Cold and copper exposure in Pseudomonas fluorescens BA3SM1

Environmental metal pollution is increasing as a result of many anthropogenic activities and can represent a serious threat to living organisms. In the marine environment, the tidal flat area is particularly exposed to metal pollution as well as being naturally subjected to wide variations in salinity, temperature, water turbulence, and light. Consequently, bacteria living in this specific environment have to acclimatize to numerous stresses by developing various resistance strategies making them promising agents for bioremediation.

 In a previous study by Poirier et al. (2013), a proteomic analysis of the response of the Pseudomonas fluorescens BA3SM1 strain to metal stress showed that it acclimatizes to metals by inducing defence mechanisms such as cell aggregation/biofilm formation, modification of envelope properties, decrease in metal uptake, metal export, protection against oxidative stress, metal sequestration, and over-synthesis of proteins inhibited by metal. Several studies have shown that the metal tolerance of a bacterium and its performance in bioremediation processes are influenced by physical factors such as temperature. Thus, it is essential that bacterial strains used in these processes retain their ability to remove pollutants even at low temperatures.

In this paper, Poirier et al. (2014) set out to study the proteomic response to cold stress in P. fluorescens BA3SM1 in order to identify resistance mechanisms developed by this strain at low temperatures; to assess the metal biosorption ability of the bacterium and to determine the effect of cold stress on metal tolerance and metal biosorption ability of this strain.

When P. fluorescens BA3SM1 was exposed to cold stress before growth at +20◦C in nutrient broth without metal (control with pre-cold stress), a decrease in the μ_max was observed compared to the control without pre-cold stress. This growth disturbance can be explained by a down-regulation of some proteins involved in cell division, as evidenced by proteomic data. When cold stress was applied before moderate metal stress, growth disturbances induced by metal, in comparison with respective controls, were reduced for Cd and Zn while they were pronounced for Cu. These results are in accordance with other studies showing the influence of temperature on bacterial metal resistance. Under cold stress, an increase in phosphomannomutase/phosphoglucomutase and sigma factor AlgU (also called σE) biosynthesis indicates an increase in alginate and LPS biosynthesis two major molecules for biofilm formation and metal sequestration.

Several studies have shown that cold stress typically induces an antioxidant response. Consistent with these observations, proteomic data revealed that several proteins involved in oxidative stress resistance were differentially expressed under cold stress. Metals are known to trigger reactive oxygen species production which can lead to cellular damage. Consequently, cold stress enables bacteria to be better armed to combat oxidative stress generated by metals.

For high metal concentrations, growth of pre-cold-stressed cells was more affected than that of non-pre-cold-stressed cells, compared to respective controls. Therefore, resistance mechanisms developed under cold stress prove inefficient to counteract effects induced by high metal concentrations. In this case, pre-cold stress probably becomes a handicap because it leads to multiple stresses in mesophilic bacteria.

The low Cu biosorption observed in this study seems to show that P. fluorescens BA3SM1 develops resistance strategies to keep Cu outside the cell and avoid its adsorption on cells. An extracellular Cu sequestration through the synthesis of specific proteins released in the environment could be proposed.

 Consequently, P. fluorescensBA3SM1 appears to be a promising agent for removing Cd and Zn in polluted environments since it allows an efficient biosorption of these two metals compared to other strains, especially during the lag phase and exponential growth phase even at low temperatures. It is able to develop various resistance strategies to maintain its metabolism under cold stress. Some of these strategies were also set up under metal stress. Pre-cold stress seems to be a good means to help this strain to combat toxicity of some metals at least to moderate concentrations. This property is very interesting as most bacteria in the temperate regions are mesophiles and relatively inactive in the winter. Moreover, its presence in marine sediment certainly permits a reduction both in metal bioavailability in this environment, and consequently in toxicity to other marine organisms. In this study, biosorption ability of P. fluorescens BA3SM1 was evaluated in a nutrient rich medium, probably improving the establishment of metal resistance mechanisms. Further experiments need to be carried out in lower nutrient levels to better assess the strain’s biosorption capabilities.

Poirier, I., Kuhn, L., Caplat, C., Hammann, P., & Bertrand, M. (2014). The effect of cold stress on the proteome of the marine bacterium Pseudomonas fluorescens BA3SM1 and its ability to cope with metal excess. Aquatic Toxicology.

http://www.sciencedirect.com/science/article/pii/S0166445X14002951