Batrachochytrium dendrobatidis
Encyclopedia
Batrachochytrium dendrobatidis (or Bd for short) is a chytrid fungus that causes the disease
chytridiomycosis
. In the decade after it was first discovered in amphibians in 1998, the disease devastated amphibian populations
around the world, in a global decline towards multiple extinctions, part of the Holocene extinction.
Some amphibian species appear to have an innate capacity to withstand chytridiomycosis infection. Even within species that generally succumb, some populations survive, possibly demonstrating that these traits or allele
s of species are being subjected to evolutionary selection
. Another explanation for such occurrences, explained below, could be that some forms of the fungus are not pathogenic.
words batracho (frog) and chytr (earthen pot), while the specific epithet is derived from the genus of frogs from which the original confirmation of pathogenicity was made (Dendrobates
).
analysis of the ssu
-rDNA
has corroborated the view, with the closest match to Chytridium confervae
.
) recovered from their infections when incubated at a temperature of 37°C.
ized skin of amphibian
s. The fungus in the epidermis has a thallus bearing a network of rhizoid
s and smooth-walled, roughly spherical, inoperculate (without an operculum
) sporangia. Each sporangium produces a single tube to discharge spores.
(19-20 µm long), and possess a core area of ribosome
s often with membrane-bound spheres of ribosomes within the main ribosomal mass. A small spur
has been observed, located at the posterior of the cell body, adjacent to the flagellum, but this may be an artifact in the formalin-fixed specimens. The core area of ribosomes is surrounded by a single cisterna
of endoplasmic reticulum
, two to three mitochondria, and an extensive microbody
–lipid globule complex. The microbodies closely appose and almost surround four to six lipid globules (three anterior and one to three laterally), some of which appear bound by a cisterna. Some zoospores appear to contain more lipid globules (this may have been a result of a plane-of-sectioning effect, because the globules were often lobed in the zoospores examined). A rumposome has not been observed.
lies adjacent to the kinetosome. Nine interconnected props attach the kinetosome to the plasmalemma, and a terminal plate is present in the transitional zone. An inner ring-like structure attached to the tubules of the flagellar doublets within the transitional zone has been observed in transverse section. No roots associated with the kinetosome have been observed. In many zoospores, the nucleus lies partially within the aggregation of ribosomes and was invariably situated laterally. Small vacuoles and a Golgi
body with stacked cisternae occurred within the cytoplasm outside the ribosomal area. Mitochondria, which often contain a small number of ribosomes, are densely staining with discoidal cristae.
released from the zoosporangium. The zoospores are known be active only for a short period of time, and can travel short distances of one to two centimeters. However, the zoospores are capable of chemotaxis
, and can move towards a variety of molecules that are present on the amphibian surface, such as sugars, protein
s and amino acid
s. Bd also contains a variety of proteolytic enzymes and esterase
s that help it digest amphibian cells and use amphibian skin as a nutrient source. Once the zoospore reaches its host, it forms a cyst underneath the surface of the skin, and initiates the reproductive portion of its life cycle. The encysted zoospores develop into zoosporangia, which may produce more zoospores that can reinfect the host, or be released into the surrounding aquatic environment. The amphibians infected with these zoospores are shown to die from cardiac arrest.
and subsequently spread to other parts of the world by trade in African clawed frog
s (Xenopus laevis). In this study, 697 archived specimens of three species of Xenopus, previously collected from 1879 to 1999 in southern Africa were examined. The earliest case of chytridiomycosis was found in a X. laevis specimen from 1938. The study also suggests that chytridiomycosis had been a stable infection in southern Africa from 23 years prior to finding any infected outside of Africa.
Bullfrogs (Rana catesbiana), also widely distributed, are also thought to be carriers of the disease due to their inherent low susceptibility to Bd infection. The bullfrog often escapes captivity and can establish feral
populations where it may introduce the disease to new areas. It has also been shown that Bd can survive and grow in moist soil and on bird feathers, suggesting that Bd may also be spread in the environment by birds and transportation of soils.
Infections have been linked to mass mortalities of amphibians in North America
, South America
, Central America
, Europe
and Australia
. Bd has been implicated in the extinction of the sharp-snouted day frog (Taudactylus acutirostris) in Australia.
