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Antimicrobial Agents and Chemotherapy, April 2001, p. 1305-1308, Vol. 45, No. 4
0066-4804/01/$04.00+0   DOI: 10.1128/AAC.45.4.1305-1308.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Multidrug Resistance Is Mediated by Large Plasmids Carrying a Class 1 Integron in the Emergent Salmonella enterica Serotype [4,5,12:i:]

Beatriz Guerra, Sara M. Soto, Jose M. Argüelles, and M. Carmen Mendoza^*

Área de Microbiología, Departamento de Biología Funcional, Universidad de Oviedo, 33006 Oviedo, Principado de Asturias, Spain

Received 7 August 2000/Returned for modification 24 October 2000/Accepted 10 January 2001

	ABSTRACT

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A multidrug-resistant Salmonella enterica serotype [4,5,12:i:] clone carried a class 1 integron harboring dfrA12 and aadA2 gene cassettes and bla_TEM-1, aac(3)-IV, cmlA1, and tetA genes located in large plasmids of about 140 kb (carrying spv) or 120 kb (lacking spv). Several segregants, lacking multidrug resistance, contained a plasmid smaller than the parental one and no longer hybridized with probes for the lost resistances. The genes mediating resistance to ampicillin, chloramphenicol, and tetracycline in the [4,5,12:i:] clone are different from those found in the pentadrug-resistant serotype Typhimurium DT104 clone.

	TEXT

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Multidrug-resistant Salmonella enterica serotype [4,5,12:i:] isolates emerged in 1997 in Spain (5). DNA fingerprinting analysis of 29 isolates (27 collected in the Principality of Asturias between May 1997 and April 2000 and the reference strains CNM4IC and CNM9IC) had shown that they fall into a single genetic lineage or clone which seems to be closely related to some contemporary serotype Typhimurium lineages causing human salmonellosis (reference 10 and unpublished data). All [4,5,12:i:] isolates expressed resistance to ampicillin (AMP), chloramphenicol (CHL), gentamicin (GEN), streptomycin (STR), sulfadiazine (SUL), tetracycline (TET), and trimethoprim (TMP) except two isolates which were TET susceptible. These isolates harbored two or three small cryptic plasmids and one large plasmid carrying (pUO-SVR3 [plasmid of University of Oviedo-Salmonella virulence resistance]) or lacking (pUO-SR4) spv loci (10) (Table 1). The objective of this study was to investigate the molecular basis of the multidrug resistance in serotype [4,5,12:i:] and ascertain the relations between resistance genes, integrons, and plasmids. For this, several experimental approaches were made, and the results were compared with those obtained for Typhimurium DT104 LSP14/92 (9), used as the type strain of a pentadrug-resistance epidemic clone of concern in many countries (3, 7, 8, 9, 11, 15).

View this table: [in this window] [in a new window]   TABLE 1.   Features of multidrug-resistant Salmonella serotype [4,5,12:i:] isolates used in curing experiments and their segregants

Determination of resistance genes. To determine the genes for AMP, GEN, and STR resistance, the [4,5,12:i:] isolates were tested, by disk diffusion assay (13) and taking into account results described elsewhere (2, 16, 17), with (i) amoxicillin-clavulanic acid (AMC; 30 µg), carbenicillin (CAR; 100 µg), cephalothin (30 µg), ceftazidime (30 µg), cefotaxime (30 µg), imipenem (10 µg), oxacillin (OXA; 1 µg), and piperacillin (PIP; 100 µg) and (ii) amikacin (30 µg), apramycin (APR; 100 µg), 5-episisomycin (5EPI; 10 µg), fortimycin (100 µg), kanamycin (30 µg), netilmicin (NET) and its derivatives 2'-NET- and 6'-NET (100 µg), tobramycin (TOB; 10 µg), and spectinomycin (SPT; 10 µg). PCR amplication was performed using specific primers (Table 2). All [4,5,12:i:] isolates showed resistance to AMP-CAR-OXA, GEN-APR-5EPI-NET-2'NET-6'NET-TOB, and STR-SPT and generated amplification products with the bla_TEM, aac(3)-IV, and aadA primers. Typhimurium LSP14/92 showed resistance to AMP-CAR-PIP-OXA-AMC and STR-SPT and generated the expected amplification products with the bla_CARB and aadA primers. PCR for other resistance genes using [4,5,12:i:] isolates generated amplification products with the cmlA, dfrA12, tetA, and sul1 primers, while Typhimurium LSP14/92 generated products with the floR, tetG, and sul1 primers. Sequencing of the 460-bp bla_TEM and 435-bp cmlA amplicons generated by LSP389/97, conducted described elsewhere (9), confirmed the presence of bla_TEM-1-like and cmlA1 genes (accession numbers AF126482.1 and U12338, respectively). These data support the finding that the genes implicated in AMP, CHL, and TET resistance in the [4,5,12:i:] clone differ from those reported for the pentadrug-resistant Typhimurium DT104 clone.

