Pseudomonas fluorescens is a common Gram-negative, rod-shaped bacterium. It
belongs to the Pseudomonas genus; 16S rRNA analysis has placed P. fluorescens
in the P. fluorescens group within the genus, to which it lends its name.
General characteristics P. fluorescens has multiple flagella. It
has an extremely versatile metabolism, and can be found in the soil and in
water. It is an obligate aerobe, but certain strains are capable of using
nitrate instead of oxygen as a final electron acceptor during cellular
respiration. Optimal temperatures for growth of
Pseudomonas fluorescens are 25-30 degrees Celsius. It tests positive for
the oxidase test. Pseudomonas fluorescens is also a nonsaccharolytic
bacteria. Heat-stable lipases and proteases are
produced by Pseudomonas fluorescens and other similar pseudomonads. These
enzymes cause milk to spoil, by causing bitterness, casein breakdown, and
ropiness due to production of slime and coagulation of proteins.
= The name= The word Pseudomonas means 'false unit',
being derived from the Greek words pseudo and monas. The word was used
early in the history of microbiology to refer to germs. The name 'fluorescens'
refers to the microbe's secretion of a soluble fluorescent pigment called
pyoverdin, which is a type of siderophore.
= Genome sequencing projects= The genomes of P. fluorescens strains
SBW25, Pf-5 and PfO-1 have been sequenced.
Biocontrol properties Some P. fluorescens strains present
biocontrol properties, protecting the roots of some plant species against
parasitic fungi such as Fusarium or Pythium, as well as some phytophagous
nematodes. It is not clear exactly how the
plant-growth promoting properties of P. fluorescens are achieved; theories
include: the bacteria might induce systemic
resistance in the host plant, so it can better resist attack by a true pathogen
the bacteria might outcompete other soil microbes, e.g., by siderophores, giving
a competitive advantage at scavenging for iron
the bacteria might produce compounds antagonistic to other soil microbes,
such as phenazine-type antibiotics or hydrogen cyanide
To be specific, certain P. fluorescens isolates produce the secondary
metabolite 2,4-diacetylphloroglucinol, the compound found to be responsible for
antiphytopathogenic and biocontrol properties in these strains. The phl
gene cluster encodes factors for 2,4-DAPG biosynthesis, regulation,
export, and degradation. Eight genes, phlHGFACBDE, are annotated in this
cluster and conserved organizationally in 2,4-DAPG-producing strains of P.
fluorescens. Of these genes, phlD encodes a type III polyketide synthase,
representing the key biosynthetic factor for 2,4-DAPG production. PhlD shows
similarity to plant chalcone synthases and has been theorized to originate from
horizontal gene transfer. But phylogenetic and genomic analysis has
revealed that the entire phl gene cluster is ancestral to P. fluorescens,
many strains have lost the capacity, and it exists on different genomic regions
among strains. There is experimental evidence to
support all of these theories, in certain conditions; a good review of the
topic is written by Haas and Defago. Several strains of P. fluorescens, such
as Pf-5 and JL3985, have developed a natural resistance to ampicillin and
streptomycin. These antibiotics are regularly used in biological research as
a selective pressure tool to promote plasmid expression.
The strain referred to as Pf-CL145A has proved itself a promising solution for
the control of invasive zebra mussels and quagga mussels. This bacterial
strain is an environmental isolate capable of killing >90% of these mussels
by intoxication, as a result of natural product(s) associated with their cell
walls, and with dead Pf-145A cells killing the mussels equally as well as
live cells. Following ingestion of the bacterial cells mussel death occurs
following lysis and necrosis of the digestive gland and sloughing of stomach
epithelium. Research to date indicates very high specificity to zebra and
quagga mussels, with low risk of non-target impact. Pf-CL145A has now
been commercialized under the product name Zequanox, with dead bacterial cells
as its active ingredient. Medical Properties
By culturing Pseudomonas fluorescens, Mupirocin can be produced, which has
been found to be useful in treating skin, ear, and eye disorders. Mupirocin
free acid and its salts and esters are agents currently used in creams,
ointments, and sprays as a treatment of Methicillin-resistant Staphylococcus
aureus infection. P. fluorescens demonstrates hemolytic
activity and, as a result, has been known to infect blood transfusions.
It is also used in milk to make yogurt. United States Patents: 6489358, 4873012,
6156792 Disease
P. fluorescens is an unusual cause of disease in humans, and usually affects
patients with compromised immune systems. From 2004 to 2006, there was an
outbreak of P. fluorescens in the United States, involving 80 patients in six
states. The source of the infection was contaminated heparinized saline flushes
being used with cancer patients. Metabolism
Pseudomonas fluorescens produces phloroglucinol, phloroglucinol
carboxylic acid and diacetylphloroglucinol.
= Biodegradation capacities= 4-Hydroxyacetophenone monooxygenase is
an enzyme found in P. fluorescens that transform piceol, NADPH, H+ and O2 into
4-hydroxyphenyl acetate, NADP+ and H2O. Further Reading
Appanna, Varun P.; Auger, Christopher; Thomas, Sean C.; Omri, Abdelwahab.
"Fumarate metabolism and ATP production in Pseudomonas fluorescens exposed to
nitrosative stress". Antonie van Leeuwenhoek 106: 431–438.
doi:10.1007/s10482-014-0211-7. References
Further Reading Cabrefiga, J.; Frances, J.; Montesinos,
E.; Bonaterra, A.. "Improvement of a dry formulation of Pseudomonas fluorescens
EPS62e for fire blight disease biocontrol by combination of culture
osmoadaptation with a freeze-drying lyoprotectant". Journal of Applied
Microbiology 117: 1122-1131. doi:10.1111/jam.12582. Retrieved 2
November 2014. External links
www.pseudomonas.com, the Pseudomonas genome database
Migula's Systematic Bacteriology UV fluorescent Pseudomonas video