Inside the cellular control center of eukaryotes
Intracellular bacteria including major pathogens generally live within their host cell’s cytoplasm or in cytoplasmic vacuoles. However, some can invade more unusual intracellular niches, such as the eukaryotic nucleus. The nucleus provides a rich pool of nutrients and protection against host cytoplasmic defense mechanisms; bacteria located in this compartment can directly manipulate the host by interfering with nuclear processes.
Phylogenetically diverse intranuclear bacteria have been discovered in various protist, arthropod, marine invertebrate, and mammal hosts. Although targeting the same cellular compartment, they have apparently developed fundamentally different infection strategies.
We currently study the intranuclear amoeba symbiont Nucleicultrix amoebophila, a member of a novel clade of protist symbionts affiliated with the Rickettsiales and Rhodospirillales. The symbiont traffics within 6 hours post infection to the host nucleus; maximum infection levels are reached after 96 to 120 hours, at which time point the nucleus is pronouncedly enlarged and filled with bacteria. Yet, the presence of the symbiont has no apparent negative impact on the fitness of its original Hartmannella sp. host, and transmission of the symbionts occurs preferentially vertically upon host cell division.
Intranuclear symbiosis is an exceptional phenomenon, and amoebae represent an ideal model system to investigate evolution and underlying molecular mechanisms of these unique microbial associations.