A wide variety of amphibian hosts have been identified as being susceptible to infection by Bd, including wood frogs (Rana sylvatica), the mountain yellow-legged frog (Rana muscosa) the southern two-lined salamander (Eurycea cirrigera), San Marcos Salamander (Eurycea nana
) Texas Salamander (Eurycea neotenes
) Blanco River Springs Salamander (Eurycea pterophila) Barton Springs Salamander (Eurycea sosorum) Jollyville Plateau Salamander (Eurycea tonkawae) Ambystoma jeffersonianum, the western chorus frog (Pseudacris triseriata), the southern cricket frog (Acris gryllus
), the eastern spadefoot toad (Scaphiopus holbrooki), the southern leopard frog (Rana sphenocephala), the Rio Grande Leopard frog (Lithobates berlandieri), and the Sardinian newt (Euproctus platycephalus).
s of two Bd isolates were sequenced
, and scientists have begun using this genetic information to help understand the molecular basis of the Bd life cycle and amphibian pathogenicity. Analysis of global gene expression
using whole-genome arrays has revealed that greater than 55% of the approximately 9000 genes in the Bd genome undergo differential expression between the sessile sporangium stage and the infectious zoospore stage. Expression of a variety of metalloproteases (enzymes that can break down keratin-containing tissue, like amphibian tissue) is believed to contribute to pathogenicity by enabling cutaneous infection.
compounds, such as indole-3-carboxaldehyde and violacein, that inhibit the growth of Bd even at low concentrations. Similarly, the bacterium Lysobacter gummosus found on the red-backed salamander (Plethodon cinereus), produces the compound 2,4-diacetylphloroglucinol that is inhibitory to the growth of Bd.
Understanding the interactions of microbial communities present on amphibians’ skin with fungal species in the environment can reveal why certain amphibians, such as the frog Rana muscosa, are susceptible to the fatal effects of Bd and why others, such as the salamander Hemidactylium scutatum, are able to coexist with the fungus. As mentioned before, the antifungal
bacterial species Janthinobacterium lividum, found on several amphibian species, has been shown to prevent the effects of the pathogen even when added to another amphibian that lacks the bacteria (Bd-susceptible amphibian species). Interactions between cutaneous microbiota and Bd can be altered to favor the resistance of the disease, as seen in past lab studies concerning the addition of the violacein-producing bacteria J. lividum to amphibians that lacked sufficient violacein, allowing them to inhibit infection. Although the exact concentration of violacein (antifungal
metabolite produced by J. lividum) needed to inhibit the effects of Bd is not fully confirmed, violacein concentration can determine whether or not an amphibian will experience morbidity (or mortality) caused by the chytrid fungus Bd. The frog Rana muscosa, for example, has been found to have very low concentrations of violacein on its skin, yet the concentration is so small that it is unable to facilitate increased survivability of the frog; furthermore, Janthinobacterium lividum has not been found to be present on the skin of Rana muscosa. This implies that the antifungal bacteria J. lividum (native to other amphibians' skin, such as Hemidactylium scutatum) is able to produce a sufficient amount of violacein to prevent infection by Bd and allow coexistence with the potentially deadly fungus.
Studies conducted by Dr. Reid Harris and colleagues of the Department of Biology of James Madison University in Virginia have shown that the addition of the anti-chytrid (antifungal
) bacteria Janthinobacterium lividum to the skin of Bd-susceptible amphibians (i.e. Rana muscosa juveniles) increases the concentration of the antifungal metabolite violacein, which in turn decreases the mortality rate due to infection by Bd and also increases survivability. The removal of resident skin bacteria of the amphibians precedes the application of Janthinobacterium lividum and exposure to Bd zoospores (in the majority of experiments that have been previously been conducted), which reduces bacterial species on the amphibians' skin and also reduces possible interactions between J. lividum and other species of bacteria present on the skin. This allows for a standard condition of the amphibians’ skin that can then be compared to the J. lividum treatment of an experiment, thereby yielding simpler and more attributable survival/inhibition results concerning the newly added bacterial species (J. lividum). To reiterate, the majority of research done in this area has been concerned with prevention by applying J. lividum to amphibians before infection (by Bd) and after removal of their original skin bacteria. However, little research has been conducted to see if the addition of Janthinobacterium lividum without initial removal of the amphibians’ cutaneous microbiota is still as effective against the pathogen. Further research is needed to explore conditions and treatments that will include the original cutaneous bacterial species of the amphibians (that is to say, excluding the bacterial removal procedures commonly done before applying the antifungal bacteria) that will determine whether or not the addition of J. lividum will still increase survivability by inhibiting the fungus even without the initial removal of the resident skin bacteria. This would allow for a more practical method of bioaugmentation when treating a Bd-susceptible amphibian population in nature.
