JOURNAL OF CLINICAL MICROBIOLOGY, Feb. 1997, p. 369–373 0095-1137/97/$04.0010 Copyright q 1997, American Society for Microbiology
Vol. 35, No. 2
Characterization of Aeromonas spp. Isolated from Humans with Diarrhea, from Healthy Controls, and from Surface Water in Bangladesh ¨ HN,1* M. JOHN ALBERT,2 M. ANSARUZZAMAN,2 N. A. BHUIYAN,2 S. A. ALABI,2 INGER KU ¨ LLBY1 M. SIRAJUL ISLAM,2 P. K. B. NEOGI,2 G. HUYS,3 P. JANSSEN,3 K. KERSTERS,3 AND R. MO Microbiology and Tumor Biology Centre, Karolinska Institute, S-171 77 Stockholm, Sweden1; International Centre for Diarrhoeal Disease Research, Bangladesh, Dhaka-2, Bangladesh2; and Laboratorium voor Microbiologie, Universiteit Ghent, B-9000 Ghent, Belgium3 Received 5 July 1996/Returned for modification 8 October 1996/Accepted 4 November 1996
Aeromonas isolates from patients with diarrhea in Bangladesh (n 5 69), from healthy controls (n 5 11), and from surface water (n 5 40) were analyzed with respect to their hybridization groups (HGs) by the aid of fatty acid methyl ester (FAME) characterization and DNA fingerprinting by AFLP, biochemical phenotypes (PhenePlate [PhP] types), and the production of hemolysin and cytotoxin. The aim of the investigation was to find out whether certain strains carrying virulence factors predominated among patient isolates. According to FAME and/or AFLP analysis, most human isolates were allocated to DNA HGs 4 (Aeromonas caviae) and 1 (A. hydrophila). Most environmental strains were allocated to HG8 (A. veronii biogroup sobria) and HG4 (A. caviae), and only one was of HG1. According to PhP typing, the diversity among patient isolates was lower than that among other strains, and two dominating PhP types (types BD-1 and BD-2) were identified in 29 and 30% of the patient isolates, respectively. PhP type BD-1 was also common among the environmental isolates, whereas PhP type BD-2 was only identified in two of the other isolates. Twenty-five of 26 isolates belonging to HG1 were of the same PhP type (BD-2), whereas isolates of other common HGs were more diverse according to their PhP types. Hemolytic and cytotoxin-producing strains occurred more frequently among the environmental isolates than among patient isolates. However, the hemolytic and cytotoxic activities among human isolates was strongly correlated to the HG1/BD-2 type, which, in addition, showed high cytotoxin titers (median values, 1/512 compared to 1/128 for cytotoxin-positive isolates belonging to other types). Thus, the HG1/BD-2 type may represent a pathogenic A. hydrophila type that is able to produce diarrhea in humans. vironments (15), and also, a high prevalence of 12.1% in stools from infants with diarrhea, compared to only 1.6% in stools from healthy control infants, has been reported in Bangladesh (10). In order to investigate whether certain pathogenic strains may be circulating among patients with diarrhea in Bangladesh, we have compared Aeromonas isolates from patients with diarrhea, from healthy controls, and from surface water sources in Bangladesh. All isolates have been phenotyped with a high-resolution biochemical fingerprinting system (the PhenePlate [PhP] system), most of them have been assigned to HGs by determination of fatty acid methyl ester (FAME) profiles, and some of them have also been assigned to HGs with the aid of the DNA fingerprinting technique AFLP. In addition, all isolates were assayed for the production of hemolysin and cytotoxin.
