against the fungal pathogen Lagenindium callinectes (Gil-Turnes and Fenical 1992;
Gil-Turnes et al. 1989).
Most described examples of defensive mutualisms involve bacterial partners. In
comparison, far less is known about the role that fungal species can play in providing
protective benefits to host organisms. This is partly because, relative to bacteria,
fungi remain hugely understudied in the context of the microbiome. They can also be
difficult to culture, making it hard to characterise their function. However, genomic
studies as well as bioactivity assays using fungal isolates have demonstrated that
many fungal species encode a diverse secondary metabolism capable of making a
large arsenal of different antimicrobials (Rateb and Ebel 2011). Fungal species are
also hugely abundant in the microbiomes of many species and in some cases, such as
sponges, are transmitted maternally across generations suggesting that they can be
closely associated with their host organisms (Maldonado et al. 2005). One of the few
examples of a defensive fungal symbiont is the association between leaf rolling
weevils (Euops chinensis) and the fungal species Penicillium herquei. The Penicillium symbiont is added to the leaves in which the weevils roll their eggs and larvae.
Here, it has been shown to produce the antimicrobial scleroderolide which inhibits
the growth of microbial pathogens (Wang et al. 2015). With greater study and the
development of new techniques, further examples of defensive partnerships involving fungi may come to light.
Antibiotic production is a key mechanism by which microbes are able to protect
their host against infection. However, this often also works in addition to, or in
combination with, other mechanisms. By colonising a host and taking up resources
such as space and nutrients, symbiotic microbes can also competitively exclude
pathogenic or parasitic microorganisms which use the same resources. For example,
in mice, a bacterial species called Bacteroides thetaiotaomicron, is known to
consume carbohydrates that are required for the growth of Citrobacter rodentium
(a mucosal pathogen of mice). By preventing access to a key resource,
B. thetaiotaomicron is able to exclude the pathogenic species from the mouse
intestinal lumen (Buffie and Pamer 2013).
Members of the microbiome can additionally contribute to the defence of their
host by priming the host immune system so that it can efficiently respond to
pathogenic attack. For example, evidence from mouse models suggests that commensal bacteria in the intestinal tract can enhance host immunity by directing the
development of immune cell populations involved in both innate and adaptive
immune processes. These bacteria also promote the production of antimicrobials
and pro-inflammatory factors by cells in the gut (Buffie and Pamer 2013; Hooper
et al. 2012). Similarly, bacterial symbionts that colonise plant roots, such as Bacillus,
Streptomyces and Pseudomonas species, have also been shown to prime the plant
immune system, resulting in an elevated and accelerated response to pathogenic
infection (Pieterse et al. 2014; Kurth et al. 2014). The plant host recognises residues
on the surface of these beneficial microbial species (called microbial associated
molecular patterns, or MAMPs) which leads to the activation of signaling cascades
involved in mounting an immune response. These pathways are then primed to
respond to pathogenic invasion (Pieterse et al. 2014; Selosse et al. 2014).
502
S. Worsley
Gil-Turnes et al. 1989).
Most described examples of defensive mutualisms involve bacterial partners. In
comparison, far less is known about the role that fungal species can play in providing
protective benefits to host organisms. This is partly because, relative to bacteria,
fungi remain hugely understudied in the context of the microbiome. They can also be
difficult to culture, making it hard to characterise their function. However, genomic
studies as well as bioactivity assays using fungal isolates have demonstrated that
many fungal species encode a diverse secondary metabolism capable of making a
large arsenal of different antimicrobials (Rateb and Ebel 2011). Fungal species are
also hugely abundant in the microbiomes of many species and in some cases, such as
sponges, are transmitted maternally across generations suggesting that they can be
closely associated with their host organisms (Maldonado et al. 2005). One of the few
examples of a defensive fungal symbiont is the association between leaf rolling
weevils (Euops chinensis) and the fungal species Penicillium herquei. The Penicillium symbiont is added to the leaves in which the weevils roll their eggs and larvae.
Here, it has been shown to produce the antimicrobial scleroderolide which inhibits
the growth of microbial pathogens (Wang et al. 2015). With greater study and the
development of new techniques, further examples of defensive partnerships involving fungi may come to light.
Antibiotic production is a key mechanism by which microbes are able to protect
their host against infection. However, this often also works in addition to, or in
combination with, other mechanisms. By colonising a host and taking up resources
such as space and nutrients, symbiotic microbes can also competitively exclude
pathogenic or parasitic microorganisms which use the same resources. For example,
in mice, a bacterial species called Bacteroides thetaiotaomicron, is known to
consume carbohydrates that are required for the growth of Citrobacter rodentium
(a mucosal pathogen of mice). By preventing access to a key resource,
B. thetaiotaomicron is able to exclude the pathogenic species from the mouse
intestinal lumen (Buffie and Pamer 2013).
Members of the microbiome can additionally contribute to the defence of their
host by priming the host immune system so that it can efficiently respond to
pathogenic attack. For example, evidence from mouse models suggests that commensal bacteria in the intestinal tract can enhance host immunity by directing the
development of immune cell populations involved in both innate and adaptive
immune processes. These bacteria also promote the production of antimicrobials
and pro-inflammatory factors by cells in the gut (Buffie and Pamer 2013; Hooper
et al. 2012). Similarly, bacterial symbionts that colonise plant roots, such as Bacillus,
Streptomyces and Pseudomonas species, have also been shown to prime the plant
immune system, resulting in an elevated and accelerated response to pathogenic
infection (Pieterse et al. 2014; Kurth et al. 2014). The plant host recognises residues
on the surface of these beneficial microbial species (called microbial associated
molecular patterns, or MAMPs) which leads to the activation of signaling cascades
involved in mounting an immune response. These pathways are then primed to
respond to pathogenic invasion (Pieterse et al. 2014; Selosse et al. 2014).
502
S. Worsley