Photobacterium phosphoreum

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A Microbial Biorealm page on the genus Photobacterium phosphoreum

Classification

Higher order taxa

No rank: Cellular Organisms

Superkingdom: Bacteria

Phylum: Proteobacteria

Class: Gammaproteobacteria

Order: Vibrionales

Family: Vibrionaceae

Genus: Photobacterium

Species: Photobacterium phosphoreum

Species

NCBI: Taxonomy

Photobacterium phosphoreum

Description and significance

Photobacterium phosphoreum was first isolated from the aquatic environment in the late 1880’s by the Dutch microbiologist Martinus Beijerinck (1851-1931).[7] It is a Gammaproteobacteria which are Gram-negative, usually motile rods, are mesophilic and chemoorganotrophic, have falcultative fermentative metabolism and are found in aquatic habitats in association with eukaryotes.[7]

Photobacterium phosphoreum is one of many organisms that produce bioluminescence in marine organisms. P. phosphoreum is a light organ symbiont, living in the gut of the fish where metabolites are provided in exchange for bioluminescence, which is used for communication, prey attraction, and predator avoidance.[7] P. phosphoreum is typical of deep sea fishes, but can also be found in dim and dark versions.[2] The gut of the fish, where P. phosphoreum is cultivated, is connected to some light organ on the fish which can be controlled with shutter apparatuses.[2] P. phosphoreum is psychrotolerant and often thrives in low temperatures but can be inhibited at temperatures above 25 degrees Celsius.[6, 8] Bioluminescence has been strongly linked to cell density, and bacteria living freely in the ocean are not bioluminescent as they are in the light organs of the host organism.[10]

P. phosphoreum also acts as the most important spoilage bacterium of packed chilled fish fillets. It enhances growth in packed fish products and causes the spoiled, fishy flavor.[6]

Genome structure

The gene products of luxA, α-luciferase, are necessary for the light-emitting reaction of all known luminous bacteria.[1] Unlike the Vibrio lux system, P. phosphoreum has a new gene, luxF which is located in the lux operon between luxB and luxE. The functions of the luxA to E gene products are known to be involved in the luminescent pathway, therefore luxF likely has the same function.[10] The organization of the lux operon in P. phosphoreum is as follows: luxC luxD luxA luxB luxF luxE.[10]

In P. phosphoreum, luxA and luxB gene products are luciferase subunits and were shown to catalyze light emission in the presence of FMNH2, O2, and aldehyde. The luxC, luxD, and luxE gene products are fatty acid reductase subunits and are responsible for aldehyde biosynthesis. The new gene, luxF, was found to code for a new polypeptide of 26kDa.[9]

Cell structure and metabolism

Photobacterium phosphoreum is Gram-negative, motile rods, and is psychrophilic and chemoorganotrophic, can grow in anaerobic conditions, and emits a blue-green light.[7] Bioluminescence of Photobacterium phosphoreum is caused by an oxidation reaction. Reduced flavin mononucleotide (FMNH2), and long chain fatty acids are oxidized by molecular oxygen to flavin mononucleotide (FMN) and a corresponding fatty acid.[8, 10] The catalyst of this reaction is the enzyme luciferase, whose structure is heterodimeric and has molecular weights of 76,000 ± 4,000.[8] The light reaction:[5, 10]

FMNH2 + RCHO + O2 --> FMN + RCOOH + H2O + light (490-495 nm)

P. phosphoreum has demonstrated that its luciferase synthesis is constitutive.[8] Luminescence has, however, been shown to be maximal in deeper waters where oxygen concentrations are lower and growth is limited. At these depths, growth of the cells stop but since luciferase synthesis does not halt, the cells have higher luciferase concentration and thus display more intense luminescence.[8]

P. phosphoreum is biochemically advantageous in growth-limiting low oxygen concentrations with nonfermentable carbon sources. In these conditions, luminous bacteria use luciferase to transfer electrons, where other organisms have lost their ability to do so. By using luciferase, P. phosphoreum is able to reoxidize reduced coenzymes and other molecules for metabolism, giving it a major metabolic advantage.[3, 8]

P. phosphoreum has also been known to spoil packed, chilled salmon by using trimethylamine oxide (TMAO) as a terminal electron acceptor, facilitating its growth in packed fish products. Then, P. phosphoreum reduces TMAO to trimethylamine (TMA) which give the product its distinctive spoiled, fishy flavor.[6]

Ecology

Photobacterium phosphoreum is a common symbiont of deep sea fishes, unlike its Vibrio relatives that are commonly found in squids at shallow and medium depths.[6] There has been evidence that P. phosphoreum also has a close partnership with zooplankton, as it has been found attached to the external surfaces of zooplankton.[7]