Aeromonas spp. comprise a group of bacteria that is widespread in natural water habitats such as fresh water, sewage, and wastewater (1). This group of bacteria has also been associated with a wide variety of infections in humans (9, 17, 20). Although they are known to produce many different extracellular toxins, such as cytotoxin and enterotoxins (11), the mechanism of their virulence is still uncertain. It has been shown that pathogenicity can be associated with certain phenospecies and/or DNA hybridization groups (HGs) of Aeromonas spp. (16, 33) and also that among isolates within the same HG, clinical isolates produce virulence factors more frequently than environmental isolates do (21). These findings indicate that virulence within the genus Aeromonas may be a clonal property. Bacteria belonging to Aeromonas spp. have been identified as common enteric pathogens from several countries, with a reported isolation rate in stools from patients with diarrhea of 1.4% (Nigeria) (30), 8.7% (tourists from Morocco with traveler’s diarrhea) (8), 3% (Peru) (3), and 1.8% (India) (5). Most of these studies reported Aeromonas hydrophila HG1 as the most frequently occurring Aeromonas species (3, 5, 30); however, among tourists to Morocco with traveler’s diarrhea, A. veronii biotype sobria and A. caviae were the most common species isolated (8). In Bangladesh, Aeromonas spp. are abundant in aquatic en-
MATERIALS AND METHODS Isolation and preliminary identification of Aeromonas sp. strains. Altogether, 80 fecal Aeromonas isolates from humans were included in the study. Sixty-nine strains were isolated from 62 patients with diarrhea treated at the Dhaka Treatment Centre of the International Centre for Diarrheal Disease Research, Bangladesh (ICDDR,B), during 1993 and 1994. The patients were under the age of 5 years and presented with routine cases of diarrhea; these cases were assumed to be epidemiologically unrelated. Twenty-five of these patients also harbored, besides Aeromonas spp., other pathogens, such as Campylobacter jejuni, Salmonella spp., Shigella spp., or non-O1, non-O139 Vibrio cholerae. Eleven control isolates originated from nine healthy children who were part of a case-control study of diarrhea during 1993 and 1994. Stool samples were routinely plated on taurocholate-tellurite-gelatin agar, MacConkey agar, Salmonella-Shigella agar, and ampicillin blood agar for isolation of various enteric bacterial pathogens. Colonies suspected of representing pathogens were further identified by standard biochemical tests at ICDDR,B (2). Aeromonas spp. were identified accord-
* Corresponding author. Mailing address: Microbiology and Tumor Biology Centre, Karolinska Institute, S-171 77 Stockholm, Sweden. Phone: 46 8 728 7155. Fax: 46 8 345 704. E-mail:
[email protected] .se. 369
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ing to reactions toward esculin, KCN, salicin, arabinose, glucose, VogesProskauer, lysine, arginine, and ornithine (4). Normally, one isolate per individual was saved. The Aeromonas isolates from seven patients and two healthy controls displayed more than one biochemical pattern, and in those cases, one isolate representing each phenotypic group was saved for further analysis. Forty environmental strains were isolated from 10 ponds in Tangail, Bangladesh, from September to November 1994. The isolates were collected from duckweed (n 5 16), fish (n 5 11), and water (n 5 13). The samples were enriched in alkaline peptone water before subculturing on the plating medium described above (15). Both clinical and environmental isolates were stored in T1N1 soft agar (1.0% Trypticase, 1.0% NaCl, 0.5% agar) at 258C. Determination of virulence factors. For analysis of the cytotoxic effect, culture filtrates were prepared in double-strength tryptic soy broth (Difco, Detroit, Mich.) as a shaker culture at 378C for 24 h (29). The supernatant obtained after filtration through a 0.45-mm-pore-size membrane filter (Sartorius, Go ¨ttingen, Germany) was tested for cytotoxicity with a Vero cell line (6). Serial doubling dilutions of filtrates starting from 1:2 were tested. Known cytotoxin-positive and -negative strains of Aeromonas spp. were included as controls. Isolates showing a cytotoxic effect at a dilution of 1/8 or more were regarded as clearly positive. Hemolytic activity was measured on sheep erythrocytes by using supernatants from Aeromonas cultures grown in heart infusion broth (36). Known positive (El Tor V. cholerae O1) and negative (Classical V. cholerae O1) strains were included as controls. Biochemical fingerprinting with the PhP system. The PhP-48 plates (BioSys Inova, Stockholm, Sweden) consist of 96-well microplates