Deoxyribonucleic AcidBaseComposition and Homology Studies ofLeptospira

Fourdistinct genetic groups ofleptospiras weredemonstrated amongselected pathogenic and"biflexa" serological types. Pathogenic leptospiras could bedivided into twogroups onthebasis ofpercent guanine + cytosine (GC)intheir deoxyribonucleic acid(DNA).Onegrouphad36 1%,theother39 4 1%.The biflexa strains hadDNA of39+ 1%GC,butwerefurther separated into twogroups onthebasis ofDNA-annealing tests. Strains within groups hadahighdegree ofspecific duplex formation (75% binding ormorewithreference tothehomologous DNA).There waslittle ornogenetic relatedness between strains ofthefourgroups (less than10%DNA homology). Thethermal elution midpoint ofheterologous DNA duplexes wasalways lower thanthehomologous reaction. Theserological relationships amongstrains werenotmeaningful interms ofrelatedness determined by specific duplex formation. Thediversity ofserological types inthegenus Leptospira andits bearing onlaboratory diagnosis, epidemiology, andprophylaxis ofleptospirosis haveserved todirect considerable attention to serological studies. Otherschema forthedifferentiation ofleptospiras havebeendeveloped, and itisevident thatthegenusiscomprised oftwo majorgroups orcomplexes oforganisms, the pathogenic andthe"saprophytic," "water," or "biflexa" leptospiras. Thebiflexa organisms are omnipresent infreshsurface waters. Unlike pathogenic leptospiras, theyhaverarely been associated withanimal hosts andreportedly do notproduce infections inlaboratory rodents (2, 3,22,23,27). Thetwocomplexes alsodiffer in nutritional requirements andother phenotypic properties. Nonetheless, theseparate speciation ofthetwocomplexes hasbeendenied byaWorld Health Expert Groupwhorecommended that "thegenusLeptospira beconsidered asmonospecific until itispossible tocircumscribe species withconfidence" (42). Taxonomic considerations ofleptospiras have heretofore beenbased onphenotypic characteristics whichpresumably relate tooverall genetic character. Itisdoubtful, however, that themeasureofevenlarge numbers ofphenotypic characters (asinAdansonian orcomputer taxonomy) 1Present address: Department ofMicrobiology andTropical Medicine, Georgetown University SchoolofMedicine, Washington, D.C.20009.