Cytotoxic Enterotoxin Produced byAeromonas hydrophila: Relationship ofToxigenic Isolates toDiarrheal Disease

strains could beshowntohaveanyplasmid byagarose gelelectrophoresis. Cytotoxin production correlated withapositive lysine decarboxylase phenotype (98%) orapositive Voges-Proskauer phenotype (94%), compared to27%lysine decarboxylase-positive and23%Voges-Proskauer-positive, cytotoxin-negative isolates (P<0.001 for both). Infecal samples, cytotoxin production correlated withdiarrheal disease; of 40diarrheal isolates, 32(80%) weretoxigenic compared to9(41%) of22nondiarrheal isolates (P= 0.004). Itappears thatA.hydrophila canbeacauseof diarrhea andthatthisenteropathogenic potential ismediated byacytotoxic enterotoxin. Aeromonas hydrophila hasbeenrecognized asproducing avariety ofextracellular products, including enzymes andhemolysins (3,28). This organism hasbeenconsidered tobeagastrointestinal pathogen onthebasis ofreports ofisolation frompatients withgastroenteritis (1,5, 15,17,25). Nevertheless, there havenotbeen anywell-controlled epidemiological studies of therole ofA.hydrophila inhumandisease. In addition, theorganism hasbeenisolated from upto3%ofhealthy people (26). A possible explanation oftheenteropathogenic potential of A.hydrophila hascomewiththefinding thatit produces anenterotoxin. Thissubstance, presentinthecell-free supernatants ofbrothcultures, canbeidentified byproduction offluid secretion intheisolated rabbit leal loop (1, 16). Brothculture supernatants fromA.hydrophila cause cell death inseveral tissue culture systems (13, 16). Similar effects intissue culture cells havebeenreported forthegnterotoxins producedbyShigella dysenteriae andClostridium perfringens (14). Thelatter enterotoxins have beenclassified as"cytotoxic" todifferentiate themfromthe"cytotonic" enterotoxins suchas those produced byEscherichia coli andVibrio cholerae,whichcauseroundingandincreased steroidproduction,butnotcelldeath,inmouse adrenaltumorcells(14).Onegroup,however, hasclaimedthatA.hydrophilaproducesboth acytotoxinandacytotonicenterotoxinwhich resemblesthecholeraandE.coliheat-labile enterotoxinsbyitseffectonmouseadrenaltumorcells(16,28).Intheirstudy,heatingthe brothculturesupernatantsofA.hydrophilaat 56°Cfor10mininactivatedthecytotoxictoxin, consequentlyallowingthecytotonictoxintobe detectedbyitsroundingeffectonmouseadrenal cells.Theseauthorsclaimedalsotohaveseparatedacytotoxin,acytotonicenterotoxin,anda hemolysinfromA.hydrophilacell-freefiltrates byisoelectricfocusing.BothstudiesinvestigatingthetoxigenicityofalargenumberofA. hydrophilastrainsisolatedfromclinicaland environmentalsourceshavesuggestedthatall suchstrainsaretoxigenic(1,16). Untilrecently,noattempthasbeenmadeto correlatethebiochemicalcharacteristicsofA. hydrophilawithtoxigenicity.Ljunghetal.(16) tested11enterotoxigenicstrainsinmorethan90 reactionsbutcouldnotcorrelateanybiochemicalreaction withenterotoxigenicity.