each with two sets of 48 dehydrated reagents (24, 25, 27). The isolates to be tested were precultivated on nutrient agar plates. A small amount of bacteria was suspended in a medium containing 0.1% proteose peptone and 0.01% bromothymol blue, and 150 ml of each bacterial suspension was dispensed into each of 48 wells of a PhP-48 plate. The inoculated plates were incubated at 378C for 2 days, and the absorbance value (A620) of each well was measured with a microplate reader after 7, 24, and 48 h of incubation. After the last measurement, the biochemical fingerprint was calculated as the mean value of the three readings for each reaction. The biochemical fingerprints of the isolates were compared pairwise, and a similarity matrix consisting of the correlation coefficients between all possible pairs was constructed. The similarity matrix was clustered according to the unweighted pair group method using average linkages (UPGMA) (34). Isolates with a level of similarity greater than 0.97 (the identity level) were assigned to the same PhP type (24). Isolates not identical to any other strains were named single PhP types. All data processing, including optical readings and calculations of correlation coefficients, as well as clustering and printing of dendrograms, was performed with the PhP software (BioSys Inova). Identification of Aeromonas spp. by gas-liquid chromatographic analysis of FAME profiles. Previously, a collection of genotypically characterized Aeromonas strains was used to generate a database (AER48C) of FAME profiles representative of all currently described phenospecies and HGs within this genus (12). The strains were grown on Trypticase soy agar containing 3% (wt/vol) Trypticase soy broth (BBL, Cockeysville, Md.) and 1.5% (wt/vol) Bacto Agar (Difco) at 288C for 48 h. FAMEs were extracted and analyzed according to the standardized protocol of the Microbial Identification System (MIDI; Microbial ID, Inc., Newark, Del.). The instrumentation and procedures have been described elsewhere (31). Identification of FAME profiles was performed with the MIDI software package (version 3.8). The identification of unknown Aeromonas isolates was performed by comparing them with the isolates in the AER48C library, as described previously (13). Identification of Aeromonas spp. by AFLP. DNA fingerprints were produced by AFLP, a recently developed genomic fingerprinting method based on the selective amplification of restriction fragments (35). In short, the technique consists of digestion of total cellular DNA with two restriction endonucleases (ApaI and TaqI), ligation of double-stranded restriction half-site-specific adaptors to all restriction fragments, amplification of a selected subset of these fragments with primers containing the same sequence as their corresponding adaptors, and electrophoresis of the PCR products on a regular 5% polyacrylamide sequencing gel. Selection during PCR is achieved by adding to the 39 ends of both primers one or more selective bases which are complementary to the DNA sequences adjacent to the restriction sites. The reaction conditions are such that only perfectly matched primers will initiate DNA synthesis. In addition, only one of the two PCR primers is radioactively labeled so that a limited number of PCR products will be visualized by autoradiography. The same Aeromonas reference strains used for FAME analysis were used to create a database of AFLP patterns (14), which was used for the identification of Aeromonas isolates. All procedures relating to the AFLP technique, including the production of bacterial cells, extraction and purification of genomic DNA, preparation of template DNA by digestion of genomic DNA and subsequent ligation of the restriction half-site-specific adaptors, selective amplification reactions, electrophoretic separation of PCR products, and analysis of banding patterns, as well as the sequences of the adaptors and PCR primers used in this study to generate AFLP banding patterns of Aeromonas strains, have been reported elsewhere (18).
J. CLIN. MICROBIOL. TABLE 1. Putative virulence factors among human and environmental Aeromonas strains No. (%) of isolates positive for:
Source (no. of isolates assayed)
Hemolysin
Cytotoxina
Patients with diarrhea (68) Healthy controls (11) Environmental strains (39)
31 (46) 3 (27) 25 (64)
34 (50) 3 (27) 24 (61)
All isolates (118)
59 (50)
61 (52)
0.10
0.26
P valueb
Titer, $1/8. b For isolates from patients with diarrhea versus environmental strains. a
Statistical analysis. Chi-square analysis and Mann-Whitney paired ranking tests were used (Statgraphics software, version 2.6; STSC, Statistical Graphics Corporation, Rockville, Md.).
