Related Experiment Video
Updated: Aug 13, 2026
Visualizing Bacteria in Nematodes using Fluorescent Microscopy
Published on: October 19, 2012
From extracellular to intracellular: the establishment of a symbiosis
Abstract:
The colonization of host cells by modern symbionts is surveyed. The morphological distinction between extracellular and intracellular symbionts is not sharp, and the various kinds of association can be arranged in a graded series of increasing morphological integration of the symbiont into the host cell. Apart from some aggressive parasitic infections, the great majority of symbionts are enclosed by a host membrane in a vacuole. Those not enclosed in a host vacuole usually cannot be cultivated outside the cell. It is therefore surmised that encirclement by a vacuolar membrane would only disappear, if at all, in the later stages of the evolution of intracellular symbiosis. Recognition mechanisms between host and symbiont occur, but have been little studied. In some associations, recognition at surface contact occurs, and there is evidence for the involvement of lectins in certain cases. In other associations, recognition may occur wholly or in part after the entry of symbiont into host cells. After entry, special mechanisms for the biotrophic transfer of nutrients from symbiont to host develop. Both the symbiont population size and its rate of increase are strictly regulated by the host cell; symbiont metabolism may be controlled likewise. Rates of evolution of intracellular symbionts are probably very rapid, owing in part to responses of the host cell to its symbiont.
Insights
Modern symbionts integrate into host cells through a spectrum of associations. Host cells regulate symbiont growth, metabolism, and nutrient transfer, driving rapid symbiont evolution.
Area of Science:
- Microbiology
- Cell Biology
- Evolutionary Biology
Background:
- Symbiotic relationships involve microorganisms living within host cells.
- The integration of symbionts into host cells varies morphologically, forming a continuum.
- Understanding host-symbiont interactions is crucial for comprehending cellular evolution.
Purpose of the Study:
- To survey the colonization of host cells by modern symbionts.
- To explore the morphological integration and recognition mechanisms between hosts and symbionts.
- To investigate the regulatory processes and evolutionary rates of intracellular symbionts.
Main Methods:
- Review and synthesis of existing literature on symbiont colonization.
- Analysis of morphological distinctions and integration levels between symbionts and host cells.
- Examination of host-symbiont recognition, nutrient transfer, and regulatory mechanisms.
Main Results:
- A graded series of symbiont integration into host cells exists, from extracellular to intracellular.
- Most symbionts are vacuole-enclosed, with vacuolar membrane loss potentially occurring in later evolutionary stages.
- Host cells actively regulate symbiont population size, growth, and metabolism, facilitating biotrophic nutrient transfer.
- Recognition mechanisms, involving surface contact or post-entry events, are essential for symbiosis.
- Intracellular symbionts exhibit rapid evolutionary rates, influenced by host cell responses.
Conclusions:
- Host cell regulation is a key factor in the establishment and maintenance of intracellular symbiosis.
- The evolution of intracellular symbiosis involves complex recognition and integration processes.
- Host-symbiont interactions significantly impact the evolutionary trajectory of symbionts.
Related Concept Videos
00:58Symbiosis
02:32Plasmodesmata
00:50Exocytosis
Exocytosis is the opposite of endocytosis, which brings molecules inside the cell. Sometimes, the released materials are signaling molecules. For example, neurons typically use exocytosis to release neurotransmitters. Cells also use exocytosis to insert proteins such as ion channels into their cell membranes, secrete proteins for use in the extracellular matrix, or...
01:20Plasmodesmata
Intercellular junctions are a feature of fungal, plant, and animal cells. However, different types of junctions are found in different kinds of cells. Intercellular junctions found in animal cells include tight junctions, gap junctions, and...
01:26Microbial Interactions: Cooperation
01:25Colonisation of Pathogens