Ethics statement
This study was approved by the ethics committee of Okayama University Graduate School of Medicine, Dentistry, and Pharmaceutical Sciences and Okayama University Hospital (Authorization Number: 1603–059) and Brain Attack Center Ota Memorial Hospital (Authorization Number: 121). All enrolled patients provided written informed consent for the use of their resected tissue and oral samples.
Participants
The study focused on 12 patients who visited Brain Attack Center Ota Memorial Hospital between April 2016 and March 2018, and who were diagnosed with internal carotid artery stenosis. The patients were ≥ 40 years of age, underwent carotid endarterectomy, had more than ten teeth, and consented to participate.
Samples
We harvested atheromatous plaques from the carotid artery walls extracted during carotid endarterectomy. Bacteria in the gingival pocket were collected using absorbent paper points (United Dental Manufactures Inc., Johnson City, TN, USA). The three paper points were inserted into patients’ gingival pockets and were harvested after one minute. Each item was put them into an Eppendorf tube containing PBS. Later, bacteria from the tongue surface were collected using forensic swabs (Sarstedt AG & Co. Nümbrecht, Germany) by wiping the dorsum portion of each tongue several times. Each swab was placed in a tube and immediately stored at -80°C until DNA purification. Blood samples were collected from each patient and serum was prepared as previously described [43].
Oral examination
Periodontal examinations were performed to evaluate the average pocket probing depth (PPD) and rate of bleeding on probing (BOP) for each teeth of each patient. The patients were then divided into three groups according to the Japanese Society for Periodontology Clinical Practice Guideline for the Periodontal Treatment [44]. Briefly, patients with <5% of deep PPD (>4 mm) were defined as periodontally healthy (control, n = 4; H1-H4), patients (n = 4) with 6-23% deep PPD were defined as mild periodontitis and were excluded from further analysis, and patients with >24% of deep PPD were designated as severe periodontitis (periodontitis, n = 4; P1-P4).
DNA purification
APs were extensively minced using a scalpel and suspended in phosphate buffered saline (PBS). The collected material from paper points and swabs were resuspended using PBS. One milliliter of each resuspended bacterial sample was transferred to 2 ml Lysing Matrix B tubes (MP Biomedicals, Santa Ana, CA, USA) containing 0.1 mm silica beads and 500 μl ATL buffer (Qiagen, Hilden, Germany). The contents of each tube were homogenized using FastPrep 24 (MP Biomedicals) for 45 s at 6.5 m/s. Bacterial DNA was extracted using the QIAamp DNA Microbiome Kit (Qiagen) according to the manufacturer's instructions. The quality and quantity of the DNA were verified using the NanoDrop 2000 spectrophotometer (Thermo Fisher Scientific, Wilmington, DE, USA) and the PicoGreen dsDNA assay kit (Life Technologies, Grand Island, NY, USA).
Library preparation, sequencing, and analysis of 16S rRNA
The V3 and V4 regions of the 16S rRNA was amplified using the forward primer 5’-TCGTCGGCAGCGTCAGATGTGTATAAGAGACAGCCTACGGGNGGCWGCAG, and the reverse primer 5’-GTCTCGTGGGCTCGGAGATGTGTATAAGAGACAGGACTACHVGGGTATCTAATCC. Thermal cycling conditions were 98°C for 3 min; 25 cycles of 98°C for 30 s, 55°C for 30 s, and 72°C for 30 s; and a final extension at 72°C for 5 min. After PCR clean-up step using AMpure XP Beads (Beckman Coulter, Inc., Brea, CA, USA), a second PCR was performed to add sequencing adapters and dual-index barcodes to the amplicon target to distinguish amplicons from each sample using the same reaction conditions with only eight cycles instead. After the PCR clean-up step, the quality and quantity of the amplicon were verified using KAPA library quantification kit (KAPA Biosystems Inc., Wilmington, MA, USA). Aliquots (5 μL) of the diluted amplicon from each library were combined to form pooling libraries. Six pM of pooling libraries with PhiX was sequenced using the MiSeq® system (Illumina Inc., San Diego, CA, USA). The obtained sequence was analyzed using the CLC Microbial Genomics Module of the CLC Genomics Workbench (CLC bio, Aarhus, Denmark). Briefly, we extracted 5911 bacteria having sequences that passed both those being retained and operational taxonomic units (OTUs) selected based on a 97% similarity with a minimum of 10 reads representing each OTU [45]. Principal component analysis (PCA) and clustering analysis were performed using R statistical software [39]. We also performed co-occurrence analysis for the 13 highly detected operational taxonomic units from control and periodontitis samples using the Quantitative Insights Into Microbial Ecology approach [46].
Plasma IgG antibody titer test against periodontal bacteria
Plasma IgG antibody titer against periodontal bacteria was determined as described previously [22, 43, 47, 48]. Bacterial antigens used were sonicated preparations of Aggregatibacter actinomycetemcomitans (Aa) Y4, Aa ATC29523, Aa SUNY67, Eichenerra corrodens (Ec) FDC1073, Fusobacterium nucleatum (Fn) ATCC25586, Prevotella intermedia (Pi) ATCC25611, P. nigrescens (Pn) ATCC33563, Capnocytophaga ochracea (Co) S3, Porphyromonas gingivalis (Pg) FDC381, Pg SU63, Treponema denticola (Td) ATCC35405, and Tannerella forsythia (Tf)ATCC43037. The sera from five healthy participants without periodontitis (24-29 years of age) were pooled and used to calibrate the analyses. Standard titration curves were prepared using serial dilutions of this pooled control serum. The absorbance of each sample after reaction was defined as an ELISA unit (EU), with 100 EUs corresponding to a 1: 3,200 dilution of the calibrator sample [43]. According to the formula for clinical use, the mean ± 2 standard deviations of the controls, based on the reported dataset of IgG titers to individual pathogens among five healthy individuals, was defined as the standard value of 1.
Statistical analysis
The statistical analysis was performed using the Mann-Whitney U Test. A P-value of 0.05 was considered significant and was determined using SPSS Ver. 23 (SPSS Inc., Chicago, IL, USA) for all the experimental results.