A recent study has postulated that Daphnia magna
eats the spores of the fungus.
carbaryl
(a cholinesterase
inhibitor) increase susceptibility of foothill yellow-legged frogs (Rana boylii) to chytridomycosis. In particular, the skin peptide defenses were significantly reduced after exposure to cabaryl, suggesting that pesticides may inhibit this innate immune defense, and increase susceptibility to disease.
Disease
A disease is an abnormal condition affecting the body of an organism. It is often construed to be a medical condition associated with specific symptoms and signs. It may be caused by external factors, such as infectious disease, or it may be caused by internal dysfunctions, such as autoimmune...
chytridiomycosis
Chytridiomycosis
Chytridiomycosis is an infectious disease of amphibians, caused by the chytrid Batrachochytrium dendrobatidis, a non-hyphal zoosporic fungus. Chytridiomycosis has been linked to dramatic population declines or even extinctions of amphibian species in western North America, Central America, South...
. In the decade after it was first discovered in amphibians in 1998, the disease devastated amphibian populations
Decline in amphibian populations
Dramatic declines in amphibian populations, including population crashes and mass localized extinctions, have been noted since the 1980s from locations all over the world...
around the world, in a global decline towards multiple extinctions, part of the Holocene extinction.
Some amphibian species appear to have an innate capacity to withstand chytridiomycosis infection. Even within species that generally succumb, some populations survive, possibly demonstrating that these traits or allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s of species are being subjected to evolutionary selection
Selection
In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...
. Another explanation for such occurrences, explained below, could be that some forms of the fungus are not pathogenic.
Etymology
The generic name is derived from the GreekGreek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
words batracho (frog) and chytr (earthen pot), while the specific epithet is derived from the genus of frogs from which the original confirmation of pathogenicity was made (Dendrobates
Dendrobates
Dendrobates is a genus of poison dart frogs native to South America. It once contained all poison dart frogs; until recently, frogs such as Dendrobates pumilio and Dendrobates terribilis were scientifically valid names...
).
Systematics
B. dendrobatidis is a monotypic species of the genus Batrachochytrium. The initial classification of the pathogen as a Chytrid was based on zoospore ultrastructure. DNADNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
analysis of the ssu
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
-rDNA
Ribosomal DNA
Ribosomal DNA codes for ribosomal RNA. The ribosome is an intracellular macromolecule that produces proteins or polypeptide chains. The ribosome itself consists of a composite of proteins and RNA. As shown in the figure, rDNA consists of a tandem repeat of a unit segment, an operon, composed of...
has corroborated the view, with the closest match to Chytridium confervae
Chytridiales
Fungi of the order Chytridiales, like other members of its phylum, may either have a monocentric thallus or a polycentric rhizomycelium. When the ribosomal genes of members classified in this order were first examined using molecular techniques, it was discovered that the order contained some...
.
Physiology
B. dendrobatidis can grow within a wide temperature range (4-25°C), with optimal temperatures being between 17-25°C. The wide temperature range for growth, including the ability to survive at 4°C gives the fungus the ability to overwinter in its hosts, even where temperatures in the aquatic environments are low. The species does not grow well above temperatures of 25°C, and growth is halted above 28°C. Infected red-eyed treefrogs (Litoria chlorisLitoria chloris
Litoria chloris, also commonly known as the Red-eyed Tree Frog or Orange Eyed Tree Frog, is a species of tree frog native to eastern Australia; ranging from north of Sydney to Proserpine in mid-northern Queensland.-Description:...
