are ubiq- from 1 to 428C (65, 74). uitousin water and in many foods. Their role in foodborne Aeromonadsare considered opportunistic pathogens of illnessis still not rmlyestablished. The purpose of this bothaquatic and terrestrial animals (25, 67). Since1960, it reviewis to provide a generaloverview of Aeromonas spp. hasbeen speculated that these bacteria may be pathogenic infoods and the role they may play in foodborne illness. to humans (67, 72). In1984, the Food and Drug Admin- GENERAL CHARACTERISTICS istrationintroduced A.hydrophila asa ‘‘new’’ foodborne pathogen (89). Aeromonadshave been considered contro- Aeromonadsare gram-negative, oxidase-positive, glu- versialhuman pathogens, since results from humanvolun- cose-fermenting,facultatively anaerobic rods, and most are teerstudies have been inconclusive (57);however,epide- motileby polar agella (82). The word Aeromonas was miologicalevidence continues to indicate that these organ- derivedfrom theGreek words aer, meaningair or gas,and (7,26, 62, 69) monas, meaningunit or monad. Cells are typically 1.0 to ismsare capable of causing gastroenteritis 4.4 mminlength and occasionally form lamentsof upto andother complications, including wound infections, sep- 8.0 mm.Aeromonads can ferment the sugars glucose, fruc- ticemia,and endocarditis (2, 14, 48). Gastroenteritisin- tose,maltose, and trehalose to acid or toacid and gas. They volvesin ammation of the gastrointestinal tract due to in- canalso hydrolyze starch, dextrin, and glycerol (55). Re- vasionof bacteriainto the intestinal mucosa. Septicemia or bloodpoisoning involves the growth of bacteria in the *Authorfor correspondence. Tel: 919-513-4598; Fax: 919-515-7124; blood (9). Studiesindicate that Aeromonas spp.can act as E-mail: [email protected]. bothinfectious and enterotoxigenic pathogens (56). Non- 576 ISONHOOD AND DRAKE J.FoodProt., Vol. 65, No. 3 motileaeromonads, namely, A.salmonicida, arethought to thermalsurvivor curves. Palumbo et al. (76, 79) reported beobligate shparasites (25). Themotile mesophilic aero- tailingor two-phase thermal survivor curves with three monads,consisting of A.hydrophila,A. sobria, and A. cav- clinicaland two food isolates of A.hydrophila. Isonhood iae, are consideredcausative agents of human gastroenter- (45) alsoreported biphasic thermal survivor curves for two itis,wound infections, and septicemia (34). strains of A.hydrophila. Thetaxonomy of the genus Aeromonas isstill devel- FOODBORNE OUTBREAKS oping.Molecular genetic evidence now indicates that Aero- monas spp.,formerly members of thefamily Vibrionaceae, Currently,reports of foodborne Aeromonas outbreaks arequite different from eachother and from thevibrios lackhard evidence that indisputably establishes the organ- (58). Earlystudies of aeromonadsclassi ed themesophilic, ismas a causativeagent of foodborne illness. Kirov (57) pathogenic,and nonpathogenic strains as A.hydrophila and reviewedincidences of Aeromonas-relatedfoodborne gas- thepsychrophilic strains that were shpathogens as A. sal- troenteritisthat occurred between 1977 and 1991. Most out- monicida(58, 72). Inthe early 1980s, Popoff (82) used breaksinvolved seafood, such as oysters, sashimi, prawns, Downloaded from http://meridian.allenpress.com/jfp/article-pdf/65/3/575/1674790/0362-028x-65_3_575.pdf by guest on 01 October 2021 DNA-DNA hybridizationto show that at leastnine distinct andshrimp. Although Aeromonas was implicated,viruses HGs existedamong the mesophilic species A.hydrophila. were notruled out as a possiblecause of the illness. The Amongthe HGs, threeseparate phenotypic groups were onlyreport presenting good evidence leading to Aeromonas foundby using 8 to18 biochemical tests. These three asthe cause of gastroenteritis involved a 38-year-oldman ‘‘phenospecies’’ were named A.hydrophila,A. sobriae, and whoconsumed a ready-to-eatshrimp cocktail. Through ri- A. caviae. Mostclinical laboratories have accepted pheno- botyping,the patient’ s stooland the shrimp were foundto typicclassi cation of the mesophilic aeromonads. A. hy- containidentical Aeromonas spp.This report offers thebest drophila isnow limited to HG1, HG2, andHG3 (58). A evidenceknown from astudyin which molecular tech- decadeago, investigators used DNA-DNA hybridization niqueswere usedto identify Aeromonas spp.as the caus- studiesto apply the classi cation schemes of genomospe- ativeagent in foodborne gastrointestinal infection (47). cies,genospecies, or hybridization groups, which were de- Saad et al. (84) reportedan increased rate of Aeromonas nedas having at least 70% DNA homologywith the des- spp.isolated from humanstools during the summer months ignatedtype strain (72). Thesehybridization groups are andhypothesized an association with the incidence of mo- geographicallydispersed. Moreover ,noevidence has sug- tileaeromonads in fresh vegetables.The increased frequen- gesteda signicant correlation between these hybridization cyof human stool– associated aeromonad isolation in the groupsand virulence (67). WhileDNA homologyanalysis summermonths was notlinked with microorganisms iso- isthe traditional tool for speciescon rmation, 16S rRNA latedfrom nearbyriver and tap water ,butit did correlate sequencing,ampli ed fragment length polymorphism, and withlarge numbers of Aeromonas from foodsamples. fattyacid methyl ester analysis have all been proposed to VIRULENCE FACTORS characterizenew isolates (51). Aeromonas spp.have properties that are shared by bac- Therole of aeromonads and the toxins they produce teriain the families Enterobacteriaceae and Vibrionaceae andtheir relationship to virulence in cases of human gas- (55).Aeromonas spp.are not particularly fastidious and can troenteritishave not been fully elucidated. There is no an- growwell on many common laboratory media, such as imalmodel for Aeromonas gastroenteritis.Most cases of MacConkeyagar ,Hektoenenteric agar ,andxylose lysine gastroenteritisassociated with Aeromonas occurin chil- desoxycholateagar (47). Inefforts toisolate Aeromonas dren,elderly people, and immunocompromised people. A spp.,selective media are used that take advantage of unique numberof putative virulence factors have been identi ed, characteristicsof amylaseactivity and ampicillin resistance. includinghemolysins, invasins, adhesins, endotoxin (lipo- Selectivemedia such as glutamate starch penicillin agar polysaccharide[LPS]),
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