View this table: [in this window] [in a new window]   TABLE 2.   PCR primers and conditions used in this work

Detection of integrons. Detection of class 1 integrons and the resistance genes located therein was performed by PCR amplification with specific primers (Table 2). All [4,5,12:i:] isolates carried two integrons defined by PCR products of 1,900 and 150 bp, sizes indicating that only in the first could gene cassettes carrying resistance genes be inserted. Class 1 integrons in their conserved regions, in addition to the integrase (intI1) gene, usually contain genes encoding resistance to quaternary ammonium compounds and ethidium bromide (qacE) and sulfonamides (sul1) (6, 12, 14). The presence of these genes was confirmed in all [4,5,12:i:] isolates by PCR. Sequencing of the 1,900-bp amplicon from LSP389/97 confirmed the presence of the dfrA12 (dhfrXII) gene cassette near the 5' conserved region (5'CS), an open reading frame with an unknown function and a 59-bp element, and the aadA2 gene cassette near the 3'CS (accession number AF284063). Intragenic primers for dfrA12 were used to confirm that this gene, together with aadA2, was carried in the 1,900-bp amplicons from all [4,5,12:i:] isolates studied. Typhimurium LSP14/92 carried two other integrons, with variable regions previously defined as 1,200-bp pse1 and 1,000-bp aadA2 (9).

Determination of relationships between drug resistance and plasmids. In a previous work, we found that when two representative [4,5,12:i:] isolates were grown in the presence of sodium dodecyl sulfate (1%), some cells (segregants) lost AMP, CHL, GEN, and TMP resistance and their large plasmids appeared to be smaller (10). To confirm the apparent relationship between drug resistance and plasmids, as well as the possible relationship between integrons and plasmids, we used the following approaches.

(i) New curing experiments using representative isolates of the four plasmid profiles. For each experiment we analyzed about 100 colonies, finding that some colonies had lost some resistances and could be grouped in segregant types. A strong correlation between phenotype and genotype was found in all segregants, and one colony of each type was selected for the following experiments (Table 1).

(ii) Integron analysis. Using qacE1, sul1, and 5'CS/3'CS primers, all segregant types generated amplification products of about 250, 430, and 150 bp, respectively, but only some of them generated products of 1,900 bp with dfrA12 primers and expressed TMP resistance. Only some of the TMP-susceptible segregants were also STR susceptible indicating that more than one gene was implicated in STR resistance.

(iii) Plasmid analysis. None of the small cryptic plasmids was eliminated by curing, and all pUO-SVR3 segregants yielded an amplification product with spv primers. In segregants lacking the 1,900-bp dfrA12-aadA2 integron and the other resistance genes, pUO-SVR3 or pUO-SR4 derivatives were smaller than the original plasmids.

(iv) Plasmid DNA hybridization. Hybridization using DNA from parents and representative segregants was performed with probes for the presumably lost genes and spvC. The results (Fig. 1) confirmed that 150- and 1,900-bp dfrA12-aadA2 integrons, as well as bla_TEM-1-like, aac(3)-IV, cmlA1, tetA, and spvC genes, were plasmid located.

View larger version (90K): [in this window] [in a new window]   FIG. 1.   Analysis of plasmids from representative Salmonella serotype [4,5,12:i:] isolates used in curing experiments and from some of the segregates obtained. (A) Plasmid profiles; (B) hybridization of the plasmids shown in panel A with gene-specific probes. Lanes T, Typhimurium LSP14/92; lanes P1 and P2, [4,5,12:i:] LSP389/97 and LSP272/98, respectively; lanes s4 to s8, segregant types. The features of the [4,5,12:i:] organisms shown are compiled in Table 1. Arrowheads, chromosomal (Chr) DNA.

Plasmids, complete integrons, and gene cassettes are elements with potential for individual and joint horizontal transfer (4, 6, 14). The transfer of multidrug-resistant Salmonella plasmids, or the resistance gene cluster(s) therein, to other salmonellae or other pathogenic bacteria in animal reservoirs or human hosts could result in serious problems. The relative arrangement of the integrons with the other resistance genes and the spv loci in plasmids carried by [4,5,12:i:] organisms remain to be determined.

	ACKNOWLEDGMENTS

We thank M. A. González-Hevia (Laboratotio de Salud Pública, LSP, Principado de Asturias, Spain) and M. A. Usera and A. Echeitia (Centro Nacional de Microbiología, CNM, Madrid, Spain) for Salmonella [4,5,12:i:] isolates; we thank J. Ruiz and J. Vila for the design of cmlA primers and advice regarding chloramphenicol resistance analysis.

This work was supported by a grant from the Fondo de Investigación Sanitaria (FIS 00/1084), Ministerio de Sanidad y Consumo, Spain. S. Soto is the recipient of a grant of Formación de Personal Investigador (Ref AP98), Ministerio de Educación y Cultura, Spain.

	FOOTNOTES

^* Corresponding author. Mailing address: Área de Microbiología. Departamento de Biología Funcional, Facultad de Medicina, Universidad de Oviedo, Calle Julián Clavería 6, 33006 Oviedo, Spain. Phone: 34-985-103560. Fax: 34-985-103148. E-mail: camf{at}sauron.quimica.uniovi.es.

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Antimicrobial Agents and Chemotherapy, April 2001, p. 1305-1308, Vol. 45, No. 4
0066-4804/01/$04.00+0   DOI: 10.1128/AAC.45.4.1305-1308.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

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