||Caption: Soil bacterium, Pseudomonas fluorescens and the soil fungus, Pythium ultimum. Shown here are the rod-shaped bacteria of Pseudomonas fluorescens (nonpathogenic strain) and the filamentous hyphae of Pythium ultimum (vegetative stage, pathogenic strain). P. fluorescens can suppress plant diseases caused by the fungus Pythium spp. Pseudomonas fluorescens is Gram-negative, aerobic, rod prokaryote that encompasses a group of common pseudomonads, which are nonpathogenic saprophytes that colonize soil, water and plant surfaces. A number of strains of P. fluorescens suppress plant diseases by protecting the seeds and roots from fungal infection. This disease protection is a result of the bacteria producing a number of secondary metabolites including antibiotics, siderophores and hydrogen cyanide that inhibit fungal growth. Competitive exclusion of pathogens as the result of rapid colonization of the rhizosphere may also be an important factor in disease control. P. fluorescens strains produce a soluble, greenish fluorescent pigment (yellow-green siderophores), under conditions of low iron availability. P. fluorescens is primarily an obligate aerobe, however some strains can utilize nitrate (NO3) as an electron acceptor in place of oxygen (O2). It is motile by means of multiple polar flagella. Genetic techniques such as conjugation, transposition, and gene replacement are well established for this soil bacterium. P. fluorescens is used in bioremediation applications due to various strains have the inherent metabolic diversity to partially or completely degrade pollutants such as styrene, TNT and polycyclic aromatic hydrocarbons. Pythium ultimum is a plant pathogenic fungus with a wide host range and worldwide distribution. P. ultimum is a ubiquitous soil borne pathogen which causes damping-off and root rot in plants. Almost all plants are susceptible to Pythium root rot. Root tips, important in taking up nutrients and water, are attacked and killed first. Pythium spp. also can rot the base of cuttings. P. ultimum (and two associated species, Pythium irregulare and Pythium aphanidermatum) are often found in field soil, sand, pond / stream water, and dead roots of previous crops. Pythium spp. can cause severe root rot because it has few competitors to check its activity.