Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

238j 12 Practical Applications of Rhizospheric Bacteria
plasticizers form the bulk of the plastic material; therefore, more attention is being
given to their degradation. Once the plasticizers have modified bonds/linkages
(ester and/or amide), the microorganism possessing respective enzymes can easily
act upon them [1]. In view of the above, this study was conducted, wherein graft
copolymers of LDPE are used for in vitro degradation studies.
12.3.1
Growth in the Presence of Polymer
Among the 12 isolates screened for tolerance to LDPE, LDPE-g-PMMA and LDPE-g-
PMH, three (namely S2, PN15 and PN13) had shown maximum growth and were
thus selected for further studies. Moreover, based on 16S rRNA sequencing, these
isolates have been characterized as B. pumilus, B. cereus and other Bacillus species,
respectively.
Another important observation was that the bacteria isolated from the same site
had varying levels of tolerance to the polymers mentioned above. This observation
strongly supports the exploration of bacterial diversity to identify novel microbes that
can help biodegrade the plastic waste accumulated in the environment. It is evident
from these data that PN15 has shown maximum tolerance to LDPE-g-PMMA, S2 for
LDPE-g-PMH and PN13 for both.
12.3.2
Biodegradation Studies
12.3.2.1 B. cereus
B. cereus had maximum tolerance (10 mg ml 1 ) to the polymers studied. The UV–
visible spectrum of 1/10th-diluted Davis minimal broth in the presence of B. cereus
and LDPE-g-PMMA was compared with that of the medium in the absence of the
compound. After 2 days, the lmax shifted from 289 to 298 nm in the presence of the
bacteria, while it remained unaffected when bacteria were grown in its absence
(Figure 12.1). Furthermore, in the presence of the bacteria, lmax changed to 215 nm
on day 4 and finally fell below 200 nm on day 11 of the experiment. As Figure 12.1
makes it evident, the cultures had reached the stationary growth phase between days
7 and 11 and hence no further biological activity was expected in the culture.
12.3.2.2 Bacillus sp.
When this strain was used for biodegradation of LDPE-g-PMMA, it was observed
that for the first 2 days virtually no change occurred in l max and it remained constant
at 289 nm. However, on day 3 lmax shifted to 257 nm and remained constant until
day 11 except for a minor change on day 4 – 263 nm (Figure 12.2).
On days 1 and 2 of Bacillus sp. growth on LDPE-g-PMH, there was no change in
lmax of the compound, whereas on day 3 it changed to 221 nm and further to 224 nm
on day 4 and finally stabilized at 227 nm from days 7 to 11. This culture entered
stationary growth phase between days 7 and 11, which corresponds to the duration
when no shift in lmax took place (Figure 12.3).