Travellers returning from the island of Zanzibar colonized with MDR Escherichia coli strains: Assessing the impact of local people and other sources

Objectives: Many travellers to low-income countries return home colonized at the intestinal level with extended-spectrum cephalosporin-resistant (ESC-R) and/or colistin-resistant (CST-R) Escherichia coli (Ec) strains. However, nothing is known about the local sources responsible for the transmission of these pathogens to the travellers. Methods: We compared the ESC-R- A nd CST-R-Ec strains found in the pre-(n = 23) and post-trip (n = 37) rectal swabs of 37 travellers from Switzerland to Zanzibar with those (i) contemporarily isolated from local people, poultry, retailed chicken meat (n = 31), and (ii) from other sources studied in the recent past (n = 47). WGS and core-genome analyses were implemented. Results: Twenty-four travellers returned colonized with ESC-R-(n = 29) and/or CST-R-(n = 8) Ec strains. Almost all ESC-R-Ec were CTX-M-15 producers and belonged to heterogeneous STs/core-genome STs (cgSTs), while mcr-positive strains were not found. Based on the strains' STs/cgSTs, only 20 subjects were colonized with ESC-R- A nd/or CST-R-Ec that were not present in their gut before the journey. Single nucleotide variant (SNV) analysis showed that three of these 20 travellers carried ESC-R-Ec (ST3489, ST3580, ST361) identical (0-20 SNVs) to those found in local people, chicken meat, or poultry. Three further subjects carried ESC-R-Ec (ST394, ST648, ST5173) identical or highly related (15-55 SNVs) to those previously reported in local people, fish, or water. Conclusions: This is the first known study comparing the ESC-R- A nd/or CST-R-Ec strains obtained from travellers and local sources using solid molecular methods. We showed that for at least one-third of the returning travellers the acquired antibiotic-resistant Ec had a corresponding strain among resident people, food, animal and/or environmental sources.