) recovered from their infections when incubated at a temperature of 37°C.
Morphology
B. dendrobatidis infects the keratinKeratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...
ized skin of amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
s. The fungus in the epidermis has a thallus bearing a network of rhizoid
Rhizoid
Rhizoids are thread-like growths from the base or bottom of a plant, found mainly in lower groups such as algae, fungi, bryophytes and pteridophytes, that function like roots of higher plants ....
s and smooth-walled, roughly spherical, inoperculate (without an operculum
Operculum (botany)
An operculum, in botany, is a term generally used to describe a structure within a plant, moss, or fungus acting as a cap, flap, or lid. In plants, it may also be called a bud cap.Examples of structures identified as opercula include:...
) sporangia. Each sporangium produces a single tube to discharge spores.
Zoospore structure
Zoospores of B. dendrobatidis, which are typically 3-5 µm in size, have an elongate–ovoidal body with a single, posterior flagellumFlagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
(19-20 µm long), and possess a core area of ribosome
Ribosome
A ribosome is a component of cells that assembles the twenty specific amino acid molecules to form the particular protein molecule determined by the nucleotide sequence of an RNA molecule....
s often with membrane-bound spheres of ribosomes within the main ribosomal mass. A small spur
Spur
A spur is a metal tool designed to be worn in pairs on the heels of riding boots for the purpose of directing a horse to move forward or laterally while riding. It is usually used to refine the riding aids and to back up the natural aids . The spur is used in every equestrian discipline...
has been observed, located at the posterior of the cell body, adjacent to the flagellum, but this may be an artifact in the formalin-fixed specimens. The core area of ribosomes is surrounded by a single cisterna
Cisterna
A cisterna comprises a flattened membrane disk that makes up the Golgi apparatus. A typical Golgi has anywhere from 3 to 7 cisternae stacked upon each other like a stack of dinner plates, but there are usually around 6...
of endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...
, two to three mitochondria, and an extensive microbody
Microbody
A microbody is a cytoplasmic organelle of a more or less globular shape that comprises degradative enzymes bound within a single membrane. Microbodies are specialized as containers for metabolic activity.-Types:...
–lipid globule complex. The microbodies closely appose and almost surround four to six lipid globules (three anterior and one to three laterally), some of which appear bound by a cisterna. Some zoospores appear to contain more lipid globules (this may have been a result of a plane-of-sectioning effect, because the globules were often lobed in the zoospores examined). A rumposome has not been observed.
Flagellum structure
A nonfunctioning centrioleCentriole
A Centriole is a barrel-shaped cell structure found in most animal eukaryotic cells, though it is absent in higher plants and most fungi. The walls of each centriole are usually composed of nine triplets of microtubules...
lies adjacent to the kinetosome. Nine interconnected props attach the kinetosome to the plasmalemma, and a terminal plate is present in the transitional zone. An inner ring-like structure attached to the tubules of the flagellar doublets within the transitional zone has been observed in transverse section. No roots associated with the kinetosome have been observed. In many zoospores, the nucleus lies partially within the aggregation of ribosomes and was invariably situated laterally. Small vacuoles and a Golgi
Golgi
Golgi may refer to:*Camillo Golgi , Italian physician and scientist after which the following terms are named:**Golgi apparatus , an organelle in the eukaryotic cell...
body with stacked cisternae occurred within the cytoplasm outside the ribosomal area. Mitochondria, which often contain a small number of ribosomes, are densely staining with discoidal cristae.
Life cycle
Bd has two primary life stages - a sessile, reproductive zoosporangium and a motile, uniflagellated zoosporeZoospore
A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some algae, bacteria and fungi to propagate themselves.-Flagella:...
released from the zoosporangium. The zoospores are known be active only for a short period of time, and can travel short distances of one to two centimeters. However, the zoospores are capable of chemotaxis
Chemotaxis
Chemotaxis is the phenomenon in which somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemicals in their environment. This is important for bacteria to find food by swimming towards the highest concentration of food molecules,...
