Fifty-eight multidrug-resistant Salmonella enterica strains of 20 serotypes, isolated from animal sources in Italy, were analyzed for tet(A) and strA-strB, conferring tetracycline and streptomycin resistance, respectively. The strA and strB genes were highly prevalent in Salmonella strains of our collection, being detected in 84% of the streptomycin-resistant strains. In many strains, the strA and strB genes were linked to a particular Tn5393-derivative transposon characterized by the presence of the insertion sequence IS1133, previously identified only in the plant pathogen Erwinia amylovora. Sixty-eight percent of the tetracycline-resistant strains were tet(A) positive, indicating that this gene is widely diffused in Salmonella strains circulating in animals in Italy. Most of the tet(A) genes were localized within a deleted Tn1721 transposon variant. Two prevalent repN and repI1 resistance plasmids were identified in Salmonella isolates of our collection.

A conjugative IncL/M plasmid (pSEM) conferring resistance to gentamicin, amikacin, kanamycin, sulfonamides, and expanded-spectrum cephalosporins was found in pathogenic strains of Salmonella enterica serotype Typhimurium. Resistance to aminoglycosides was encoded by a sul1-type class 1 integron (In-t3). An extendedspectrum beta-lactamase gene, bla SHV-5 , was identified 3.5 kb downstream of the integrase (intI1) gene of In-t3. Nucleotide sequence analysis of the 5.3-kb bla SHV-5 -In-t3 region of pSEM highlighted striking similarities with IncL/M plasmids isolated from nosocomial gram-negative pathogens, conferring resistance to expandedspectrum cephalosporins and aminoglycosides.Although antibiotic resistance is becoming a major threat to human health worldwide, information concerning the dissemination and geographical distribution of antibiotic-resistant bacterial pathogens remains scattered (8,14,26). During the last decade many hospital outbreaks caused by extended-spectrum beta-lactamase (ESBL)-producing Enterobacteriaceae have been reported, and most of the ESBL-producing strains were found to carry the bla TEM-1 , bla TEM-2 , and bla SHV-1 gene derivatives (5,6,9,11,19,27). These genes are prevalently located on large conjugative plasmids of the incompatibility (Inc) groups IncC, IncFI, IncH12, and IncL/M (3,5,7,13,18,20,23,28).More recent reports have highlighted the emergence of ESBL-producing strains endowed with an extremely wide spectrum of antibiotic resistance, including resistance to sulfonamides, trimethoprim, streptomycin, kanamycin, gentamicin, and amikacin (17,21,23). However, the molecular mechanisms involved in the acquisition and/or transmission of ESBL-encoding genes are poorly understood.In previous work we identified an IncL/M plasmid, referred to as pSEM, in eight epidemiologically unrelated, multipledrug-resistant strains of Salmonella enterica serotype Typhimurium. This plasmid contains a sul1-type class 1 integron (In-t3) carrying the aacA4, aacC1, and aadA1 gene cassettes and conferring resistance to sulfonamides, kanamycin, gentamicin, and amikacin (29). The streptomycin and spectinomycin-resistance determinant, aadA1, is poorly expressed in In-t3 because it is located in a distal position relative to the main integron promoter P ant (29).The Salmonella strains harboring pSEM were analyzed in more detail with regard to their antibiotic resistance profile. Microdilution susceptibility tests demonstrated that all eight strains were resistant to ceftazidime (Ն16 g/ml), cefotaxime (Ն32 g/ml), ceftriaxone (Ն64 g/ml), cefamandole (Ն32 g/ ml), cefoxitin (Ն8 g/ml), and aztreonam (Ն16 g/ml) but sensitive to imipenem (Ͻ4 g/ml).To clone the gene responsible for resistance to expandedspectrum cephalosporins, an EcoRI genomic library was constructed in the pUC18 vector using total DNA extracted from the prototypic Salmonella 202 strain. The library was introduced by transformation in Escherichia coli DH5␣ competent cells, and selection was performed on Luria-Bertani agar plates containing ceftaz...

The presence and genetic content of integrons were investigated for 37 epidemiologically unrelated multiple-drug-resistant strains ofSalmonella enterica serotype Typhimurium from humans. All isolates were resistant to ampicillin, chloramphenicol, kanamycin, streptomycin, sulfonamides, and trimethoprim, as well as to tetracycline and/or nalidixic acid; 20% of them were also resistant to gentamicin and amikacin. Three different class 1 integrons (In-t1, In-t2, and In-t3) were identified by Southern blot hybridization, PCR, and DNA sequencing, and these integrons were found to carry theaadB, catB3, oxa1,aadA1a, aacA4, and aacC1 gene cassettes. Integrons In-t1 (aadB and catB3) and In-t2 (oxa1 and aadA1a) were both located on a conjugative IncFI plasmid of 140 kb. In-t3 (aacA4,aacC1, and aadAIa) was located on an IncL/M plasmid of 100 kb which was present, in association with the IncFI plasmid, in gentamicin- and amikacin-resistant isolates. Despite the extensive similarity at the level of the antibiotic resistance phenotype, integrons were not found on the prototypic IncFI plasmids carried by epidemic Salmonella strains isolated during the late 1970s. The recent appearance and the coexistence of multiple integrons on two conjugative plasmids in the sameSalmonella isolate are examples of how mobile gene cassettes may contribute to the acquisition and dissemination of antibiotic resistance.

The results of this study will be useful for planning strategies to prevent and control diarrheal diseases in our country.