Chromosomal DNA, iron-transport systems, outer membrane proteins, and enterotoxin (heat labile) production in Salmonella typhi strains.

We examined a representative collection ofSalmonella typhi strains fromChile, Peru,Mexico, India, and England forthepresenceofseveral properties. Ailstrains hadaconserved pattern ofouter membraneproteins, asdetermined bysodiumdodecyl sulfate-polyacrylamide gelelectrophoresis. Theelectrophoresis profiles of chromosomal DNA digested withEcoRIandPstIrestriction enzymeswere similar forallthestrains. A conserved pattern ofhybridization was observed whendigested chromosomal DNA was hybridized withDNA probes forthe36-kilodalton porin, enterobactin synthesis, andenterobactin receptor genes.AUlthestrains produced enterobactin butnotaerobactin inbioassays. Noneofthestrains produced heat-labile toxin, as measured byan enzyme-linked immunosorbent assay.Colony andSouthern hybridizations withDNA probes foraerobactin synthesis anditsreceptor andheat-labile toxin geneswerenegative. Theseresults indicate that S.typhi strains fromdifferent origins havesimilar phenotypic andgenetic properties and,as hasbeen suggested, constitute a clone. Typhoid fever hasalmost totally disappeared asa causeof morbidity inindustrialized countries because ofimproved sanitary conditions inthepreparation offoodandwater, sewagedisposal, andtreatment ofcarriers (8). Nonetheless, typhoid fevercontinues tobean important public health problem insome developing countries, wherethetransmissionofthedisease ischaracterized byepidemics superimposed on anendemic background ofSalmonella typhi infections (8). Massive outbreaks oftyphoid fever during thepast 17years,withhundreds ofthousands ofcases,inMexico City, Mexico, Santiago, Chile, andLima,Peru(5), have usually beenduetobreakdowns inthetreatment ofwater, contaminated food, andimproper disposal ofsewage(5). Theconserved antigenic andmetabolic properties among different isolates ofS.typhi, aswellastherecentdemonstration ofa conserved isoenzyme pattern (17,18),have suggested thatallS.typhi strains worldwide are clonally related. Nonetheless, rRNAgenerestriction pattern analysis hasrecently provided a tool todifferentiate among S.typhi strains (3). Tounderstand S.typhi virulence, toelucidate the hostimmuneresponsetoitsantigens, andtodesign new vaccines anddiagnostic assays,itisimportant todetermine ifother S.typhi properties are constant among different S. typhiisolates (19). Included intheseproperties are the presenceofgenescoding forputative virulence factors and surface antigens. Twosuch genes arethose that codeforthe outermembraneproteins oftheenterochelin-mediated iron uptake systemandthe36-kilodalton (kDa)porin(6,11). Boththeporin andtheironuptakeproteins are important humanimmunogens andevokehightiters ofantibody in typhoid fever patients (6,11).We haveusedsuitable DNA probesandassays todetermine whetherthese genes and their products are conserved among S.typhistrains of different geographical origin.