Nosocomial outbreak caused by Salmonella enterica serotype Livingstone producing CTX-M-27 extended-spectrum beta-lactamase in a neonatal unit in Sousse, Tunisia

Abstract

In this study, we report an outbreak of Salmonella enterica serotype Livingstone resistant to extended-spectrum cephalosporins that occurred in a neonatal ward of the maternity department of Farhat Hached Hospital, Sousse, Tunisia, in 2002. A total of 16 isolates were recovered from 16 babies hospitalized in the ward during the period 1 to 16 July. All these babies developed diarrhea, and three of them developed septicemia. All the isolates demonstrated resistance to ceftriaxone and ceftazidime due to the production of an extended-spectrum beta-lactamase (ESBL). The isolates were also resistant to aminoglycosides (kanamycin, tobramycin, netilmicin, gentamicin, and amikacin) and sulfamethoxazole-trimethoprim. DNA profiles were determined by pulsed-field gel electrophoresis using the XbaI and SpeI endonucleases and by ribotyping with PstI digestion. They yielded the same patterns, showing that the outbreak was caused by a single clone. The ESBL was identified as CTX-M-27 by sequencing of PCR products and by isoelectric focusing. The ESBL resistance was transferred by a 40-kb conjugative plasmid. The mobile insertion sequence ISEcp1 was found to be located upstream of bla(CTX-M-27) in the same position as that known for a bla(CTX-M-14) sequence. A new gene named dfrA21, encoding resistance to trimethoprim and carried by a 90-kb plasmid, was characterized. The dfrA21 gene was inserted as a single resistance cassette in a class I integron. The babies were treated with colistin, and all except two recovered. The outbreak came to an end when appropriate actions were taken: patient isolation, hand washing, and disinfection of the ward.

FIG. 1.

PFGE of XbaI-digested genomic DNA from S. enterica serotype Livingstone isolates. Lane M, S. enterica serotype Braenderup H9812 used as a molecular size marker (band sizes in kilobase pairs); lanes 1 to 15, CTX-M-27-producing outbreak isolates 1, 2, 3, 4, 7, 6, 8, 9, 10, 11, 12, 13, 14, 15, and 16, respectively (isolate 5 is not shown here); lane 16, comparison isolate Tun-1; lane 17, comparison isolate Tun-2; lane 18, comparison isolate Tun-3; lane 19, comparison isolate 01-8221; lane 20, comparison isolate 03-5725; lane 21, comparison meat isolate Tun-4; lane 22, comparison isolate 00-2311; lane 23, serotype Livingstone reference strain 336K.

FIG. 2.

Schematic representation showing the ribotypes obtained by automated ribotyping (RiboPrinter; Qualicon) of S. enterica serotype Livingstone outbreak isolates and comparison strains, using Taxotron software.

FIG. 3.

Alignment of the deduced amino acid sequences of the novel dihydrofolate reductase type A21 gene with those of dihydrofolate reductase type 12 (GenBank accession no. AY126944) and type 13 (GenBank accession no. Z50802). Dashes indicate amino acids identical to those of dihydrofolate reductase type A21.