Bacterial strains, cells, and culture
S. Typhimurium SL1344, Typhimurium SL1344 ∆sseK3 mutant (with deletion of sseK3), and sseK3-complemented bacterial strains used in this study were available in our laboratory. The ∆sseK3 mutant was constructed using counter-selectable suicide vectors. The sseK3 gene was cloned into the pBR322 plasmid for complementation studies. RAW264.7 macrophage cells were obtained from the American type culture collection (ATCC, Manassas, VA), and cultured in Dulbecco's modified Eagle medium (DMEM)/high-glucose medium (HyClone, USA) containing 10% fetal calf serum (FCS) in an incubator at 37 ℃ and 5% CO2.
Adherence and invasion assay
Adhesion and invasion of RAW264.7 cells was assessed as previously described [56, 57]. A 24-well cell culture plate was inoculated with 1×105 RAW264.7 cells per well. The WT, ΔsseK3 mutant and sseK3-complemented strains were then added to RAW264.7 cells at a multiplicity of infection (MOI) of 100:1, with three replicate wells per strain. To allow complete contact between the bacteria and RAW264.7 cells, the plates were centrifuged at 1000 rpm and incubated in 5% CO2 for 2 h at 37 ℃. For the adherence assay, the supernatants were aspirated, and the cells were washed three times with PBS. Subsequently, the cells were digested with 0.25% trypsin, plated in a gradient dilution and counted. For the invasion assay, the supernatants were aspirated, cells were washed three times with PBS, gentamicin-containing medium (100 μg/mL) was added, and the cells were incubated at 37 ℃ with 5% CO2. After incubation, the supernatants were aspirated, and the cells were washed three times with PBS. Subsequently, the cells were lysed using 0.1% Triton X-100, plated with a gradient dilution and counted.
Flow cytometry assay
A 6-well cell culture plate was inoculated with 1×106 RAW264.7 cells per well and incubated for 16 h. WT, ΔsseK3 mutant and sseK3-complemented strains were incubated with RAW264.7 cells at a multiplicity of infection (MOI) of 100:1, with three replicate wells per strain. To allow complete contact between bacteria and RAW264.7 cells, the plates were centrifuged at 1000 rpm. Gentamicin-containing medium (100 μg/mL) was then added and the plates incubated at 37 ℃ with 5% CO2. After incubation, the supernatants were aspirated, and the cells were washed three times with PBS. The percentage of cells undergoing apoptosis was detected by flow cytometry using an Annexin V-FITC/PI apoptosis detection kit (KeyGEN BioTECH, Jiangsu, China). Cells from the infected and mock groups were digested with 0.25% trypsin, washed three times with ice-cold phosphate buffered saline (PBS), and suspended in 500 μL binding buffer. 5 μL Annexin V-FITC and 5 μL Propidium Iodide (PI) were added, and the solution was incubated in a dark room for 15 min at room temperature and immediately analyzed by flow cytometry (Beckman Coulter, Inc., Fullerton, CA, US).
Caspase-3, caspase-8, and caspase-9 activity assay
Caspase-3, caspase-8, and caspase-9 activity was measured using a Caspase-3 Assay Kit, Caspase-8 Assay Kit, and Caspase-9 Assay Kit (Beyotime, Shanghai, China), respectively. A 6-well cell culture plate was inoculated with 1×106 RAW264.7 cells per well and incubated for 16 h. WT, ΔsseK3 mutant, and sseK3-complemented strains were incubated with the RAW264.7 cells at a MOI of 100:1, with three replicate wells per strain. The plates were centrifuged at 1000 rpm and gentamicin-containing medium (100 μg/mL) was added and incubated at 37 ℃ with 5% CO2. The supernatants were then aspirated, and the cells were washed three times with PBS. Subsequently, the cells of the infected and mock groups were digested by trypsinization without EDTA and washed three times with ice-cold lysis buffer, and treated with 100μL lysis buffer on ice. After incubation for 15 minutes, the concentration of protein was detected using the Bradford protein assay kit (Beyotime, Shanghai, China). Subsequently, the cell lysates were incubated with Ac-DEVD-pNA for 4 h at 37℃, the absorbance was read at 405 nm in a microplate spectrophotometer (Infinite 200 PRO NanoQuant, Tecan, Switzerland).
Glycolysis assay
The glycolysis levels were measured using pyruvic acid, lactic acid, and ATP analysis kits, which were purchased from Nanjing Jiancheng Bioengineering Institute (Nanjing, China). WT, ΔsseK3 mutant, and sseK3-complemented groups were treated as above similar methods. After incubation with gentamicin-containing medium (100 μg/mL), the supernatants were aspirated, and the cells were washed three times with PBS. Cells were then analyzed using the kits according to manufacturers’ instructions at 2 h, 4 h, 6 h, and 8 h. The protein concentration in each group was detected using the Bradford protein assay kit (Beyotime, Shanghai, China), and absorbance values for pyruvic acid analysis, lactic acid, and ATP analysis were read at 505 nm, 530 nm, and 636 nm, respectively in a microplate spectrophotometer (Infinite 200 PRO NanoQuant, Tecan, Switzerland).
Statistical analysis
Data were presented as the mean ± standard deviation (SD) of three independent experiments. Two-way analysis of variance (ANOVA) with a post-hoc test (Bonferroni’s multiple-comparison test) was used to compare and assess statistical significance among all groups. P < 0.05 was considered statistically significant.