EVOLUTION OF DRUG-RESISTANCE IN SALMONELLA-PANAMA ISOLATES IN CHILE

In a search for Salmonella isolates in the environment in Chile in 197 5, drug-susceptible strains of Salmonella panama were recovered for th e first time from river water and vegetables in the vicinity of Santia go, Two to 3 years later, antibiotic-resistant S. panama began to appe ar in a variety of sources (meat, animals, vegetables, etc.), giving r ise to a human epidemic that involved the entire nation, Of 139 clinic al isolates studied, 7 were drug susceptible, 11 were resistant only t o nitrofurans, and 3 were streptomycin, spectinomycin, and nitrofuran resistant; none of these 21 isolates harbored plasmid DNA. Most isolat es (n = 107) were resistant to nitrofurans (chromosomal) and to strept omycin, spectinomycin, sulfonamides, tetracycline, and mercuric and te llurite salts; this multidrug resistance was encoded on a 218-kb plasm id classified in a number of strains as being in the IncHI(2) group, F rom 1982 to 1993, 11 isolates acquired an additional self-transferable plasmid coding for resistance to any one of ampicillin (61 kb), ampic illin and trimethoprim (65 kb), ampicillin, trimethoprim, streptomycin , and sulfonamides (71 kb), ampicillin, gentamicin, kanamycin, and tet racycline (120 kb), or a nontransferable plasmid of similar to 6 kb en coding resistance to ampicillin or kanamycin, With the exception of am picillin or ampicillin and trimethoprim resistance, S. panama isolates from foodstuffs, mainly pork meat products, and animals had resistanc e patterns that were the same as those found in clinical specimens, Re markably, strains from goats and goat cheese and from shellfish isolat ed in particular rural regions were either drug susceptible or resista nt only to streptomycin-spectinomycin encoded on a mobile genetic elem ent and to nitrofurans. The report describes the arrival of a suscepti ble S. panama strain, its spread all over the country, and the evoluti on of progressively complex resistance patterns.