Enterotoxin production in relation to taxonomic grouping and source of isolation of Aeromonas species

Abstract

A total of 19 of 20 (95%) strains of Aeromonas hydrophila biovar hydrophila and 16 of 17 (94%) strains of Aeromonas sobria isolated from a variety of clinical and environmental sources were found to be enterotoxin positive. Only 2 of 18 (11%) A. hydrophila biovar anaerogenes and 2 of 13 (15%) unidentified Aeromonas strains from a similar variety of sources produced enterotoxin. No association was apparent between the source of isolation, in particular diarrheal stools, and enterotoxigenicity; 41% of the isolates from diarrheal stools were enterotoxin negative. A strong correlation was noted between ability to produce enterotoxin and positive results in six characters: lysine decarboxylase and Voges-Proskauer reactions, production of gas from glucose, gluconate oxidation, xanthine hydrolysis, and hemolysis of human erythrocytes. In the majority of cases (35 of 39 strains), enterotoxigenicity was detected using cell-free filtrates of brain heart infusion broth cultures grown at 36°C for 15 h; however, the other four positive isolates were detected after growth in the same broth at 30°C or in Casamino Acids-yeast extract broth at 30 or 37°C. It is recommended that for enterotoxin tests, strains should be grown in both media at both temperatures. The infant mouse test was found to be a simple and reliable method for detection of the enterotoxin. The toxin proved to be heat labile and not neutralized by cholera antitoxin. Aeromonads are ubiquitous waterborne bacteria belonging to the Vibrionaceae. The taxonomy of Aeromonas spp. has been in a state of flux for some years, and only now is it becoming better understood. At present, the most frequently used classification of motile Aeromonas spp. is that of Popoff and Véron (29), who divided them into Aeromonas hydrophila biovar hydrophila, A. hydrophila biovar anaerogenes, and a new species, Aeromonas sobria. More recently, it has been proposed that A. hydrophila biovar anaerogenes should be renamed Aeromonas caviae (28), but this remains to be validated. Even now, many laboratories are unable to distinguish these species readily, and nonspecific references to A. hydrophila in reports until very recently can be interpreted as encompassing both biovars of A. hydrophila (sensu strictu) and A. sobria. These species, usually under the name A. hydrophila, have been isolated frequently from soil, foods, and a variety of animals, mostly those associated with water, such as fish, reptiles, and amphibians; in fact, these species have long been recognized as pathogenic agents causing hemorrhagic septicemias in these types of cold-blooded animals (12, 13, 17, 23). Their ability to cause opportunistic infections in compromised human hosts has been accepted for some time, and reports of various types of A. hydrophila infections, particularly in swimming-related injuries, in previously healthy persons are increasing. References to such reports are listed by other authors (6, 17, 32). Several investigators have reported the isolation of A.

AEROMONAS HYDROPHILA and AEROMONAS SOBRIA AS POTENTIAL FOOD POISONING SPECIES: A REVIEW1

“…hydrophila enterotoxin was not neutralized by specific antisera to cholera toxin. Champsaur et al (1982) and Turnbull et al (1984) reported similar results for enterotoxins from clinical and environmental isolates of A . sobria and A. hydrophila.…”

“…Most isolates are cytotoxic to cell cultures, producing responses similar to those noted with Clostridiumperfringens enterotoxin (Hostacka et al 1982;Cumberbatch et al 1979;Jiwa 1983). Additionally, a number of investigators have reported that some isolates of Aeromoms elaborate a heat-labile, cytotonic enterotoxin that has activity similar to that noted with cholera or E. coli heat-labile enterotoxins (Ljungh et al 1981(Ljungh et al , 1982a(Ljungh et al , 1982bLjungh and Wadstrom 1983;Jiwa 1983;Turnbull et al 1984;Campbell et al 1984). Ljungh and coworkers (Ljungh et al 1981(Ljungh et al , 1982a(Ljungh et al , 1982bLjungh and Wadstrom 1983) isolated two hemolysins and a distinct enterotoxin from A .…”

THE OCCURRENCE, HEMOLYTIC ACTIVITY AND ANTIBIOTIC SUSCEPTIBILITY OF MOTILE AEROMONAS SPP. ISOLATED FROM MEAT AND MILK SAMPLES IN TURKEY

“…Among the virulence factors are enterotoxin, cytotoxin, and hemolysin. The production of cytotoxin and hemolysin is related to A. hydrophila and A. sobria (Turnbull et al 1984), and the production of hemolysin is associated with enterotoxigenicity (Janda et al 1985). In addition to Aeromonasassociated disease in the very young, the old and the immunocompromised often require antimicrobial therapy.…”

CHARACTERIZATION OF AEROMONAS SPECIES ISOLATED FROM FOOD

“…These factors are highly restricted to both A. hydrophila and A. sobria comparing to A . caviae (Turnbull et al 1984;Kirov et al 1986). There is a strong association (91 %) between enterotoxigenicity and gastroenteritis.…”

“…There is a strong association (91 %) between enterotoxigenicity and gastroenteritis. Enterotoxin producing strains possess three or more of the following properties: they are Voges-Proskauer positive, do not hydrolyse arabinose, are positive for lysine decarboxylase, ferment sorbitol, are able to grow at 42 or 43C, produce large amount of hemolysin Turnbull et al 1984;Kirov et al 1986;Callister and Agger 1987). In contrast, Singh and Sanyal (1992) found that enterotoxin production did not correlate with these properties in addition to citrate utilization either singly or in combination.…”