3.2 The antibacterial property and antibacterial mechanisms of
the CDs
To evaluate the antibacterial activity of the CDs, E. coli, S.
aureus P. aeruginosa , S. typhimurium , and V.
parahaemolyticus strains were utilized as the model bacteria. The
antibacterial property of the CDs for each bacterium was tested in the
dark using the spread plate method. As shown inFigure S4 , the number of
colonies decreased drastically with the increase in the concentration of
the CD. A broad-spectrum antibacterial capacity of the CDs was therefore
demonstrated.
Due to the electrostatic action, the positively charged CDs may complex
with the membrane upon adsorption on bacteria, which alters the fluidity
of the lipid bilayer and destroy the membrane structure for
antibacterial purposes.[31] ONPG was used to
assess the permeability of the cytoplasmic membranes ofE. coli and S.
aureus . It is recognized that when the cytoplasmic membrane is
disrupted, β -galactosidase is released from the cell and
catalyzes the hydrolysis of ONPG to produce the yellow
o-nitrophenol.[32] As shown in Figure 3a ,
the absorbance at 420 nm increased gradually with the increase in the CD
concentration and incubation time, although the rate of increase slowed
down after around 30 min. This indicates that the permeability of the
cytoplasmic membranes of E. coli was positively correlated with
the concentration of the CDs and incubation time. Similar results were
attained for the S. aureus group (Figure 3b ).
Noteworthy, higher concentrations of the CD were required for S.
aureus to cause a consequence of cytoplasmic membrane damage comparable
to that of E. coli . This result can be ascribed to the thicker
cytoderm of S. aureus that is composed of peptidoglycan, in
comparison with that of E. coli .[33] The
above results suggest that the CDs disrupt the bacterial cytoplasmic
membrane.
To directly visualize the disrupted bacterial structure by the CDs, the
morphological change in E. coli and S. aureus were studied
using electron microscopies. As shown in the SEM images, E. coliin the control group possessed a smooth surface with an intact cell wall
and a well-stereo-shaped body, while the bacterial cell walls treated
with the CDs became rough and displayed obvious wrinkles and damage
(Figure 3c ). The TEM images further confirmed the disruption ofE. coli structure caused by the CDs (Figure 3d ).E. coli treated with the CDs demonstrated significant
morphological change, and the intracellular content appeared to oozed
out. Similar phenomena were observed for S. aureus . Both the SEM
and TEM characterizations reveal that S. aureus treated with the
CDs had severe surface structural damage and clear leakage of
intracellular content. The above result indicates that the disruption of
the membrane structure by the CDs may be responsible for the bacteria
leakage and the bacteria-killing effect (Figure 3e ).