Ofloxacin

The S. marcescens NJ01 strain was a kind gift from Yong Yu Li (Fujian Agriculture and Forestry University, China). S. marcescens NJ01, a pathogenic bacterium isolated from diseased tomato leaves, can cause infection in people who consume infected tomatoes. Both 3-PPA and ofloxacin were purchased from Shanghai Yuanye Biotechnology Co., Ltd. (Shanghai, China).

Polymyxin B and ficin enzyme were obtained from Sigma-Aldrich (St. Louis, MO, USA). Polymyxin E was obtained from Kangmanlin (Nanjing, China). The following antibiotics were purchased from Yuanye Bio-technology Co., Ltd (Shanghai, China): erythromycin, lincomycin, vancomycin, ampicillin, ofloxacin, or Meilun (Dalian, China): linezolid, nisin, penicillin G, ciprofloxacin, tigecycline, and chlortetracycline.

The MICs of 3-PPA and quinolone antibiotics (ofloxacin) (Shanghai Yuanye Biotechnology Co., Ltd., Shanghai, China) were determined following the method developed by the Clinical Laboratory Standards Committee (CLSI, 2020). The S. marcescens NJ01 strain was inoculated in 5 mL of Luria-Bertani (LB) broth (Qingdao Hope Biotechnology Co., Ltd., Qingdao, China), then incubated at 180 rpm and 28°C for 17 h (overnight). The overnight culture was diluted to an optical density at 620 nm (OD₆₂₀) of 0.05 (13 (link)). Subsequently, different concentrations of drugs were added, and dimethyl sulfoxide (DMSO, Saiguo Biotechnology Co., Ltd., Guangzhou, China) was used as a negative control. The culture (200 μL) was added to a 96-well plate (Corning, USA) and incubated at 28°C and 180 rpm for 24 h. Absorbance was determined at 620 nm. All experiments were repeated at least three times.
3-PPA (12.5, 25.0, and 50.0 μg/mL) was added to the diluted solution and cultured at 28°C and 180 rpm for 24 h. DMSO was used as a negative control and did not exceed 1% of the bacterial solution. The culture medium was taken every 3 h to measure OD₆₂₀, and a growth curve was constructed.