Protein as chemical cue: non-nutritional growth enhancement by exogenous protein in Pseudomonas putida KT2440

Abstract

Research pertaining to microbe-microbe and microbe-plant interactions has been largely limited to small molecules like quorum sensing chemicals. However, a few recent reports have indicated the role of complex molecules like proteins and polysaccharides in microbial communication. Here we demonstrate that exogenous proteins present in culture media can considerably accelerate the growth of Pseudomonas putida KT2440, even when such proteins are not internalized by the cells. The growth enhancement is observed when the exogenous protein is not used as a source of carbon or nitrogen. The data show non-specific nature of the protein inducing growth; growth enhancement was observed irrespective of the protein type. It is shown that growth enhancement is mediated via increased siderophore secretion in response to the exogenous protein, leading to better iron uptake. We highlight the ecological significance of the observation and hypothesize that exogenous proteins serve as chemical cues in the case of P.putida and are perceived as indicator of the presence of competitors in the environment. It is argued that enhanced siderophore secretion in response to exogenous protein helps P.putida establish numerical superiority over competitors by way of enhanced iron assimilation and quicker utilization of aromatic substrates.

Figure 1. Effect of BSA on growth of P.putida.

a) Effect of BSAaddition (0.5%) on growth of P.putida in complete media as well as in media devoid of either carbon source (glucose) or nitrogen source (ammonia). b) Residual concentration of BSA in supernatant at different time intervals. Concentration was determined using HPLC (mean±SD). c) HPLC chromatogram overlay showing identical peak area of BSA at different time intervals (BSA concentration 0.05% and 0.5%, w/v).

Figure 2. Influence of different concentrations of BSA on growth rate of P.putida (mean±SD).

Figure 3. Effect of exogenous proteins (extracted from granular sludge) on growth of P.putida (mean±SD).

Figure 4. Effect of BSA on siderophore secretion by P.putida.

a) Per capita siderophore secretion by P.putida in presence or absence of BSA. b) Influence of increasing concentration of BSA on siderophore secretion by P.putida. c) Maximum siderophore secretion in response to increasing concentration of BSA (mean±SD).

Figure 5. BSA accelerates aromatic substrate utilization by inducing siderophore secretion.

a) Influence of BSA on various aromatic substrate utilization (BA: benzyl Alcohol, Benzo; Benzoate, 3MB; 3-Methyl benzoate). b) Influence of exogenously added BSA and Fe⁺³ on growth of mutant (P.putida, ΔPpsD) on benzyl alcohol as sole carbon source.

Figure 6. Influence of model proteases on a) growth of P.putida and b) Siderophore (pyoverdine) secretion by P.putida.