RESULTS Occurrence of virulence factors. A total of 58 (49%) of all isolates were positive by cytotoxin and hemolysin assay. Among the human strains, isolates from patients with diarrhea more often produced hemolysin and cytotoxin than those from healthy controls (Table 1), but these properties were found to occur even more frequently among the environmental isolates than among patient isolates (Table 1). Hemolysin production was strongly correlated to cytotoxin production; only 1 of 61 cytotoxin-producing isolates (titer, $8) was negative by the hemolysin test, and only 1 of 57 cytotoxin-negative isolates (titer, ,8) was positive by the hemolysin test. PhP types. The diversities of the PhP types were calculated separately for each group of isolates, and it was found that patient isolates showed the lowest level of diversity, whereas the environmental strains showed the highest level of diversity (Table 2). The PhP types of the human and environmental isolates were clustered separately, yielding one dendrogram for human isolates (Fig. 1) and one for environmental isolates (Fig. 2). Among the human isolates, two different PhP types dominated (named BD-1 and BD-2); these were found in 23 (29%) and 24 (30%) isolates, respectively, and in 19 (31%) and
TABLE 2. Diversity index of PhP, occurrence of types BD-1 and BD-2, and expression of putative virulence factors among human and environmental Aeromonas strains Source (no. of isolates assayed)a
Diversity index
No. (%) of isolates of PhP type: BD-1
BD-2
Other
Patients with diarrhea (69) Cytotoxin-positive strains
0.83
20 (29) 6 (30)
23 (33) 21 (91)
26 (38) 7 (27)
Healthy controls (11) Cytotoxin-positive strains
0.89
3 (27) 0
1 (9) 1
7 (64) 2 (29)
Environmental strains (40) Cytotoxin-positive strains
0.92
9 (23) 0
1 (3) 1
30 (75) 23 (77)
32 (27) 6 (19)
25 (21) 23 (92)
63 (52) 32 (51)
Total (120) Cytotoxin-positive strains
a P value for prevalence of PhP type BD-2 in isolates from patients with diarrhea versus environmental strains, ,0.001. P value for cytotoxin-positive isolates in PhP type BD-2 versus all other isolates, ,0.0001.
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FIG. 1. Dendrogram showing UPGMA clustering of the PhP types obtained from Aeromonas isolates from patients with diarrhea and from healthy controls. BD-1 to BD-12 denote PhP types found in more than one individual, and S’s denote the PhP types found in only one individual. The HGs according to FAME and AFLP analysis are also given, and the presence of hemolysin and the cytotoxin titer of each isolate are provided. a, HGs were determined by AFLP analysis; nd, not determined; NT, nontypeable; C, healthy control; ID, identity.
22 (35%) of the patients with diarrhea, respectively. Most other human strains were of a minor PhP type or single PhP types. PhP type BD-1 was also the most common type among the environmental isolates (Fig. 2), whereas PhP type BD-2 was only identified in one of the environmental isolates. Two other PhP types (types E2 and E3) that were common among the environmental isolates were not found at all among human isolates. HGs. HGs were determined by FAME analysis of 106 isolates and by AFLP analysis of 29 isolates that were of special interest or for which the results needed to be further confirmed. All three groups of isolates (patient, human, and environmental isolates) yielded similar diversity indices (Table 3). Most human isolates were allocated to the genospecies A. caviae HG4 and A. hydrophila HG1 (Table 3). Among the environmental strains, most were allocated to HGs 8 and 4 (Fig. 2). Correlation between HGs, PhP types, and virulence factors. The most common HGs found among the isolates were 1, 4, and 8. According to PhP typing, all isolates except one human isolate belonging to HG1 were identical and were assigned to PhP type BD-2 (Fig. 1 and 2). The human isolates belonging to HG4 were more diverse according to their PhP types, with a main cluster forming related PhP types BD-1, BD-3, BD-7,
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FIG. 2. Dendrogram showing UPGMA clustering of the PhP types obtained from environmental Aeromonas isolates. The HGs according to FAME and AFLP analysis are also given, and the presence of hemolysin and the cytotoxic effect of each isolate are provided. a, v, the results varied; nd, not determined; NT, nontypeable; ?, isolates could not be assigned to any special HG group. b, HGs were determined by AFLP analysis. c, main PhP types of patient isolates. d, Swedish reference strain. ID, identity.
BD-8, and BD-9 (Fig. 1), and all environmental isolates belonging to HG4 were assigned to PhP type BD-1 (Fig. 2). HG8 isolates both from human and from environmental sources were of several different PhP types. The occurrence of cytotoxic activity was calculated separately for the two most common PhP types (Table 2) and for
TABLE 3. Diversity indices of HGs, occurrence of HGs, and expression of putative virulence factors among human and environmental Aeromonas strains Source (no. of isolates assayed)a
Diversity index
Patients with diarrhea (62) Cytotoxin-positive strains
0.66
Healthy controls (7) Cytotoxin-positive strains
0.64
Environmental strains (36) Cytotoxin-positive strains
0.67
Total (105) Cytotoxin-positive strains
No. (%) of isolates in HG: 1
4
8
Other
24 (39) 22 (92)
27 (44) 9 (33)
3 (5) 1 (33)
8 (13) 1 (13)
1 (14) 1 (100)
4 (57) 1 (25)
1 (14) 1 (100)
1 (14) 0
2 (6) 2 (100)
8 (22) 16 (44) 0 13 (81)
10 (28) 7 (70)
39 20 10 (26) 15 (75)
19 8 (42)
27 25 (92)
a P value for prevalence of HG1 in isolates from patients with diarrhea versus environmental isolates, ,0.001. P value for cytotoxin-positive isolates in HG1 versus all other isolates, ,0.0001.
