Pseudomonas fluorescens is a common, rod-shaped, Gram-negative bacterium highly adaptable to various environments. It is distinguished by its versatile metabolism and its ability to thrive in cooler temperatures, a characteristic known as psychrotrophism. The species is frequently studied for its environmental roles, including interactions with plants and capacity for bioremediation. It produces a soluble fluorescent pigment called pyoverdin, which helps the bacterium scavenge iron from its surroundings.
Distribution in General Natural Environments
The primary habitat for P. fluorescens is the natural environment, where it is globally distributed across terrestrial and aquatic ecosystems. It is one of the most frequently isolated bacterial species from soil, existing both freely and in close association with organic matter. In soil, the bacterium plays a role in nutrient cycling by degrading a wide range of compounds. It is an obligate aerobe, generally requiring oxygen to grow, though some strains can utilize nitrate as an alternative electron acceptor when oxygen is scarce. The optimal growth temperature is typically between 25 and 30 degrees Celsius, but it remains active at temperatures as low as 4 degrees Celsius.
Beyond terrestrial habitats, the species is widely found in aquatic systems, including freshwater, marine environments, and groundwater. Its ability to form biofilms on submerged surfaces helps it persist in flowing water bodies. In aquaculture systems, P. fluorescens is a naturally occurring bacterium that can sometimes act as an opportunistic pathogen for fish, especially in environments with high organic loads.
Specialized Habitats Around Plant Roots
A highly concentrated location for P. fluorescens is the rhizosphere, the narrow zone of soil directly influenced by the secretions of plant roots. This habitat is considered a “hot-spot” of microbial activity due to the abundant release of carbon-rich compounds from the plant. Plants can release between 11 and 40 percent of their photosynthetic yield into this area in the form of root exudates.
These root exudates, which include low-molecular-weight compounds like amino acids, organic acids, and sugars, act as a nutritional feast that selectively recruits specific microbes. P. fluorescens exhibits chemotaxis, a directed movement toward these compounds, allowing it to colonize the root surface and surrounding soil at higher densities than in bulk soil.
The rhizosphere environment demands unique adaptive traits from the bacteria. For example, some root exudates contain compounds like choline and glycine betaine, which serve as osmoprotectants, helping the bacteria cope with water stress. This adaptation highlights the reciprocal relationship where the plant provides nutrients, and the bacterium often promotes plant growth or provides biocontrol against pathogens. Certain strains are known to produce the secondary metabolite 2,4-diacetylphloroglucinol (2,4-DAPG) near the roots, which suppresses plant diseases.
Presence in Human and Industrial Systems
The cold-tolerant nature of P. fluorescens allows it to colonize environments managed by humans, particularly those maintained under refrigeration. This psychrotrophism makes it a common spoilage organism in the food industry, especially for refrigerated meat and dairy. It can grow effectively at temperatures below 7 degrees Celsius, which are typical cold chain storage conditions.
In milk and dairy products, P. fluorescens is a primary spoilage agent, causing defects like off-flavors, discoloration, and premature coagulation. The spoilage is caused by the heat-stable enzymes it produces, specifically lipases and proteases. These enzymes remain active and degrade milk proteins and fats even after pasteurization, leading to quality issues in processed products.
The bacterium is also found in man-made water systems and can be an opportunistic colonizer in clinical environments. It forms biofilms on surfaces such as water pipes and medical devices, which can be difficult to remove and act as a persistent source of contamination. While generally considered non-pathogenic to healthy humans, P. fluorescens can cause infections, such as bacteremia, in immunocompromised individuals. These infections are often linked to the transfusion of contaminated blood products or the use of contaminated intravenous equipment.