Abstract
The two-component system TctD-TctE is important for regulating the uptake of tricarboxylic acids in Pseudomonas aeruginosa TctD-TctE accomplishes this through derepression of the gene opdH, which encodes a tricarboxylic acid-specific porin. Previous work from our lab revealed that TctD-TctE in P. aeruginosa also has a role in resistance to aminoglycoside antibiotics. The aim of this study was to further characterize the role of TctD-TctE in P. aeruginosa in the presence of citric acid. Here it was found that deletion of P. aeruginosa PA14 TctD-TctE (ΔtctED) resulted in a 4-fold decrease in the biofilm bactericidal concentrations of the aminoglycosides tobramycin and gentamicin when citric acid was present in nutrient media. Tobramycin accumulation assays demonstrated that deletion of TctD-TctE resulted in an increase in the amount of tobramycin retained in biofilm cells. The PA14 wild type responded to increasing concentrations of citric acid by producing less biofilm. In contrast, the amount of ΔtctED mutant biofilm formation remained constant or enhanced. Furthermore, the ΔtctED strain was incapable of growing on citric acid as a sole carbon source and was highly reduced in its ability to grow in the presence of citric acid even when an additional carbon source was available. Use of phenotypic and genetic microarrays found that this growth deficiency of the ΔtctED mutant is unique to citric acid and that multiple metabolic genes are dysregulated. This work demonstrates that TctD-TctE in P. aeruginosa has a role in biofilm development that is dependent on citric acid and that is separate from the previously characterized involvement in resistance to antibiotics.IMPORTANCE Nutrient availability is an important contributor to the ability of bacteria to establish successful infections in a host. Pseudomonas aeruginosa is an opportunistic pathogen in humans causing infections that are difficult to treat. In part, its success is attributable to a high degree of metabolic versatility. P. aeruginosa is able to sense and respond to varied and limited nutrient stress in the host environment. Two-component systems are important sensors-regulators of cellular responses to environmental stresses, such as those encountered in the host. This work demonstrates that the response by the two-component system TctD-TctE to the presence of citric acid has a role in biofilm formation, aminoglycoside susceptibility, and growth in P. aeruginosa.
Highlights
Pseudomonas aeruginosa is a member of the gammaproteobacteria and an opportunistic pathogen in humans
Interest in studying TctD-TctE originated from previous findings from our lab demonstrating that the loss of expression of these proteins in the ΔtctED deletion strain resulted in biofilms that were more susceptible to a subset of antibiotics, the aminoglycosides tobramycin and gentamicin [25]
We were able to build on previous findings and further characterize the aminoglycoside susceptibility of the ΔtctED deletion strain in the presence of citric acid
Summary
Pseudomonas aeruginosa is a member of the gammaproteobacteria and an opportunistic pathogen in humans. Salmonella enterica serovar Typhimurium, another pathogenic member of the gammaproteobacteria, contains an operon that encodes genes, named the tricarboxylic transporters (tct), which are responsible for the uptake of tricarboxylic carbon sources, such as citric acid [22]. Within this operon are two genes for a TCS: tctD, which encodes a transcriptional regulator, and tctE, which encodes a histidine sensor kinase [22, 23]. We demonstrate for the first time that TctD-TctE has a role in regulating biofilm formation in P. aeruginosa
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