Editing Pathogen transmission (section)

==Beneficial microorganisms==

The mode of transmission is also an important aspect of the biology of beneficial microbial symbionts, such as [[coral#Intracellular symbionts|coral]]-associated dinoflagellates or [[human microbiota]]. Organisms can form symbioses with microbes transmitted from their parents, from the environment or unrelated individuals, or both.{{cn|date=June 2021}}

===Vertical transmission===

[[Vertical transmission (symbiont)|Vertical transmission]] refers to acquisition of symbionts from parents (usually mothers). Vertical transmission can be intracellular (e.g. transovarial), or extracellular (for example through post-embryonic contact between parents and offspring). Both intracellular and extracellular vertical transmission can be considered a form of non-genetic [[Hologenome theory of evolution|inheritance]] or [[maternal effect|parental effect]]. It has been argued that most organisms experience some form of vertical transmission of symbionts.<ref>{{cite journal | vauthors = Funkhouser LJ, Bordenstein SR | title = Mom knows best: the universality of maternal microbial transmission | journal = PLOS Biology | volume = 11 | issue = 8 | pages = e1001631 | date = 2013 | pmid = 23976878 | pmc = 3747981 | doi = 10.1371/journal.pbio.1001631 | doi-access = free }}</ref> Canonical examples of vertically transmitted symbionts include the nutritional symbiont [[Buchnera (bacterium)|Buchnera]] in aphids (transovarially transmitted intracellular symbiont) and some components of the [[Gut flora#Acquisition of gut flora in human infants|human microbiota]] (transmitted during passage of infants through the birth canal and also through breastfeeding).{{cn|date=June 2021}}

===Horizontal transmission===

Some beneficial symbionts are acquired [[Horizontal transmission|horizontally]], from the environment or unrelated individuals. This requires that host and symbiont have some method of recognizing each other or each other's products or services. Often, horizontally acquired symbionts are relevant to [[secondary metabolism|secondary]] rather than primary metabolism, for example for use in defense against pathogens,<ref>{{cite journal | vauthors = Kaltenpoth M, Engl T |title=Defensive microbial symbionts in Hymenoptera |journal=Functional Ecology |date=2013 |volume=28 |issue=2 |doi=10.1111/1365-2435.12089 |pages=315–327|hdl=11858/00-001M-0000-000E-B76B-E |hdl-access=free }}</ref> but some primary nutritional symbionts are also horizontally (environmentally) acquired.<ref>{{cite journal | vauthors = Nussbaumer AD, Fisher CR, Bright M | title = Horizontal endosymbiont transmission in hydrothermal vent tubeworms | journal = Nature | volume = 441 | issue = 7091 | pages = 345–348 | date = May 2006 | pmid = 16710420 | doi = 10.1038/nature04793 | s2cid = 18356960 | bibcode = 2006Natur.441..345N }}</ref> Additional examples of horizontally transmitted beneficial symbionts include  [[Bobtail squid#light organ|bioluminescent bacteria associated with bobtail squid]] and [[Rhizobia|nitrogen-fixing bacteria in plants]].{{cn|date=February 2023}}

===Mixed-mode transmission===

Many microbial symbionts, including [[Gut flora#Acquisition of gut flora in human infants|human microbiota]], can be transmitted both vertically and horizontally. Mixed-mode transmission can allow symbionts to have the "best of both worlds" – they can vertically infect host offspring when host density is low, and horizontally infect diverse additional hosts when a number of additional hosts are available. Mixed-mode transmission make the outcome (degree of harm or benefit) of the relationship more difficult to predict, because the [[Fitness (biology)|evolutionary success]] of the symbiont is sometimes but not always tied to the success of the host.<ref name="auto"/>