and become so widespread as defensive symbionts (Kaltenpoth 2009). It is possible
that many actinobacterial species began as commensals or mild parasites, competing
with other microbial species for host resources. The production of antimicrobials
during this microbial warfare may have, in turn, become beneficial to the host by
preventing pathogenic infection. This may have driven the evolution of host mechanisms to ensure that Actinobacteria were consistently able to colonise the
microbiome (Kaltenpoth 2009). The spore-forming capabilities of Actinobacterial
species may have also aided this process by enabling species to resist environmental
stressors that may be experienced in the absence of the host during inter-individual
and inter-generational transmission (Kaltenpoth 2009).
The spore-forming capabilities of actinobacterial species may be particularly
important during symbiosis with solitary ‘Beewolf’ digger wasps. Female solitary
digger wasps (in the genera Pilanthus, Trachypus and Philanthinus) lay their eggs in
burrows, which they dig into the soil and provision with a paralysed honey bee; the
developing larvae feed on the bee before spinning cocoons (Kaltenpoth 2009;
Kaltenpoth et al. 2005). The brood chambers are humid and damp providing optimal
growth conditions for a variety of fungi and bacteria. However, to prevent developing infections, the mother wasp coats the brood chamber walls with secretions
containing a species of Actinobacteria, called Candidatus Streptomyces philanthi,
which the wasp cultures in specialised antennal glands (Kaltenpoth et al. 2005,
2006). These bacteria then become incorporated into the larval cocoon and produce
antibiotics on the cocoon surface (Kroiss et al. 2010). Remarkably, the Streptomyces
symbiont can remain viable on the cocoon wall as spores for up to nine months
before the larva emerges, despite the cocoon surface being a very poor environment
with limited nutrients availability (Kaltenpoth et al. 2010). This long-term survival
also allows the Streptomyces bacteria to be vertically transmitted across wasp
generations as they are then taken-up by the fully-developed females that emerge
from the cocoons (Kaltenpoth 2009; Kaltenpoth et al. 2010).
23.2.3 Competitive Exclusion and Modulation of the Host
Immune System
Actinobacteria are important symbionts for many organisms, however, defensive
microbes are not limited to this bacterial phylum. In fact, a large number of other
bacteria, across a wide range of phyla, are also known to provide their hosts with
protection against disease. For example, hoopoe birds (Upupa epops) are known to
culture high densities of Enterococcus bacteria in their uropygial (preen) glands;
these bacteria produce volatile antimicrobial substances that are known to inhibit the
growth of feather-degrading microorganisms (Martin-Platero et al. 2006; MartinVivaldi et al. 2010). Similarly, embryos of several species of crustacean are coated in
a dense growth of Gram-negative bacteria that produce antifungal compounds
23 Defensive Microbiomes: A Widespread Phenomenon in Nature
501
that many actinobacterial species began as commensals or mild parasites, competing
with other microbial species for host resources. The production of antimicrobials
during this microbial warfare may have, in turn, become beneficial to the host by
preventing pathogenic infection. This may have driven the evolution of host mechanisms to ensure that Actinobacteria were consistently able to colonise the
microbiome (Kaltenpoth 2009). The spore-forming capabilities of Actinobacterial
species may have also aided this process by enabling species to resist environmental
stressors that may be experienced in the absence of the host during inter-individual
and inter-generational transmission (Kaltenpoth 2009).
The spore-forming capabilities of actinobacterial species may be particularly
important during symbiosis with solitary ‘Beewolf’ digger wasps. Female solitary
digger wasps (in the genera Pilanthus, Trachypus and Philanthinus) lay their eggs in
burrows, which they dig into the soil and provision with a paralysed honey bee; the
developing larvae feed on the bee before spinning cocoons (Kaltenpoth 2009;
Kaltenpoth et al. 2005). The brood chambers are humid and damp providing optimal
growth conditions for a variety of fungi and bacteria. However, to prevent developing infections, the mother wasp coats the brood chamber walls with secretions
containing a species of Actinobacteria, called Candidatus Streptomyces philanthi,
which the wasp cultures in specialised antennal glands (Kaltenpoth et al. 2005,
2006). These bacteria then become incorporated into the larval cocoon and produce
antibiotics on the cocoon surface (Kroiss et al. 2010). Remarkably, the Streptomyces
symbiont can remain viable on the cocoon wall as spores for up to nine months
before the larva emerges, despite the cocoon surface being a very poor environment
with limited nutrients availability (Kaltenpoth et al. 2010). This long-term survival
also allows the Streptomyces bacteria to be vertically transmitted across wasp
generations as they are then taken-up by the fully-developed females that emerge
from the cocoons (Kaltenpoth 2009; Kaltenpoth et al. 2010).
23.2.3 Competitive Exclusion and Modulation of the Host
Immune System
Actinobacteria are important symbionts for many organisms, however, defensive
microbes are not limited to this bacterial phylum. In fact, a large number of other
bacteria, across a wide range of phyla, are also known to provide their hosts with
protection against disease. For example, hoopoe birds (Upupa epops) are known to
culture high densities of Enterococcus bacteria in their uropygial (preen) glands;
these bacteria produce volatile antimicrobial substances that are known to inhibit the
growth of feather-degrading microorganisms (Martin-Platero et al. 2006; MartinVivaldi et al. 2010). Similarly, embryos of several species of crustacean are coated in
a dense growth of Gram-negative bacteria that produce antifungal compounds
23 Defensive Microbiomes: A Widespread Phenomenon in Nature
501