Objectives: In a previous study with a Salmonella typhimurium strain containing cloned ampC-ampR from Enterobacter cloacae, it was suggested that ampC expression must be kept at low levels by AmpR to maintain normal growth and virulent phenotype. The purpose of this study was to determine whether findings obtained with a laboratory model can be extended to a virulent clinical isolate of S. typhimurium expressing the plasmid-encoded blaCMY-7. Methods: Disc induction assays were carried out to investigate inducibility of blaCMY-7. Primer extension and sequence analyses were carried out to map the transcriptional start site of blaCMY-7 and determine the relative expression. Growth and invasion potential of Salmonella strains were monitored by optical density, viable counts and cell invasion assays. Results: Sequence analysis confirmed the absence of ampR upstream of blaCMY-7 therefore confirming the negative results observed using the disc induction assay. Primer extension analysis mapped the start site of blaCMY-7 transcription within an ISEcp1-like element. The relative expression of blaCMY-7 was ∼ 965-fold higher than the expression of a wild-type Citrobacter freundii chromosomal ampC and ∼4.1-fold higher than ampC expression from a derepressed mutant of C. freundii. Growth and the capacity to invade mammalian cells were not compromised for either the clinical isolate or the S. typhimurium transconjugant containing blaCMY-7. However, a Salmonella transformant containing blaCMY-7 exhibited a compromised phenotype with respect to growth and invasion of mammalian cells. Conclusion: These findings indicate that the biological cost of high-level AmpC production can be compensated by plasmid-encoded factors and not by regulating ampC expression.
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