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.
Highlights
Typhoid fever is a gastrointestinal infectious disease caused by Salmonella enterica serovar Typhi that is transmitted through the ingestion of contaminated food or water
Luria-Bertani (LB) broth and agar were purchased from Conda (Madrid, Spain) and Junsei (Tokyo, Japan), respectively. 2,3,5-triphenyl tetrazolium chloride (TTC) and 4-nitrophenyl phosphate disodium hexahydrate were purchased from Sigma-Aldrich
Human convalescent sera from cholera patients showed that bactericidal activity against V. cholerae was inversely proportional to the dilution fold of serum, suggesting that serum bactericidal assay (SBA) titer could be determined by a microtiter plate reader
Summary
Typhoid fever is a gastrointestinal infectious disease caused by Salmonella enterica serovar Typhi that is transmitted through the ingestion of contaminated food or water. Risk factors for the disease are high in developing countries due to poor hygiene and sanitation, and the global burden of typhoid fever in 2010 was estimated at 26.9 million cases [1]. Typhoid fever is treated with antibiotics but it has become complicated by the emergence of multi-drug resistant strains of S. The average cost of medical care for typhoid fever is estimated at $4500 per patient in the United States [3]. Reasonably-priced vaccine would be a cost-effective approach to prevent typhoid fever [4]
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