''Pasteuria ramosa'' is a gram-positive, endospore-forming bacterium in the

Bacillus ''Bacillus'' (Latin "stick") is a genus of Gram-positive, rod-shaped bacteria, a member of the phylum ''Bacillota'', with 266 named species. The term is also used to describe the shape (rod) of other so-shaped bacteria; and the plural ''Bacilli ...

Clostridia The Clostridia are a highly polyphyletic class of Bacillota, including '' Clostridium'' and other similar genera. They are distinguished from the Bacilli by lacking aerobic respiration. They are obligate anaerobes and oxygen is toxic to them. Sp ...

clade within

Bacillota The Bacillota (synonym Firmicutes) are a phylum of bacteria, most of which have gram-positive cell wall structure. The renaming of phyla such as Firmicutes in 2021 remains controversial among microbiologists, many of whom continue to use the earl ...

. It is an obligate pathogen of

cladocera The Diplostraca or Cladocera, commonly known as water fleas, are a superorder of small crustaceans that feed on microscopic chunks of organic matter (excluding some predatory forms). Over 1000 species have been recognised so far, with many more ...

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

s from the genus ''

Daphnia ''Daphnia'' is a genus of small planktonic crustaceans, in length. ''Daphnia'' are members of the order Anomopoda, and are one of the several small aquatic crustaceans commonly called water fleas because their saltatory swimming style resembl ...

''. Daphnia is an umbrella term for a number of small freshwater creatures including '' D. magna, P. ramosa'''s most popular host target''.'' Other hosts include D. pulex'', D. longispina'', '' D. dentifera'', and '' Moina rectirostris''. An established and widely used

coevolution In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well ...

ary model of host-pathogen interactions exists with ''P. ramosa'' and '' D. magna.

Growth and sporulation

''P. ramosa'' is an

obligate {{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym ''facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe, an organism that cannot survive without oxygen * Obligate anaerobe, an organism that ...

pathogen and it can only grow inside its host. Transmission between hosts takes place through the

endospore An endospore is a dormant, tough, and non-reproductive structure produced by some bacteria in the phylum Bacillota. The name "endospore" is suggestive of a spore or seed-like form (''endo'' means 'within'), but it is not a true spore (i.e., no ...

stage, and is strictly horizontal. These endospores are highly resistant to different environmental stresses, including freezing temperatures, and can remain in the environment for decades without any deleterious effects. The infection can be explained in 5 steps: (1.) Encounter (2.) Activation (3.) Attachment (4.) Proliferation and (5.) Termination.Duneau, D., Luijckx, P., Ben-Ami, F. ''et al.'' Resolving the infection process reveals striking differences in the contribution of environment, genetics and phylogeny to host-parasite interactions. ''BMC Biol'' 9, 11 (2011). https://doi.org/10.1186/1741-7007-9-11 The process starts when a ''Daphnia'' has ingested a spore of ''P. ramosa'' during filter feeding. The spore receives a signal to begin germination, and attaches to the host esophagus. The pathogen then enters the body cavity of the host by penetrating the esophagus wall. Once inside the body cavity, the bacterium begins to propagate in cauliflower like colonies. Propagation of spores is usually observed in the haemocoel and musculature. After the infection has spread throughout the host, the bacterium begins to sporulate. The spores are shed into the environment from the dead host and can remain in the sediment for decades while maintaining their infectivity. Additionally, these spores may be ingested by their immune hosts and pass through the gut of the ''Daphnia'' unharmed by any immune cells. This makes the spores very difficult to kill.

Pathogenicity

The infection success of ''P. ramosa'' depends on its ability to attach to the host esophagus and to spread into its body cavity where the propagation of the pathogen takes place. Propagation of the spores take place over a period of 10-20 days and ultimately leads to death of the infected host and the release of millions of created spores into the surrounding area. The attachment step of the infection depends on the genotypes of the host and the bacterium, meaning that only certain host genotypes can be infected by certain strains of the bacterium. Although the process through which the genotypic interactions occur is unclear, environmental factors, such as temperature, play a large role in the castration of Daphnia. Studies have shown that female ''Daphnia'' are sterilized at warmer temperatures (20-25°C), but still have the ability to reproduce at lower temperatures (10-15°C). This difference in temperatures can be observed in different seasons and can lead to a high amount of variability between Daphnia, a crucial part of its ability to coevolve with ''P. ramosa.'' During ''P. ramosa'' infection, the size of the Daphnia increases significantly. This phenomenon is known as pathogen-induced gigantism. In addition, the lifespan of the host is significantly reduced.

Coevolutionary model with ''Daphnia magna''

''P. ramosa'' has coevolved with its host ''Daphnia magna''. The mode of

in this system fits the model with negative frequency dependent selection where the rare genotype is favored since the more common host genotype is more likely to become the target of a specialized pathogen.

References

{{Taxonbar, from=Q23817021 Bacillales Pathogenic bacteria