Prevalence and Distribution of Aeromonas hydrophila in the United ...

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736 HAZEN ET AL.Inc., Plainview, N.Y.). Only nine of these isolates werenot confirmed to be A. hydrophila.Water quality. Salinity (per mille), conductivity(reciprocal microohms per centimeter), and temperature(degrees Celsius) were measured in situ with amodel 33 S-C-T meter (Yellow Springs InstrumentCo., Yellow Springs, Ohio); an Accumet model 150portable pH meter (Fisher Scientific Co., Raleigh,N.C.) was used to measure pH. Turbidity was measuredwith a Mini-Spec 20 spectrometer (Bausch &Lomb, Inc., Rochester N.Y.) by converting percenttransmittance to Jackson turbidity units (JTU) (11);turbidity was measured simultaneously with filtration.Statistical analyses. Analyses of variance andregression analyses were performed by using the interactivedata analysis program (University of Chicago)and an HP 3000 computer (Hewlett-PackardCo., Cupertino, Calif.). Each water quality parameterwas compared with densities of A. hydrophila by usingregression analysis. Densities of A. hydrophila andconductivity were subjected to log (x + 1) transformationbefore analyses (21) because of their non-normaldistrubution. Skewness and kurtosis were used tomeasure normality. Any statistical probability equalto or less than 0.05 was considered significant.RESULTS AND DISCUSSIONA total of 147 lotic and lentic habitats weresampled for A. hydrophila (Table 1); A. hydrophilawas isolated at all but 12 of these sites(Fig. 1). Of the 12 sites where it was not isolated,2 were hypersaline lakes (Badwater Lake, GreatAPPL. ENVIRON. MICROBIOL.Salt Lake), 2 were geothermal springs (>45°C;(White Dome Geyser, Black Sands Basin), and3 were extremely polluted rivers (Wabash, Mississippi,San Antonio). Thus, of the 12 locationswhere A. hydrophila could not be isolated, 7could be considered as extreme environments.The other five sites followed no particular patternand could be due to sampling error (3%). Ofthe 30 different states in which samples weretaken, A. hydrophila was not isolated in 5; however,in 4 of the 5, only a single sample wastaken.Saline habitats had a much higher density ofA. hydrophila than did freshwater habitats,even though the variation in density among salinehabitats was much larger than that amongfreshwater sites (Table 2). Generally, A. hydrophilais not considered to be a marine bacterium(7); however, this study indicates that it is foundnaturally in marine systems which interface withfreshwater and that it can be found at all salinities,except the most extreme (>100%o). Thisobservation has been substantiated recently; A.hydrophila was implicated in causing ulcer diseasein cod (Gadus morhua), a strictly marinefish (10). Lotic habitats had significantly higherdensities of A. hydrophila than did lentic habitats(Table 2). This is somewhat surprising becauseA. hydrophila could be isolated from watershaving a turbidity of 0 to 395 Jackson tur-FIG. 1. Distribution of A. hydrophila in the United States. Open circles indicate sampling sites where A.hydrophila could not be isolated.
