Pseudomonas

The bacterial genus Pseudomonas includes plant pathogenic bacteria such as P. syringae (various pathovars), the opportunistic human pathogen P. aeruginosa, the ubiquitous soil bacterium P. putida, and some species that are known to cause problems in dairy products. However, the actions of this group of bacteria are mainly considered neutral to man. They are considered to be metabolically diverse agents of spoilage and degradation. In recent times, members of the Pseudomonas have been used as biocontrol agents.

Members of Pseudomonas are Gram-negative, aerobic (able to consume oxygen) rods. Most are flagellated so they can move around. Most produce a slime layer that cannot be phagocytosed, and which aids in the production of surface-colonising biofilms. P. fluorescens is easily recognised as it secretes large amounts of a fluorescent, yellow-green siderophore under iron-limited conditions. Many Pseudomonadaceae are capable of producing these fluorescent pigments called pyoverdines. Growth of Pseudomonads is usually accompanied by a "fruity" odor compared to Escherichia coli.

Pseudomonas have the ability to metabolise a variety of diverse nutrients. Combined with the ability to form biofilms, they are thus able to able to survive in a variety of unexpected places. For example, they have been found in areas where pharmaceuticals are prepared. A simple carbon source, such as soap residue or cap liner-adhesives is a suitable place for the Pseudomonads to thrive. Other unlikely places where they have been found include antiseptics such as quaternary ammonium compounds and bottled mineral water. Most Pseudomonas spp. are naturally resistant to penicillin and related beta-lactam antibiotics, but a number are sensitive to piperacillin, imipenem, tobramycin, or ciprofloxacin.

This ability to thrive in harsh conditions is a result of their hardy cell wall that contains porins. Their resistance to most antibiotics is attributed to efflux pumps called ABC transporters, which pump out some antibiotics before they are able to act.

The Pseudomonads can also grow in the human ear canal causing pain and a discharge. It can be treated with Ciprofloxacin and Gentisone HC ear drops though this treatment is not recommended for children.

Certain strains of fluorescent pseudomonas are important biological components of agricultural soils that are suppressive to diseases caused by pathogenic fungi on crop plants. The biocontrol abilities of such strains depend essentially on aggressive root colonization, induction of systemic resistance in the plant, and the production of diffusible or volatile antifungal antibiotics. Evidence that these compounds are produced in situ is based on their chemical extraction from the rhizosphere and on the expression of antibiotic biosynthetic genes in the producer strains colonizing plant roots. Well-characterized antibiotics with biocontrol properties include phenazines, 2,4-diacetylphloroglucinol, pyoluteorin, pyrrolnitrin, lipopeptides, and hydrogen cyanide. In vitro, optimal production of these compounds occurs at high cell densities and during conditions of restricted growth.