Tributyltin-Resi stant Bacteria fromEstuarine and Freshwater Sediments

Resistance totributyltin (TBT) was examined inpopulations fromTBT-polluted sediments andnonpoliuted sediments froman estuary andfromfresh wateraswellasinpurecultures isolated fromthose sediments. The 50%effective concentrations (EC50s) forpopulations werehigher ataTBT-polluted freshwater site thanata site without TBT,suggesting that TBTselected for aTBT-resistant population. Incontrast, EC50s were significantly lower forpopulations fromaTBT-contaminated estuarine site thanforthose fromasite without TBT,suggesting thatother factors inaddition toTBTdetermine whether populations becomeresistant. EC50s forpopulations fromTBT-contaminated freshwater sediments were nearly 30timeshigher thanthoseforpopulations from TBT-contaminated estuarine sediments. We defined aTBT-resistant bacterium asone which grows on trypticase soyagarcontaining 8.4,uMTBT,aconcentration whichprevented thegrowth of90%oftheculturable bacteria fromthese sediments. Thetoxicity ofTBTinlaboratory mediawas influenced markedly bythecomposition of themediumandwhether itwas liquid orsolid. TenTBT-resistant isolates fromestuarine sediments and19from freshwater sediments were identified tothegenus level. Two isolates, eacha BaciUlus sp.,may bethefirst gram-positive bacteria isolated fromfresh waterinthepresenceofahighconcentration ofTBT.There was ahigh incidence ofresistance toheavymetals: metalresistance indices were 0.76forestuarine isolates and0.68for freshwater isolates. Theseisolates also hada highincidence ofantibiotic resistance: antibiotic resistance indices were 0.50forestuarine isolates and0.59forfreshwater isolates. Bothmetalandantibiotic resistances can be plasmid mediated, and8of10estuarine and12of19freshwater isolates contained plasmids. Butyltins areorganometallic compounds usedas stabilizersinplastics, as woodpreservatives, andas antifouling agents(49). Muchofthecurrentinterest inorganotin toxicology andchemistry hasfocused on theenvironmental fate andeffects oftributyltin (TBT), a componentofantifouling paints whichhasbeenwidely employed on boathulls in marinewaterandon other surfaces which aresubmerged in water.However, TBT'shightoxicity tonontarget organisms ledtoitsregulation inseveral European countries andthe United States (51). Although TBThasbeenregulated, itmay still beapplied tovessels longer than25m, toaluminum-hulled boats, and towaterintake pipesandothersubmerged surfaces. Becauseofcurrent input andslowdegradation, itcontinues to befoundinbothmarinesediments (26,33,52,53)and freshwater sediments (31). Concernabouttheenvironmental fateofTBT incoastal watersstimulated research into thebiodegradation ofTBTin water,sediment, andhigher marine organisms. Theresults ofthese studies indicate that TBTcanbedegraded rapidly in themarinewatercolumntodibutyltin andmonobutyltin, witha half-life ofseveral days(43). TBT degradation by photolysis aloneproceeds slowly, witha half-life of>89 days(31), butitcan bestimulated inthepresenceoflight whenmicroalgae are present (29) andnutrients areadded (34). TBT isalsometabolized byseveral marineinvertebrates andfish that use mixed-function oxygenases(28). MostTBT isfoundinsediments andthesurface microlayer (18, 31,32). Allevidence todateindicates thatbiodegradation doesnotcontribute significantly tothedisappearance ofTBT fromsediments. In fine-grained marine