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the three most common HGs (Table 3). Of the isolates belonging to PhP type BD-1, only a few produced cytotoxin and hemolysin, with all of these being patient isolates and belonging to HG4. Twenty-three of the 25 isolates belonging to PhP type BD-2 produced cytotoxin (titers, $1/8), and 19 of these showed high titers (titers, $1/256). DISCUSSION The question of whether Aeromonas spp. should be regarded as pathogens has still not been clarified. It is indisputable that Aeromonas strains may produce many different putative virulence factors, such as hemolysins, enterotoxins, or cytotoxins (11). These virulence factors occur among clinical isolates as well as among isolates from other sources, and no clear connection between the production of virulence factors and prevalence in human disease has so far been observed (23, 32), although it has been found in a study in Bangladesh that enterotoxin production correlates well with enteropathogenicity (10). In the present study we found a high prevalence of cytotoxin and hemolysin production among all Aeromonas strains studied, which is in concert with what has been reported by several other investigators (7, 20, 23, 26, 33). We also found an equal or even higher prevalence of these properties among environmental strains than among isolates from humans with diarrhea; this might have led us to the false conclusion that no special virulence traits existed among the patient isolates. The relation between environmental and patient strains is a matter of debate in many kinds of infections caused by bacterial species common in environmental sources. One way to analyze these matters is to type every single isolate from patients and from environmental sources in order to obtain a measure of the diversity among isolates from both sources, and to find out which strains predominate in each source. In fact, this approach was used in the present study. The low level of diversity obtained from the PhP types among the Aeromonas isolates from patients with diarrhea, compared to that among isolates from other sources, indicates selection for certain strains among humans with diarrhea. In addition, different types were shown to be predominating in different sources. Isolates of PhP type BD-2 and HG1 predominated in patients and isolates of PhP types E2 and E3 (mainly of HG8) predominated in the environment, whereas the common type HG4/ BD-1 was present among both environmental and patient isolates. Furthermore, isolates of the HG1/BD-2 type were able to produce relatively high levels of the investigated virulence factors. Taken together, these findings indicate that the BD-2/ HG1 type may represent a true human pathogenic Aeromonas. This is further supported by the fact that the genospecies A. hydrophila HG1 has previously been reported to be one of the most common genospecies associated with human intestinal disease (5, 21, 22) as well as with other infections (17, 19, 28), and a high prevalence of toxin-producing isolates has also been found in animal and environmental sources (7). In contrast, strains found exclusively in the environment may be relatively nonpathogenic to humans. PhP type BD-1, which was as common in the samples from the environments as in those from the human intestines, may represent strains that have entered the intestinal tracts through the ingestion of contaminated water. Whether this strain caused diarrhea in the patients from whom it was isolated or if other organisms were responsible for the diarrhea is an open question. In fact, other diarrheal pathogens (C. jejuni, Salmonella spp., Shigella spp., or non-O1, non-O139 V. cholerae) were found in almost 50% of the patients and were as common among patients harboring BD-2 as among those harboring other Aeromonas strains.
