controlled and that the structure of sclerotia makes them suitable as a microbial
carrier substrate.
Maximum colonization of sclerotia by Ralstonia pickettii (strain Sc-iso-06) was
approximately 1 Â 10
8 CFU g
À1 , which was the most extensive colonization among
isolates tested. Given also that this species accounted for 71% of the bacteria isolated
from Sc1, the highest prevalence of all isolated species, it appears that R. pickettii
readily colonizes sclerotia. In similar sclerotium colonization experiments,
Bradyrhizobium sp. (strain Sc-iso-05), Arthrobacter sp. (strain Sc-iso-19),
Methylobacterium sp. (strain Sc-iso-28), Janibacter sp. (strain Sc-iso-10), and
Nostocoida sp. (strain Sc-iso-45) were also re-isolated from sclerotia in the same
manner as R. pickettii (strain Sc-iso-06). Maximum colonization by these five isolates was in a similar range, from 1 Â 10
5 to 1 Â 10
7 CFU g
À1 . A similar result was
obtained for the one strain isolated from soil; we plan to conduct further experiments
to determine the usefulness of the soil isolate after clarifying the relationship
between isolation source and sclerotium colonization.
In contrast, colonization by Brevundimonas sp. (strain Sc-iso-30) was extremely
slow, and only very small numbers of Bacillus sp. (strain Sc-iso-25) were detected in
sclerotia even after immersion in pure culture. Given that B. cereus, a close relative
of strain Sc-iso-25, is known to form resting spores when transferred to low-nutrient
media (Clause and Berkeley 1986), it is highly likely that Sc-iso-25 was unable to
colonize the insides of sclerotia. These results indicate that the majority of isolates
tested are able to colonize sclerotia under sterile conditions where other biological
factors such as antibiotics have been excluded by autoclaving.
4.5 Conclusions
Analysis of microbial communities inside sclerotia grains collected from soil and
experiments investigating the colonization of autoclaved sclerotia indicate that
sclerotia grains can be colonized by a variety of bacteria, that this colonization can
be easily controlled, and that sclerotia have an internal structure that makes them
suitable microbial carrier substrates. The origin of the bacterial colonies inside
sclerotia grains still needs further study as to whether they are the endofungal
bacteria of sclerotia-forming fungi or bacteria that have immigrated from soil to
sclerotium. The internal structure of sclerotia may provide valuable hints for designing novel microbial carrier substrates.
74
Y. S. Nonoyama and K. Narisawa
carrier substrate.
Maximum colonization of sclerotia by Ralstonia pickettii (strain Sc-iso-06) was
approximately 1 Â 10
8 CFU g
À1 , which was the most extensive colonization among
isolates tested. Given also that this species accounted for 71% of the bacteria isolated
from Sc1, the highest prevalence of all isolated species, it appears that R. pickettii
readily colonizes sclerotia. In similar sclerotium colonization experiments,
Bradyrhizobium sp. (strain Sc-iso-05), Arthrobacter sp. (strain Sc-iso-19),
Methylobacterium sp. (strain Sc-iso-28), Janibacter sp. (strain Sc-iso-10), and
Nostocoida sp. (strain Sc-iso-45) were also re-isolated from sclerotia in the same
manner as R. pickettii (strain Sc-iso-06). Maximum colonization by these five isolates was in a similar range, from 1 Â 10
5 to 1 Â 10
7 CFU g
À1 . A similar result was
obtained for the one strain isolated from soil; we plan to conduct further experiments
to determine the usefulness of the soil isolate after clarifying the relationship
between isolation source and sclerotium colonization.
In contrast, colonization by Brevundimonas sp. (strain Sc-iso-30) was extremely
slow, and only very small numbers of Bacillus sp. (strain Sc-iso-25) were detected in
sclerotia even after immersion in pure culture. Given that B. cereus, a close relative
of strain Sc-iso-25, is known to form resting spores when transferred to low-nutrient
media (Clause and Berkeley 1986), it is highly likely that Sc-iso-25 was unable to
colonize the insides of sclerotia. These results indicate that the majority of isolates
tested are able to colonize sclerotia under sterile conditions where other biological
factors such as antibiotics have been excluded by autoclaving.
4.5 Conclusions
Analysis of microbial communities inside sclerotia grains collected from soil and
experiments investigating the colonization of autoclaved sclerotia indicate that
sclerotia grains can be colonized by a variety of bacteria, that this colonization can
be easily controlled, and that sclerotia have an internal structure that makes them
suitable microbial carrier substrates. The origin of the bacterial colonies inside
sclerotia grains still needs further study as to whether they are the endofungal
bacteria of sclerotia-forming fungi or bacteria that have immigrated from soil to
sclerotium. The internal structure of sclerotia may provide valuable hints for designing novel microbial carrier substrates.
74
Y. S. Nonoyama and K. Narisawa