, and can move towards a variety of molecules that are present on the amphibian surface, such as sugars, protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s and amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...
s. Bd also contains a variety of proteolytic enzymes and esterase
Esterase
An esterase is a hydrolase enzyme that splits esters into an acid and an alcohol in a chemical reaction with water called hydrolysis.A wide range of different esterases exist that differ in their substrate specificity, their protein structure, and their biological function.- EC classification/list...
s that help it digest amphibian cells and use amphibian skin as a nutrient source. Once the zoospore reaches its host, it forms a cyst underneath the surface of the skin, and initiates the reproductive portion of its life cycle. The encysted zoospores develop into zoosporangia, which may produce more zoospores that can reinfect the host, or be released into the surrounding aquatic environment. The amphibians infected with these zoospores are shown to die from cardiac arrest.
Varying forms
Bd has occasionally been found in forms distinct from its traditional zoospore and sporangia stages. For example, before the 2003 European heatwave that decimated populations of the water frog Rana lessonae through chytridiomycosis, the fungus existed on the amphibians as spherical, unicellular organisms, confined to minute patches (80-120 micrometers across). These organisms, unknown at the time, were subsequently identified as Bd. Characteristics of the organisms were suggestive of encysted zoospores; they may have embodied a resting spore, a saprobe, or a parasitic form of the fungus that is conditionally non-pathogenic. Once the heatwave began, the organisms assumedly changed into the more familiar disease-causing zoospores. This suggests that some populations afflicted with Bd may be free of chytridiomycosis not because of some inherent immunity, but because environmental conditions have not altered the organism into its more common, pathogenic form.Habitat and relationship to amphibians
The fungus grows on amphibian skin and produces aquatic zoospores. It is widespread and ranges from lowland forests to cold mountain tops. It is sometimes a non-lethal parasite and possibly a saprophyte. The fungus is associated with host mortality in highlands or during winter, and becomes more pathogenic at lower temperatures.Disease prevalence
It has been suggested that Bd originated in AfricaAfrica
Africa is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
and subsequently spread to other parts of the world by trade in African clawed frog
African clawed frog
The African clawed frog is a species of South African aquatic frog of the genus Xenopus. Its name is derived from the three short claws on each hind foot, which it uses to tear apart its food...
s (Xenopus laevis). In this study, 697 archived specimens of three species of Xenopus, previously collected from 1879 to 1999 in southern Africa were examined. The earliest case of chytridiomycosis was found in a X. laevis specimen from 1938. The study also suggests that chytridiomycosis had been a stable infection in southern Africa from 23 years prior to finding any infected outside of Africa.
Bullfrogs (Rana catesbiana), also widely distributed, are also thought to be carriers of the disease due to their inherent low susceptibility to Bd infection. The bullfrog often escapes captivity and can establish feral
Feral
A feral organism is one that has changed from being domesticated to being wild or untamed. In the case of plants it is a movement from cultivated to uncultivated or controlled to volunteer. The introduction of feral animals or plants to their non-native regions, like any introduced species, may...
populations where it may introduce the disease to new areas. It has also been shown that Bd can survive and grow in moist soil and on bird feathers, suggesting that Bd may also be spread in the environment by birds and transportation of soils.
Infections have been linked to mass mortalities of amphibians in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
, South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, Central America
Central America
Central America is the central geographic region of the Americas. It is the southernmost, isthmian portion of the North American continent, which connects with South America on the southeast. When considered part of the unified continental model, it is considered a subcontinent...
, Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
and Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
. Bd has been implicated in the extinction of the sharp-snouted day frog (Taudactylus acutirostris) in Australia.
A wide variety of amphibian hosts have been identified as being susceptible to infection by Bd, including wood frogs (Rana sylvatica), the mountain yellow-legged frog (Rana muscosa) the southern two-lined salamander (Eurycea cirrigera), San Marcos Salamander (Eurycea nana
Eurycea nana
The San Marcos Salamander is a small species of aquatic, lungless salamander native to the United States. Endemic to Spring Lake and a small region of the headwaters of the San Marcos River near Aquarena Springs, in Hays County, Texas...
) Texas Salamander (Eurycea neotenes
Eurycea neotenes
Eurycea neotenes is a species of entirely aquatic, lungless salamander native to the United States. It is endemic to central Texas, near Helotes, in Bexar County.- Description :...
