Sc-iso-25), the number of bacteria found inside sclerotia increased with immersion
time before reaching a near-constant state by 24 h. Of the eight isolates, Ralstonia
pickettii (strain Sc-iso-06) exhibited the highest maximum viable colony counts
(8.7 Â 10
7 CFU g
À1 ) and the highest rate of colonization (1.6 Â 10
6 CFU h
À1 ).
Five of the other isolates tested showed a similar pattern of colonization, with
maximum colonization ranging from 10
5 to 10
7 CFU g
À1 and colonization rates
ranging from 10
4 to 10
5 CFU h
À1 .
Colonization of sclerotia by Brevundimonas sp. (strain Sc-iso-30) and Bacillus
sp. (strain Sc-iso-25) was somewhat different. Colonization by Brevundimonas
sp. was slower (3.8 Â 10
2 CFU h
À1 ) than for the other isolates and maximum
colonization was lower (1.9 Â 10
5 CFU g
À1 ). In the case of Bacillus sp. (strain
Sc-iso-25), although bacteria were detected in the water used to rinse the sclerotia
surfaces after immersion, very few bacteria were detected inside the sclerotia.
Although the maximum colonization and rate of colonization varied among isolates,
almost all of the isolates except for the Bacillus sp. strain colonized the insides of
sclerotia.
4.4.3 Discussion
Seventeen species of bacteria were isolated from the insides of four sclerotia
obtained from Mt. Ontake soil, with different communities found in each sclerotium.
No systematic differences in bacterial community could be discerned, and bacterial
community composition did not appear to depend on sclerotium size. However, the
fact that a different dominant species (accounting for 45% to 71% of isolated
bacteria) was found in each sclerotium is consistent with the results of Ohta et al.
(2003) that preformed a relatively high prevalence (52%) of Sphingomonas spp.
(currently Sphingobium) isolated from sclerotia obtained from Mt. Myoko soil,
Niigata Prefecture.
The results of this study suggest that the insides of soil sclerotia are colonized by a
variety of bacteria. On the basis of this observation, we speculate that sclerotia serve
as carriers for the bacteria that are able to colonize their insides. The sclerotia
colonization experiment revealed that, in the case of most isolates, bacterial numbers
increased with immersion time before reaching a plateau by 24 h. This suggests that
movement of bacteria into autoclaved sclerotia occurs rapidly. The wide variations
in bacterial numbers inside sclerotia after up to 15 h of immersion suggest that the
colonization rate differs depending on the internal structure of individual sclerotia
and the degree of structural damage caused by autoclaving. Given the narrow range
of bacterial numbers in sclerotia after 24 h of immersion and beyond, the bacterial
population at 24 h seems to represent the maximum colonization. The sclerotium
colonization experiment, in which we focused on sclerotium structure, showed that
the colonization rate of autoclaved sclerotia differs depending on bacterial species,
and that 24 h of immersion in liquid culture is sufficient to achieve maximum
colonization. This indicates that bacterial colonization of sclerotia can easily be
4 Sclerotia Grains as Bacterial Carriers in Soil
73
time before reaching a near-constant state by 24 h. Of the eight isolates, Ralstonia
pickettii (strain Sc-iso-06) exhibited the highest maximum viable colony counts
(8.7 Â 10
7 CFU g
À1 ) and the highest rate of colonization (1.6 Â 10
6 CFU h
À1 ).
Five of the other isolates tested showed a similar pattern of colonization, with
maximum colonization ranging from 10
5 to 10
7 CFU g
À1 and colonization rates
ranging from 10
4 to 10
5 CFU h
À1 .
Colonization of sclerotia by Brevundimonas sp. (strain Sc-iso-30) and Bacillus
sp. (strain Sc-iso-25) was somewhat different. Colonization by Brevundimonas
sp. was slower (3.8 Â 10
2 CFU h
À1 ) than for the other isolates and maximum
colonization was lower (1.9 Â 10
5 CFU g
À1 ). In the case of Bacillus sp. (strain
Sc-iso-25), although bacteria were detected in the water used to rinse the sclerotia
surfaces after immersion, very few bacteria were detected inside the sclerotia.
Although the maximum colonization and rate of colonization varied among isolates,
almost all of the isolates except for the Bacillus sp. strain colonized the insides of
sclerotia.
4.4.3 Discussion
Seventeen species of bacteria were isolated from the insides of four sclerotia
obtained from Mt. Ontake soil, with different communities found in each sclerotium.
No systematic differences in bacterial community could be discerned, and bacterial
community composition did not appear to depend on sclerotium size. However, the
fact that a different dominant species (accounting for 45% to 71% of isolated
bacteria) was found in each sclerotium is consistent with the results of Ohta et al.
(2003) that preformed a relatively high prevalence (52%) of Sphingomonas spp.
(currently Sphingobium) isolated from sclerotia obtained from Mt. Myoko soil,
Niigata Prefecture.
The results of this study suggest that the insides of soil sclerotia are colonized by a
variety of bacteria. On the basis of this observation, we speculate that sclerotia serve
as carriers for the bacteria that are able to colonize their insides. The sclerotia
colonization experiment revealed that, in the case of most isolates, bacterial numbers
increased with immersion time before reaching a plateau by 24 h. This suggests that
movement of bacteria into autoclaved sclerotia occurs rapidly. The wide variations
in bacterial numbers inside sclerotia after up to 15 h of immersion suggest that the
colonization rate differs depending on the internal structure of individual sclerotia
and the degree of structural damage caused by autoclaving. Given the narrow range
of bacterial numbers in sclerotia after 24 h of immersion and beyond, the bacterial
population at 24 h seems to represent the maximum colonization. The sclerotium
colonization experiment, in which we focused on sclerotium structure, showed that
the colonization rate of autoclaved sclerotia differs depending on bacterial species,
and that 24 h of immersion in liquid culture is sufficient to achieve maximum
colonization. This indicates that bacterial colonization of sclerotia can easily be
4 Sclerotia Grains as Bacterial Carriers in Soil
73