title: Serum bactericidal assay for the evaluation of typhoid vaccine using a semi-automated colony-counting method.
abstract: Typhoid fever, mainly caused by Salmonella enterica serovar Typhi (S. Typhi), is a life-threatening disease, mostly in developing countries. Enzyme-linked immunosorbent assay (ELISA) is widely used to quantify antibodies against S. Typhi in serum but does not provide information about functional antibody titers. Although the serum bactericidal assay (SBA) using an agar plate is often used to measure functional antibody titers against various bacterial pathogens in clinical specimens, it has rarely been used for typhoid vaccines because it is time-consuming and labor-intensive. In the present study, we established an improved SBA against S. Typhi using a semi-automated colony-counting system with a square agar plate harboring 24 samples. The semi-automated SBA efficiently measured bactericidal titers of sera from individuals immunized with S. Typhi Vi polysaccharide vaccines. The assay specifically responded to S. Typhi Ty2 but not to other irrelevant enteric bacteria including Vibrio cholerae and Shigella flexneri. Baby rabbit complement was more appropriate source for the SBA against S. Typhi than complements from adult rabbit, guinea pig, and human. We also examined the correlation between SBA and ELISA for measuring antibody responses against S. Typhi using pre- and post-vaccination sera from 18 human volunteers. The SBA titer showed a good correlation with anti-Vi IgG quantity in the serum as determined by Spearman correlation coefficient of 0.737 (P < 0.001). Taken together, the semi-automated SBA might be efficient, accurate, sensitive, and specific enough to measure functional antibody titers against S. Typhi in sera from human subjects immunized with typhoid vaccines.
authors.affiliation: Clinical Immunology, Sciences Unit, International Vaccine Institute, Seoul, Republic of Korea; Department of Oral Microbiology and Immunology, DRI, and BK21 Plus Program, School of Dentistry, Seoul National University, Seoul, Republic of Korea.
authors.forename: Mi Seon
authors.initials: MS
authors.lastname: Jang
authors.affiliation: Program Management Unit, International Vaccine Institute, Seoul, Republic of Korea.
authors.forename: Sushant
authors.initials: S
authors.lastname: Sahastrabuddhe
authors.affiliation: Department of Agricultural Biotechnology and Research Institute for Agriculture and Life Sciences, Seoul National University, Seoul, Republic of Korea.
authors.forename: Cheol-Heui
authors.initials: CH
authors.lastname: Yun
authors.affiliation: Department of Oral Microbiology and Immunology, DRI, and BK21 Plus Program, School of Dentistry, Seoul National University, Seoul, Republic of Korea. Electronic address: shhan-mi@snu.ac.kr.
authors.forename: Seung Hyun
authors.initials: SH
authors.lastname: Han
authors.affiliation: Clinical Immunology, Sciences Unit, International Vaccine Institute, Seoul, Republic of Korea. Electronic address: jsyang@ivi.int.
authors.forename: Jae Seung
authors.initials: JS
authors.lastname: Yang
mesh_terms: D000818:Animals; D000907:Antibodies, Bacterial / Q000097:blood; D057205:Automation, Laboratory / Q000379:methods*; D001770:Blood Bactericidal Activity; D015169:Colony Count, Microbial / Q000379:methods*; D003165:Complement System Proteins / Q000276:immunology; D006168:Guinea Pigs; D006801:Humans; D007118:Immunoassay / Q000379:methods*; D050296:Microbial Viability; D011135:Polysaccharides, Bacterial / Q000276:immunology; D011817:Rabbits; D012485:Salmonella typhi / Q000276:immunology* / Q000502:physiology; D012680:Sensitivity and Specificity; D016896:Treatment Outcome; D014436:Typhoid-Paratyphoid Vaccines / Q000008:administration & dosage / Q000276:immunology*
chemical_list: D000907:Antibodies, Bacterial; D011135:Polysaccharides, Bacterial; D014436:Typhoid-Paratyphoid Vaccines; C054154:capsular polysaccharide, Salmonella; D003165:Complement System Proteins
keywords: Functional antibody; Salmonella enterica serovar Typhi; Serum bactericidal assay; Typhoid vaccine; anti-Vi IgG