2.3 Cytoplasmic membrane permeability
The permeability of cytoplasmic membrane was assessed by measuringβ -galactosidase activity in bacteria using ONPG, a substrate for
cytoplasmic β- galactosidase.[21] Briefly,E. coli or S. aureus were cultured separately in MHB
medium containing 2% lactose until the logarithmic growth phase. Then,
the E. coli or S. aureus suspension was centrifuged and
washed with HEPES (5 mM, pH 7.4) containing 20 mM glucose and 2 mM ONPG.
Next, 100 μL of the bacterial suspension (106 CFU/mL)
was mixed with the CDs (100 μL) at a predetermined concentration in a
96-well plate. The absorbance of the mixture at 420 nm was subsequently
measured at different times with a 5 min interval.