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Phenotypic evidence for inducible multiple antimicrobial resistance in Salmonella choleraesuis
Robert J Tibbetts1, Tsang Long Lin, Ching Ching Wu
1Department of Veterinary Pathobiology, School of Veterinary Medicine, Purdue University, 47907, West Lafayette, IN, USA. wuc@purdue.edu
Abstract:
Multiple antimicrobial resistance (MAR) in Salmonella choleraesuis is becoming a major concern. It has been demonstrated that a MAR phenotype can be induced in Escherichia coli and other members of the Enterobacteriaceae by exposing the isolates to salicylates, various antimicrobials, or organic solvents used to combat and control bacterial infection. Therefore the purpose of the present study was to determine whether this marA-associated MAR-phenotype is inducible in S. choleraesuis. Isolates used in the present study were able to withstand toxic effects of the organic solvent cyclohexane naturally, or following exposure to the inducing compounds salicylate, tetracycline, or chloramphenicol. All isolates possessed fragments of marA with the predicted size of 408 bp when amplified using marA-specific primers by PCR. The resulting PCR products that were sequenced revealed that amplified S. choleraesuis marA was 99% and 85% homologous to the published Salmonella typhimurium and E. coli marA sequences respectively. Minimum inhibitory concentrations of tetracycline (P<0.08), chloramphenicol (P<0.001), rifampin (P<0.08), and nalidixic acid (P<0.001) against cyclohexane-tolerant mutants were significantly increased when compared with wild-type S. choleraesuis. Northern hybridization signals for both marA and acrB were increased in the induced isolates when compared to uninduced controls while soxS expression did not change between induced and uninduced cultures. The results suggest that marA is present in S. choleraesuis and a MAR-phenotype is inducible in S. choleraesuis presumably due to the overexpression of marA and acrB and not to the overexpression of soxS.
Insights
Multiple antimicrobial resistance (MAR) is a growing concern in Salmonella choleraesuis. This study found that the MAR phenotype is inducible in S. choleraesuis, likely due to marA and acrB gene overexpression.
Area of Science:
- Microbiology
- Genetics
- Molecular Biology
Background:
- Multiple antimicrobial resistance (MAR) in Salmonella choleraesuis is a significant public health concern.
- A MAR phenotype can be induced in other Enterobacteriaceae by exposure to specific compounds.
- The role of inducible MAR in S. choleraesuis remains to be fully elucidated.
Purpose of the Study:
- To investigate the inducibility of the marA-associated MAR phenotype in Salmonella choleraesuis.
- To determine if S. choleraesuis isolates can develop resistance upon exposure to inducing agents.
Main Methods:
- Exposure of S. choleraesuis isolates to cyclohexane, salicylate, tetracycline, or chloramphenicol.
- Polymerase chain reaction (PCR) amplification and sequencing of the marA gene.
- Determination of minimum inhibitory concentrations (MICs) for various antibiotics.
- Northern hybridization to assess gene expression levels of marA, acrB, and soxS.
Main Results:
- S. choleraesuis isolates demonstrated tolerance to cyclohexane, with or without prior exposure to inducers.
- PCR confirmed the presence of marA gene fragments homologous to Salmonella typhimurium and Escherichia coli.
- Induced isolates showed significantly increased MICs for tetracycline, chloramphenicol, rifampin, and nalidixic acid.
- Northern hybridization revealed increased expression of marA and acrB in induced isolates, while soxS expression remained unchanged.
Conclusions:
- The marA gene is present in S. choleraesuis.
- A multiple antimicrobial resistance (MAR) phenotype is inducible in S. choleraesuis.
- Inducible MAR in S. choleraesuis is likely mediated by the overexpression of marA and acrB, not soxS.