Bological benefits are gained by both members in the symbiotic relationship between bacteria and host. P. phosphoreum colonizes the light organs of the host and plays a role via the emission of light in communication, prey attraction and predator avoidance.[7] The host has the ability to control the bioluminescence by using mechanisms such as shutters. In return, the host provides P. phosphoreum with nutrients, oxygen and a protected niche.[8]

It is believed that the spoilage of packed fish, such as salmon and cod, caused by P. phosphoreum is in part due to the bacterium’s ability to produce AHL’s (N-acylated homoserine lactones), which are communication molecules regulating bioluminescence. AHL’s were only found in nonbioluminescent strains of P. phosphoreum. This suggests that AHL may negatively regulate bioluminescence.[6] P. phosphoreum is able to remain viable on the external surfaces of migrating salmon by living under the protection of the fish’s slime.[1] The reduction of TMAO to TMA also contributes to the spoiled result of the fish’s exposure to P. phosphoreum.[6]

Pathology

Photobacterium phosphoreum has no known pathogenic activity. It is not known to cause disease in humans, plants, or animals.

Application to Biotechnology

Photobacterium phosphoreum has been used because of its reliable bioluminescence to measure toxicity in aquatic environments due to biodegradable water toxins. The responsibility, simple operation, and cost performances of bioluminescence makes it a reliable reporter for assessing aquatic samples containing toxicants such as pesticides, PCB’s, aromatic hydrocarbons, fuels, alkanes, alcohols, and heavy metals.[12] A Biochemical Oxygen Demand (BOD) system uses a bio-chip which has immobilized Photobacterium phosphoreum in micrometer-order holes.[12] This acrylic chip immobilizes P. phosphoreum with 3% or 15% sodium alginate gel.[12] Steady bioluminescence can be observed on the chip in the presence of BOD standard solution, showing that it can achieve on-site BOD detection.[12] Luminescence reduction due to physiological responses of P. phosphoreum to toxic nutrient substances is used as a reporter signal for detection and measurement of analyte.[12] BOD is widely used as an environmental indicator in wastewater treatment processes.[12] It detects the degree of pollution due to biodegradable substances in aquatic environments.[12] Traditional methods required 5 days to determine the BOD value, however the Photobacterium phosphoreum immobilized chip can produce rapid and reproducible BOD measurements.[12]

Current Research

Bioluminescent monitoring of detoxification processes

Bioluminescent assay systems are used to evaluate the detoxifying effect of humic substances on quinones (solutions of organic oxidizers).[11] Photobacterium phosphoreum and FMN-oxidoreductase-luciferase enzyme system isolated from P. phosphoreum were used as assay systems.[11] Toxicity was measured in the presence and absence of humic substances.[11] The assay system that uses P. phosphoreum” provided insight into the process of detoxification in quinone solutions.[11]

Bacterial flora of vacuum-packed cold-smoked salmon

This research includes the investigation by two different sampling strategies of the indigenous flora of freshly chilled cold-smoked salmon, at 7 degrees Celsius in vacuum packaging and after storage.[13] Identification of the bacteria were performing using a 16s rRNA sequencing of isolated bacteria and of bacterial DNA from tissue extract.[13] Originally, the flora consisted of four types of bacteria, but after storage in 7 degrees Celsius for 19 days, Lactobacillus and Photobacterium dominated.[13] Photobacterium phosphoreum was found on both freshly processed and on stored salmon.[13] Dangerously, ten percent of the bacterial flora multiplying on the salmon was unknown species.[13] The bacterial flora of fresh seafood, especially cold-packed salmon in this case, is a significant health issue that should be addressed in public health assessments.[13]

A bioluminescent signal system: detection of chemical toxicants in water

Prototype technology of a bioluminescent signal system (BSS) uses Photobacterium phosphoreum and three enzymatic bioluminescence systems to detect and signal the presence of toxicants in water systems.[14] The effects of some pesticides on the four systems have been evaluated and the maximum permissible concentration of each pesticide has been determined.[14] Sensitivities of the triple-enzyme systems are similar to those of luminous bacteria.[14] The results can then be used to construct an alarm-test bioluminescence system for detecting chemical toxicants in water by using Photobacterium phosphoreum or other intact bacteria or enzyme systems.[14] The ability to test for toxicants in water would be important in water recycling or purification processes to alleviate water shortages.[15]

The design of a detection system for biochemical luminescence

In this research project, a complementary metal oxide semiconductor (CMOS) chip was designed and with accompanied accessories hoped to detect and quantify biochemical luminescence.[16] The semiconductor chip was manufactured through a standard process, and a current mirror was designed in integrated circuit (IC) to amplify the signal current that is caused by chemiluminscence.[16] Their results illustrated that the combination of specifically designed CMOS IC and commercially available electronic devices established a simple and useful bioanalytical tool for detecting and quantifying bioluminescence.[16]