VOL. 36, 1978 A. HYDROPHILA DISTRIBUTION IN U.S. 737TABLE 2. Comparison of densities of A. hydrophilaby habitatDensity of A. hydrophila (CFUMI-I)aHabitatNo.Mean Standard RangeerrorFreshwaterLotic 161 46 3,600-0.4 96Lentic 20 8 205-0.1 26All 130 36 3,600-0.1 122Saltwater 746 688 9,000-0.1 13Total (saltwater 189 73 9,000-0.1 135and freshwater)CFU, Colony-forming units.bidity units. There was, however, no significantregression between turbidity and density of A.hydrophila.The thermal optimum for most strains of A.hydrophila is 35°C, and the thermal maximumis very close to 450C (17). In this study, A.hydrophila was isolated from waters havingtemperatures of between 4.0 and 45.00C. A. hydrophilacould not be isolated at temperaturesgreater than 450C; the highest densities occurredat 350C, along thermal gradients ranging from200 to 720C (T. C. Hazen, manuscript in preparation).Water pH did not seem to play a significantrole in A. hydrophila distribution, because thebacterium could be isolated over the entire pHrange of the samples (5.2 to 9.8). In our lab wehave found that A. hydrophila growth is unaffectedby pH's from 5 to 9 and that it is incapableof growth at a pH lower than 4 or higher than10.Regression analyses revealed significant relationshipsbetween densities of A. hydrophilaand conductivity (F = 14.5; df = 93; P < 0.001).None of the other water quality parametersshowed significant regressions with densities ofA. hydrophila. It is unlikely that conductivityalone affects the distribution and abundance ofA. hydrophila, even though inorganic ion requirementshave been demonstrated for a numberof marine bacteria (12). It is more likely thatsome unmeasured water quality parameter(s)varies proportionately with conductivity andthat it affects the density of the bacterium.Conductivity may be significant, however, as anindicator of aquatic habitats in which high densitiesof A. hydrophila occur.The cosmopolitan distribution of A. hydrophilais at least partly explained by its ability tolive under a wide variety of environmental conditionsin natural waters. Its densities, as estimatedby viable cell count, commonly rangefrom less than 1 cell per liter to several thousandcells per ml, under a wide variety of conditions.Its abundance in natural waters is clearly notcontrolled purely by allochthonous or autochthonouscarbon, because oligotrophic lakes ofthe Grand Tetons may have densities of A.hydrophila comparable to those of bayous ofLouisiana. Abundance of A. hydrophila in somany different systems would seem to indicatean important role for this bacterium in naturalaquatic processes.Epizootics in fish, caused by A. hydrophila,have been largely confined to the southeasternUnited States (5, 8, 15, 18). Densities of A.hydrophila are high in the southeast, but notsignificantly higher than in other parts of theUnited States. Biochemical and serological studiesof 361 isolates from water and fish throughoutthe United States reveal a striking similarity(Hazen and Fliermans, unpublished data); however,other investigators have reported that A.hydrophila isolated from fish is more virulentthan isolates from water, even though all isolateswere biochemically similar (2). Recent studies(4, 8, 9) have shown that host stress may be asignificant factor in the epizootiology of red-soredisease and, in combination with variability invirulence of A. hydrophila, may be of significancein limiting epizootic outbreaks to aquaticsystems in the southeastern United States.Clearly, A. hydrophila, as a potential pathogenand as an important component of the microflorain aquatic systems, requires further study.ACKNOWLEDGMENTSThis study was supported by contract EY-76-S-09-0900between the United States Department of Energy and WakeForest University. This study was also supported in part bygrant B-112-NC from the Water Resources Research Instituteto Wake Forest University and in part by contract AT (07-2)-1 between the United States Department of Energy andSavannah River Laboratory.We thank Gayle Hazen, Jim Matthews, Mark Raker, andBill Crawford for their excellent technical assistance.LITERATURE CITED1. Davis, W. A., J. G. Kane, and V. G. Garagusi. 1978.Human Aeromonas infections: a review of the literatureand a case report of endocarditis. Medicine 57:267-277.2. DeFigueiredo, J., and J. A. Plumb. 1977. Virulence ofdifferent isolates of Aeromonas hydrophila in channelcatfish. Aquaculture 11:349-354.3. Emerson, H., and C. Norris. 1905. "Red Leg"-an infectiousdisease of frogs. J. Exp. Med. 7:32-60.4. Esch, G. W., and T. C. Hazen. 1978. Thermal ecologyand stress: a case history for red-sore disease in largemouthbass (Micropterus salmoides). In J. H. Thorpeand J. W. Gibbons (ed.), Energy and environmentalstress in aquatic systems. Department of Energy symposiumseries no. CONF-771114. National TechnicalInformation Service, Springfield, Va.5. Esch, G. W., T. C. Hazen, R. V. Dimock, Jr., and J.W. Gibbons. 1976. Thermal effluent and the epizootiologyof the ciliate Epistylis and the bacterium Aeromonasin assocation with centrarchid fish. Trans. Am.Microsc. Soc. 95:687-693.
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