plant surface microbiology.pdf

7 The Functional Groups of Micro-organisms and Biotech Products 123
terium uses the soil as a natural habitat (Martin and Travers 1989). Soil survival
studies showed that the number of inoculated vegetative cells decreases
rapidly and only viable spores are found after 5 days. Vegetative cell multiplication
was not observed (Villas-Bôas et al. 2000).
Lereclus et al. (2000) suggested that during the sporulation phase the cry
regulation gene has the function of producing high quantities of Cry protein
to kill the insect, allowing the bacterium to complete its biological cycle,
(spore germination – multiplication – sporulation – dispersion). Virulence
factors, acting in unfavourable environmental conditions, enable the bacterium
to damage and invade host tissues, obtaining ideal conditions for
spore germination and cell multiplication.
3 Survival in the Soil
Knowledge of B. thuringiensis survival in the soil is important in the context
of its use for biological control. It is also relevant in the study of the interactions
with other soil microorganisms. The commercial B. thuringiensis formulas
are composed of mixtures of spores and crystals, which are released in
great quantities into the environment each year. The behaviour of these
spores and crystals in the soil has been studied in the field, greenhouse and in
sterilised and natural soils (Thomas et al. 2000; Villas Bôas et al. 2000). In
these assessments, the spores were inoculated into the soil and their recovery
was monitored.
Some reports demonstrated that, initially, the number of colony spore
forming units of B. thuringiensis declines rapidly. Vilas-Boas et al. (2000)
observed that after 24 h only 20 % of the spores survived in sterilised
soils.
Addison (1993) observed that several factors could affect the survival of the
spores, such as pH, moisture and nutrient availability. Spore viability seemed
to be little influenced by soil type or temperature. The results on nutrient
availability are controversial. Cell remains could be an extra source of nutrients
for the native microbiota and inoculated bacteria, but B. thuringiensis is
unable to use nutrients released by the lysis of inoculated dead cells (West et
al. 1985).
Competition with soil microbiota is one of the factors that most affects
spore viability in the soil. In the soil microcosm, B. thuringiensis spores have
a greater mortality index because of competition from other microorganisms
(Pruett et al. 1980).
The initial decline in the number of colony forming units after a 24-h permanence
in the soil is about 80 %. The surviving cells rapidly produce spores,
increasing the number of viable spores until the number of vegetative cells is
matched. This means that at a given moment, the number of spores will equal
that of the vegetative cells, and will remain stable for several months.