plant surface microbiology.pdf

An example for the change in water permeability of one cuticular membrane
before and after treatment with bacteria is shown in Fig. 6. The effects
on water permeability for an entire sample unit consisting of at least 12 cuticles
are given as mean values of the effects measured for individual membranes.
Some results are presented in the chapter “Interactions between Epiphyllic
Microorganisms and Leaf Cuticles” by Schreiber et al. (Chap. 9, this
Vol.).
The effects on water permeability need not necessarily be measured before
and after treatment with bacteria, but can also be measured during the incubation
with bacteria by lowering the air humidity inside the incubation box
to, e.g. 90 %. As the driving force for the water flow across the cuticle is
reduced to 1/10, periodical intervals in between weighing the chamber are
increased to 4 days in order to measure a significant weight loss.
4.6 Measuring Penetration of Microorganisms Through Cuticular
Membranes
Penetration of microorganisms through cuticular membranes can be measured
as well using the described in vitro system. The outer side of the cuticle
is inoculated with bacterial cells, whereas the inner side faces a sterile nutrition
solution. If the cuticle, located between microbial cells and nutrition
solution inside the chamber, is penetrated by bacterial cells, microbial growth
will be detectable in the nutrition solution. Thus, penetration of isolated cuticles
by bacteria can be easily monitored by transferring 50 ml of the nutrition
solution inside the chamber onto glucose-supplemented yeast extract agar
plates using a sterile syringe. Subsequent microbial growth on the agar plates
indicates that a penetration event through the cuticular membrane has
occurred. In that way, the amount of cuticles penetrated is determined in daily
intervals. The amount of cuticles penetrated after different periods of incubation
is given in percent of the total amount of inoculated membranes.
%CM =
penetrated
25 Analysing Interactions between Microorganisms and Cuticles 481
Number of CMpenetrated
Number of CMtotal
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An example for a penetration kinetic is shown in Fig. 7. The amount of penetrated
cuticular membranes increased over the incubation period of 12 days.
Some typical characteristics of a penetration kinetic can be used to describe
the barrier function of cuticles quantitatively: (1) at the end of the incubation
period there was a steady increase of penetrated cuticular membranes versus
incubation time. Rates of penetration (% CM penetrated/day) can be calculated
from the slopes of the linear regression. (2) Another meaningful parameter is
the time needed by the microorganisms to penetrate 50 % of inoculated membranes
(T 50 %). High rates of penetration and small T 50 % values indicate low