No abstract

The 47-kbp plasmid pGFT1 from Salmonella entericasubsp. enterica serovar Dublin mediated tetracycline resistance via a tet(A) gene located on an integrated copy of a Tn1721-analogous transposon. The integration site of the transposon was located within the reading frame of afip gene. Plasmid pGFT1 was shown to be conjugative and to be able to replicate and express tetracycline resistance inEscherichia coli.

During routine screening of Salmonella enterica subsp., S. enterica isolates of animal origin for plasmid-encoded tetracycline resistance, two tetracycline resistance plasmids, the 50 kbp plasmid pGFT3 of Salmonella choleraesuis and the 9.5 kbp plasmid pGFT4 of Salmonella typhimurium var. Copenhagen DT002, were detected. The respective tetracycline resistance genes (tet) were identified by hybridization and PCR analysis to belong to hybridization class A. Conjugation experiments identified plasmid pGFT3 as a conjugative plasmid. Molecular analysis of the tet(A) gene area and the flanking regions identified a complete Tn1721-like transposon on plasmid pGFT3 and a truncated Tn1721-like element on plasmid pGFT4. The complete Tn1721-like element was integrated into a transposase reading frame of a truncated Tn3 transposon also located on plasmid pGFT3. The truncated Tn1721-like element of plasmid pGFT4 lacked the entire transposase part. This Tn1721-relic was integrated in an unknown reading frame which on amino acid level showed homology to the Rop protein of Escherichia coli. A model for the deletion of the transposase part was developed on the basis of the sequences present at the termini of the truncated Tn1721-like element. ß

A total of 65 epidemiologically unrelated tetracycline‐resistant isolates of the six Salmonella enterica subsp. enterica (Salm.) serovars Dublin, Choleraesuis, Typhimurium, Enteritidis, Hadar and Saintpaul were investigated for the presence of tetracycline resistance genes. For this, specific gene probes of the tetracycline resistance genes (tet) of the hybridization classes A, B, C, D, E and G were constructed by cloning PCR‐amplified internal segments of the respective tet structural genes. These gene probes were sequenced and used in hybridization experiments with plasmid DNA or endonuclease digested whole cell DNA as targets. Only tet(A) genes were detected on plasmids in all Salm. Dublin isolates as well as in single isolates of Salm. Choleraesuis and Salm. Typhimurium. Genes of the hybridization classes B, C, D and G, but also in some cases those of class A, were located in the chromosomal DNA of the corresponding Salmonella isolates. Restriction fragment length polymorphisms (RFLPs) of tet gene carrying fragments were detected in chromosomally tetracycline‐resistant isolates. These RFLPs might represent valuable additional tools for the identification and characterization of tetracycline‐resistant Salmonella isolates.

A total of 25 epidemiologically unrelated Salmonella enterica subsp. enterica (S.) serovar Typhimurium DT009 isolates from various human and animal sources, the original S. Typhimurium DT009 Zoosaloral live vaccine strain and two Zoosaloral strains reisolated from vaccinated chickens were investigated by various molecular typing methods (I) to determine the most suitable method or combination of methods for the differentiation of DT009 field isolates and (II) to investigate which molecular methods are suitable to differentiate the Zoosaloral live vaccine strain from field isolates of the same phage type. Based on the results of plasmid profile analysis, IS200 typing and macrorestriction analysis with XbaI, SpeI and BlnI, the 28 S. Typhimurium DT009 isolates were assigned to 16 different genomic groups, one of which was exclusively represented by the original and the reisolated Zoosaloral strains. IS200 typing was the most discriminatory single method for the differentiation of the DT009 isolates followed by plasmid profile analysis and BlnI-macrorestriction analysis. The Zoosaloral vaccine strain differed from the DT009 field isolates by its unique HindIII-fragment pattern of the virulence plasmid and by its unique SpeImacrorestriction pattern. z