) Blanco River Springs Salamander (Eurycea pterophila) Barton Springs Salamander (Eurycea sosorum) Jollyville Plateau Salamander (Eurycea tonkawae) Ambystoma jeffersonianum, the western chorus frog (Pseudacris triseriata), the southern cricket frog (Acris gryllus
Acris gryllus
The Southern cricket frog is a small Hylid frog native to the Southeastern United States. It is very similar in appearance and habits to the Northern cricket frog, Acris crepitans, and was formerly conspecific...
), the eastern spadefoot toad (Scaphiopus holbrooki), the southern leopard frog (Rana sphenocephala), the Rio Grande Leopard frog (Lithobates berlandieri), and the Sardinian newt (Euproctus platycephalus).
Genomics
In 2008, the genomeGenome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
s of two Bd isolates were sequenced
DNA sequencing
DNA sequencing includes several methods and technologies that are used for determining the order of the nucleotide bases—adenine, guanine, cytosine, and thymine—in a molecule of DNA....
, and scientists have begun using this genetic information to help understand the molecular basis of the Bd life cycle and amphibian pathogenicity. Analysis of global gene expression
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...
using whole-genome arrays has revealed that greater than 55% of the approximately 9000 genes in the Bd genome undergo differential expression between the sessile sporangium stage and the infectious zoospore stage. Expression of a variety of metalloproteases (enzymes that can break down keratin-containing tissue, like amphibian tissue) is believed to contribute to pathogenicity by enabling cutaneous infection.
Immunity hypotheses
Due to the fungus' immense impact on amphibian populations, considerable research has been undertaken to devise methods to combat its proliferation. Among the most promising is the revelation that amphibians in colonies that survive the passage of the chytrid epidemic tend to carry higher levels of the bacterium Janthinobacterium lividum. This bacterium produces antifungalAntifungal
* Antifungal medication, a medication used to treat fungal infection s such as athlete's foot , ringworm, candidiasis, etc.* Antifungal protein, a protein family* an adjective referring to a fungicide compound...
compounds, such as indole-3-carboxaldehyde and violacein, that inhibit the growth of Bd even at low concentrations. Similarly, the bacterium Lysobacter gummosus found on the red-backed salamander (Plethodon cinereus), produces the compound 2,4-diacetylphloroglucinol that is inhibitory to the growth of Bd.
Understanding the interactions of microbial communities present on amphibians’ skin with fungal species in the environment can reveal why certain amphibians, such as the frog Rana muscosa, are susceptible to the fatal effects of Bd and why others, such as the salamander Hemidactylium scutatum, are able to coexist with the fungus. As mentioned before, the antifungal
Antifungal
* Antifungal medication, a medication used to treat fungal infection s such as athlete's foot , ringworm, candidiasis, etc.* Antifungal protein, a protein family* an adjective referring to a fungicide compound...
bacterial species Janthinobacterium lividum, found on several amphibian species, has been shown to prevent the effects of the pathogen even when added to another amphibian that lacks the bacteria (Bd-susceptible amphibian species). Interactions between cutaneous microbiota and Bd can be altered to favor the resistance of the disease, as seen in past lab studies concerning the addition of the violacein-producing bacteria J. lividum to amphibians that lacked sufficient violacein, allowing them to inhibit infection. Although the exact concentration of violacein (antifungal
Antifungal
* Antifungal medication, a medication used to treat fungal infection s such as athlete's foot , ringworm, candidiasis, etc.* Antifungal protein, a protein family* an adjective referring to a fungicide compound...
metabolite produced by J. lividum) needed to inhibit the effects of Bd is not fully confirmed, violacein concentration can determine whether or not an amphibian will experience morbidity (or mortality) caused by the chytrid fungus Bd. The frog Rana muscosa, for example, has been found to have very low concentrations of violacein on its skin, yet the concentration is so small that it is unable to facilitate increased survivability of the frog; furthermore, Janthinobacterium lividum has not been found to be present on the skin of Rana muscosa. This implies that the antifungal bacteria J. lividum (native to other amphibians' skin, such as Hemidactylium scutatum) is able to produce a sufficient amount of violacein to prevent infection by Bd and allow coexistence with the potentially deadly fungus.
