OBJECTIVES:Acquisition of extended-spectrum β-lactamase-producing Enterobacteriaceae (ESBL-E) by Europeans travelling individually in high-endemicity countries is common. However, how the different ESBL-E strains circulate in groups of travellers has not been studied. We investigated ESBL-E transmission within several groups of French military personnel serving overseas for 4-6 months.METHODS:We conducted a prospective study among French military personnel assigned to Afghanistan, French Guiana or Côte d'Ivoire for 4-6 months. Faecal samples provided by volunteers before leaving and after returning were screened for ESBL-E isolates. ESBL Escherichia coli from each military group was characterized by repetitive element palindromic polymerase chain reaction (rep-PCR) fingerprinting followed, in the Afghanistan group, by whole-genome sequencing (WGS) if similarity was ≥97%.RESULTS:Among the 189 volunteers whose samples were negative before departure, 72 (38%) were positive after return. The highest acquisition rates were observed in the Afghanistan (29/33, 88%) and Côte d'Ivoire (39/80, 49%) groups. Acquisition rates on return from French Guiana were much lower (4/76, 5%). WGS of the 20 strains from the Afghanistan group that clustered by rep-PCR identified differences in sequence type, serotype, resistance genes and plasmid replicons. Moreover, single-nucleotide polymorphism (SNP) differences across acquired strains from a given cluster ranged from 30 to 3641, suggesting absence of direct transmission.CONCLUSIONS:ESBL-E. coli acquisition was common among military personnel posted overseas. Many strains clustered by rep-PCR but differed by WGS and SNP analysis, suggesting acquisition from common external sources rather than direct person-to-person transmission.