It is evident that the data obtained from investigations of the frequency of virulence factors in strain collections are highly dependent on temporary variations in the distribution of individual strains, such as clonal outbreaks. For example, if there happens to be an outbreak of a strain belonging to the HG1/ BD-2 type, the proportion of virulence factor-producing Aeromonas strains in patients would become high, and Aeromonas spp. would easily be recognized as a pathogen, whereas if patients with diarrhea mainly carry nonvirulent strains of this species, the existence of a virulent clone among some of the patients will not be recognized. We thus suggest that further investigations of this kind be complemented with suitable typing methods, in order to be able to compare the proportions of different strains in different collections. ACKNOWLEDGMENTS This work was supported by the Swedish Agency for Research and Co-operation (SAREC); ICDDR,B; grant 50.0269/93 from the Swedish Council for Forestry and Agricultural Research; contract G.O.A. 91/96-2 of the Ministerie van de Vlaamse Gemeenschap, Bestuur Wetenschappelijk Onderzoek, Belgium; and Karolinska Institute Funds. REFERENCES 1. Araujo, R. M., R. Pares, and F. Lucena. 1990. The effect of terrestrial effluents on the incidence of Aeromonas spp. in coastal waters. J. App. Bacteriol. 69:439–444. 2. Aziz, K. M. S., Z. Rahim, A. S. G. Faruque, S. Huq, and A. Eusof. 1986. Aeromonas hydrophila: its isolation from acute diarrhoeal illness in rural Bangladesh. Bangladesh Med. Res. Council Bull. 12:49–58. 3. Begue, R. E., G. Castellares, K. E. Hayashi, R. Ruiz, R. Meza, C. K. English, E. Gotuzzo, J. L. Sanchez, and R. Oberst. 1994. Diarrheal disease in Peru after the introduction of cholera. Am. J. Trop. Med. Hyg. 51:585–589. 4. Carnahan, A. M., S. Behram, and S. W. Joseph. 1991. Aerokey II: a flexible key for identifying clinical Aeromonas species. J. Clin. Microbiol. 29:2843– 2849. 5. Deodhar, L. P., K. Saraswathi, and A. Varudkar. 1991. Aeromonas spp. and their association with human diarrheal disease. J. Clin. Microbiol. 29:853– 856. 6. Gentry, M. K., and J. M. Dalrymple. 1980. Quantitative microtiter cytotoxicity assay for Shigella toxin. J. Clin. Microbiol. 12:361–366. 7. Gray, S. J., D. J. Stickler, and T. N. Bryant. 1990. The incidence of virulence factors in mesophilic Aeromonas species isolated from farm animals and their environment. Epidemiol. Infect. 105:277–294. 8. Hanninen, M. L., S. Salmi, L. Mattila, R. Taipalinen, and A. Siitonen. 1995. Association of Aeromonas spp. with travellers diarrhea in Finland. J. Med. Microbiol. 42:26–31. 9. Havelaar, A. H., F. M. Schets, A. van Silfhout, W. H. Jansen, G. Wieten, and D. van der Kooij. 1992. Typing of Aeromonas strains from patients with diarrhea and from drinking water. J. App. Bacteriol. 72:435–444. 10. Hossain, M. A., K. M. Rahman, S. M. Asna, Z. Rahim, T. Hussain, and M. R. Miah. 1992. Incidence of Aeromonas isolated from diarrheal children and study of some virulence factors in the isolates. Bangladesh Med. Res. Council Bull. 18:61–67. 11. Houston, C. W., A. K. Chopra, J. M. Rose, and A. Kurosky. 1991. Review of Aeromonas enterotoxins. Experientia 47:424–426. 12. Huys, G., M. Vancanneyt, R. Coopman, P. Janssen, E. Falsen, M. Altwegg, and K. Kersters. 1994. Cellular fatty acid composition as a chemotaxonomic marker for the differentiation of phenospecies and the hybridization groups in the genus Aeromonas. Int. J. Syst. Bacteriol. 44:651–658. 13. Huys, G., I. Kersters, M. Vancanneyt, R. Coopman, P. Janssen, and K. Kersters. 1995. Diversity of Aeromonas sp. in Flemish drinking water production plants as determined by gas-liquid chromatographic analysis of cellular fatty acid methyl esters (FAMEs). J. App. Bacteriol. 78:445–455. 14. Huys, G., R. Coopman, P. Janssen, and K. Kersters. 1996. High-resolution genotypic analysis of the genus Aeromonas by AFLP fingerprinting. Int. J. Syst. Bacteriol. 46:572–580. 15. Islam, M. S., M. J. Alam, and S. Tzipori. 1992. Abundance of Aeromonas spp. in various components of pond ecosystems in Dhaka, Bangladesh. Int. J. Environ. Studies 39:297–304. 16. Janda, J. M., and R. P. Kokka. 1991. The pathogenicity of Aeromonas strains relative to genospecies and phenospecies identification. FEMS Microbiol. Lett. 15:29–33. 17. Janda, J. M., L. S. Guthertz, R. P. Kokka, and T. Shimada. 1994. Aeromonas species in septicemia—laboratory characteristics and clinical observations. Clin. Infect. Dis. 19:77–83.
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