Demonstration of primary and asymptomatic DNAaemia in participants challenged with Salmonella Typhi (Quailes strain) during the development of a human model of typhoid infection

Background: Typhoid infection remains amajor cause of global morbidity. Effective vaccination programmes and new diagnostic tests are urgently needed but are hindered by incomplete understanding of S.Typhi pathogenesis, in part due to insufficiently sensitive methods for detecting bacteria in the peripheral circulationof thoseencountering infection.Here, new insights into S.Typhi pathogenesis gained using a culture-PCR methodology during a human typhoid challenge model are described. Methods: 40 healthy adult participants were challenged with S.Typhi (Quailes strain) at doses of 1-5×103 or 1-5×104 colonyforming units. During the 2-weeks after challenge, participants were reviewed daily with clinical data and specimen collection, including blood drawn for ‘routine’ microbiological culture and a novel culture-PCR assay. For this, 5mL venous whole-blood was collected into heparin prior to 5-hour culture in 5% ox-bile tryptone soya broth. Centrifugation was performed to collect the blood pellet/bacteria and DNA was extracted using a commercial bloodspin kit. PCR using primers to amplify the fliC-d gene was performed. Results: Bacterial DNA was detected in the peripheral circulation in 57/684 (8.3%) culture-PCR and 53/674 (7.9%) routine blood culture samples. Positive culture-PCR results were detected from 12hours after oral challenge; 10/40 participants had positive culture-PCR results (but negative routine cultures) within 5 days of challenge. Seven of these participants went on to develop typhoid infection during the 2-week challenge period (typhoid diagnosis defined by development of bacteraemia or persistent fever >38 ◦C for >12-hours). DNA was detected in the peripheral circulation of 5/40 participants who were not diagnosed with typhoid infection during the challenge period. Several of these participants had mild symptoms or elevation of inflammatory markers (including C-reactive protein) only. Conclusion:Thesedata suggest that a culture-PCRmethodology targeting the fliC-d gene may be used to detect DNA in peripheral blood of those challenged withS.Typhi. Aside from unique confirmation that the mechanism of typhoid infection includes primary dissemination of bacteria in the peripheral circulation, we also demonstrate that asymptomatic infection/circulation of bacteria maybemore common than previously anticipated. Sensitive detection of S.Typhi DNA in peripheral blood samples may represent a useful additional endpoint in the evaluation of typhoid vaccines.