Sem xl20

The biofilm structures of the GBS2 were investigated by SEM XL20 scanning electron microscopy (Philips, Amsterdam, Netherlands). For biofilm formation, the overnight GBS2 cultures were diluted (1:50 ratio) into the fresh TSB broth (containing 1.0% glucose, 1.0% sodium chloride, and 1.5% milk). Sterile coverslips (18 × 18 mm) were placed into the bottom of the wells of the 12-well plates, then the dilutions were transferred onto the coverslips, and the coverslips served as the bacterial attaching surfaces. The cells in the 12-well plates were cultured for about 40–48 h at 38 °C, and then the coverslips were picked out and washed at least two times with PBS buffer. For SEM observation, the samples were prepared according to a previous study [24 (link)], Biofilm bacteria were fixed with 5% glutaraldehyde at 4 °C for about 12 h and then dehydrated by using ethanol solution (with serial concentrations: 30%, 50%, 70%, 80%, 95%, and 100%) for at least 20 min at 4 °C. After that, biofilm bacteria with the coverslips were freeze-dried for about 12 h and sputtered onto sample surface of precious metals of about 10 nm thickness. The morphology of GBS2 was observed and photographed at different magnifications.

Scanning electron microscopy (SEM XL20, Philips, Amsterdam, The Netherlands) was used to investigate the structural modifications of biofilms after treatment with Qe, Ag NPs, or QA NPs. For biofilm formation, the overnight E. coli strain ECDCM1 was diluted to a ratio of 1:50 into fresh LB medium complemented with 0.5% milk solution. The cultures were added to 12-well plates (Costar; Corning, Steuben, NY, USA), when necessary, Qe, Ag NPs, or QA NPs were added for a final concentration of 5 µg/mL. Sterile 18 mm ×18 mm coverslips were placed in the wells and served as the attaching surface for the cells. The plates were incubated for 15 h at 37 °C. After incubation, the coverslips were taken out and washed three times with PBS (pH 7.4). The preparation of the samples for SEM was performed as follows(Sun et al., 2017 (link)) . The samples were fixed overnight with 2.5% glutaraldehyde (Sangon, Shanghai, China) at 4 °C and subsequently dehydrated using serial ethanol concentrations: 30, 50, 70, 80, 95 and 100%. Each ethanol treatment lasted for 20 min at 4 °C and, finally, the cover slips were freeze-dried overnight.