Studies conducted by Dr. Reid Harris and colleagues of the Department of Biology of James Madison University in Virginia have shown that the addition of the anti-chytrid (antifungal
Antifungal
* Antifungal medication, a medication used to treat fungal infection s such as athlete's foot , ringworm, candidiasis, etc.* Antifungal protein, a protein family* an adjective referring to a fungicide compound...
) bacteria Janthinobacterium lividum to the skin of Bd-susceptible amphibians (i.e. Rana muscosa juveniles) increases the concentration of the antifungal metabolite violacein, which in turn decreases the mortality rate due to infection by Bd and also increases survivability. The removal of resident skin bacteria of the amphibians precedes the application of Janthinobacterium lividum and exposure to Bd zoospores (in the majority of experiments that have been previously been conducted), which reduces bacterial species on the amphibians' skin and also reduces possible interactions between J. lividum and other species of bacteria present on the skin. This allows for a standard condition of the amphibians’ skin that can then be compared to the J. lividum treatment of an experiment, thereby yielding simpler and more attributable survival/inhibition results concerning the newly added bacterial species (J. lividum). To reiterate, the majority of research done in this area has been concerned with prevention by applying J. lividum to amphibians before infection (by Bd) and after removal of their original skin bacteria. However, little research has been conducted to see if the addition of Janthinobacterium lividum without initial removal of the amphibians’ cutaneous microbiota is still as effective against the pathogen. Further research is needed to explore conditions and treatments that will include the original cutaneous bacterial species of the amphibians (that is to say, excluding the bacterial removal procedures commonly done before applying the antifungal bacteria) that will determine whether or not the addition of J. lividum will still increase survivability by inhibiting the fungus even without the initial removal of the resident skin bacteria. This would allow for a more practical method of bioaugmentation when treating a Bd-susceptible amphibian population in nature.
A recent study has postulated that Daphnia magna
Daphnia magna
Daphnia magna is a species of Daphnia which is native to northern and western North America. It is also widely distributed in Eurasia and in some regions of Africa.- Laboratory animal :...
eats the spores of the fungus.
Effects of pesticides
The hypothesis that pesticide use has contributed to declining amphibian populations has been suggested a several times in the literature. In 2007, this hypothesis was corroborated, as it was shown that sublethal exposure to the pesticidePesticide
Pesticides are substances or mixture of substances intended for preventing, destroying, repelling or mitigating any pest.A pesticide may be a chemical unicycle, biological agent , antimicrobial, disinfectant or device used against any pest...
carbaryl
Carbaryl
Carbaryl is a chemical in the carbamate family used chiefly as an insecticide. It is a white crystalline solid commonly sold under the brand name Sevin, a trademark of the Bayer Company. Union Carbide discovered carbaryl and introduced it commercially in 1958...
(a cholinesterase
Cholinesterase
In biochemistry, cholinesterase is a family of enzymes that catalyze the hydrolysis of the neurotransmitter acetylcholine into choline and acetic acid, a reaction necessary to allow a cholinergic neuron to return to its resting state after activation.-Types:...
inhibitor) increase susceptibility of foothill yellow-legged frogs (Rana boylii) to chytridomycosis. In particular, the skin peptide defenses were significantly reduced after exposure to cabaryl, suggesting that pesticides may inhibit this innate immune defense, and increase susceptibility to disease.
See also
- Decline in amphibian populations#Chytridiomycosis
- RanavirusRanavirusRanavirus is one of five genera of viruses within the family Iridoviridae, one of the five families of nucleocytoplasmic large DNA viruses . Ranavirus is the only genus within Iridoviridae that includes viruses that are infectious to amphibians and reptiles, and one of only three genera within...
External links
- Chytrid Fungi Online at University of AlabamaUniversity of AlabamaThe University of Alabama is a public coeducational university located in Tuscaloosa, Alabama, United States....
- Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America
- Emerging Infectious Diseases and Amphibian Population Declines
- Survival of Batrachochytrium dendrobatidis in Water: Quarantine and Disease Control Implications
- Index Fungorum