2.2.2. Phylloplane colonizing bacteria (PCB)
A field survey was conducted during 2010 and 2011 across different
brinjal growing regions of Karnataka (data not shown).
Leaves from healthy brinjal plants were collected, maintained in
a humidity chamber (relative humidity > 70%) and transferred to
the laboratory within 48 h of collection. Leaf washing technique
was followed to isolate the phylloplane bacterial strains. Five
leaves from each plant were washed with 25 mL of saline solution
(0.85%). Bacteria colonized on leaf surface were dislodged using a
sterile brush during washing. The suspension was transferred to
50 mL tubes and centrifuged at 6800g for 10 min. The pellet was
dissolved in 1 mL of saline solution and used for bacterial isolation
following serial dilution technique (up to 10–7) on Nutrient agar
(NA). After incubation at 35 ± 2 C for 36 h, morphologically different
bacterial colonies appearing on the media were selected and
pure cultured on NA slants.
Bacterial inocula were prepared by growing the bacterial isolates
in Luria Bertani (LB) broth for 24 h on a rotary shaker
(150 rpm) at 35 ± 2 C. Bacterial cells were pelleted by centrifuging
at 6800g for 10 min, further washed three times with sterile distilled
water. Final concentration of bacteria was adjusted spectrophotometrically
to 0.45 OD at A610 nm.
Phylloplane colonization assay was carried out using 25 day-old
brinjal seedlings. Briefly, brinjal seedlings were grown in pots
(9 cm diameter) containing sterilized potting mixture (soil: sand:
farm yard manure @ 2:1:1 by volume) with one seedling/pot up
to 25 days in growth chamber with 26/22 C day/night temperature,
13/11 h light/dark cycles at 65% relative humidity. The light
intensity at the plant level was 200 W/m2
. Bacterial suspension
was spray inoculated on completely open brinjal leaves till runoff.
Treated plants were incubated for 14 days and leaf samples were
harvested and analyzed for bacterial colonization. Treated and control
leaves were cut into 10 mm diameter discs using sterilized
cork borer. Discs were washed initially with sterile distilled water
and adhering bacteria were dislodged using a sterile brush. Bacterial
concentration in suspension was determined by serial dilution
technique and expressed as cfu/cm2 leaf surface area.
Phylloplane colonizing bacteria were identified through morphological
analysis, biochemical characteristics according to the
methods outlined in the Bergey’s Manual of Systemic Bacteriology
(Krieg and Holt, 1984) and Microbiology – A laboratory manual
(Cappuccino and Sherman, 2014). The PCR amplification of partial
16S rRNA gene and sequencing was done following standard methods.
The DNA sequence obtained was compared with 16S rRNA
gene sequences in the NCBI database using the BLAST search algorithm
(http://blast.ncbi.nlm.nih.gov/Blast.cgi) to determine the
closest known relatives. The nucleotide sequence was deposited
with NCBI database and accession number was obtained.