Response of Pseudomonas strains PAO1 and FRD1 to 10 mM added calcium in planktonic and biofilm conditions.. Pseudomonas aeruginosa

PAO1 and FRD1 were cultured planktonically and in biofilms with 10 mM calcium and no added calcium. The transcriptional response to calcium addition was determined for PAO1 cultured planktonically, for FRD1 cultured planktonically, for PAO1 cutured in biofilms, and for FRD1 cultured in biofilms. Pseudomonas aeruginosa is an opportunistic human pathogen that causes severe, life threatening infections in patients with cystic fibrosis (CF), endocarditis, wounds, or with artificial implants. During CF pulmonary infections, P. aeruginosa often encounters environments where the levels of calcium (Ca2+) are elevated. Previously, we showed that P. aeruginosa responds to externally added Ca2+ through enhanced biofilm formation, increased production of several secreted virulence factors, and by developing a transient increase in the intracellular Ca2+ followed by its removal to the basal sub-micromolar level. However, the molecular mechanisms responsible for regulating Ca2+-induced virulence factor production and Ca2+ homeostasis are not known. Here, we characterized the genome-wide transcriptional response of P. aeruginosa strains PAO1 and FRD1 to elevated [Ca2+] in both planktonic cultures and in biofilms. Among the genes induced by CaCl2 in PAO1 was an operon containing the two-component regulator PA2656-PA2657 (here called carS and carR), while the closely related two-component regulators, phoPQ and pmrAB were repressed by CaCl2 addition. To identify the regulatory targets of CarSR, we constructed a deletion mutant of carR, and performed transcriptome analysis of the mutant strain at low and high [Ca2+]. Among the genes regulated by CarSR in response to CaCl2 are the predicted periplasmic OB-fold protein, PA0320 and the inner membrane-anchored five-bladed -propeller protein, PA0327. Mutations in both PA0320 and PA0327 affected Ca2+ homeostasis, reducing the ability of P. aeruginosa to export excess Ca2+. In addition, a mutation in PA0327 had a pleotrophic effect in a Ca2+-dependent manner, altering swarming motility, pyocyanin production, and tobramycin sensitivity. Overall, the results indicate that the two-component system CarSR is responsible for sensing high levels of external Ca2+, and responding through its regulatory targets that modulate Ca2+ homeostasis, surface-associated motility, and production of the virulence factor, pyocyanin. Overall design: Two-condition experiment: 0 mM calcium vs. 10 mM calcium. For each of the four biological sample types, cells were grown with and without 10 mM added calcium, and co-hybridized to two color arrays. Dye swaps were performed on an additional array. Two independent biological replicates, grown on different days, were also co-hybridized on two additional arrays (dye swap). In this manner, four biological replicates were co-hybridized to four arrays, for four conditions, resulting in 16 two-color arrays deposited. Each array contained one biological growth condition at 0 at 10 mM calcium.