Carriage and within-host diversity of mcr-1.1-harboring Escherichia coli from pregnant mothers: inter- and intra-mother transmission dynamics of mcr-1.1

Background- Inter- and intra-species exchange of antimicrobial resistance genes (ARGs) primarily via mobile genetic elements take place in the gut. ARGs or antibiotic-resistant isolates can be transferred to neonate from the mother’s gut. This study evaluated carriage of mcr in pregnant mothers and their neonates along with deciphering transmission dynamics of mcr . Methods- Sample collected from pregnant mothers (rectal) and ill neonates (rectal & blood) were analyzed in terms of presence of mcr genes and its transmissibility, occurrence of plasmids and genome diversity of mcr- positive isolates, within an individual and across different individuals, to understand the exchange of mcr between isolates. Mother-to-baby transmission of mcr could not be assessed as babies of mcr -positive mothers were healthy and not included in the study. Results- mcr-1.1 was found in rectal samples of healthy pregnant mothers (0.9%), but not in the neonates. mcr - 1.1 was detected in Escherichia coli . Isolates were susceptible to most antibiotics except colistin with few virulence genes and primarily belonged to phylogroup A and B1, which are considered as commensals. Twenty-five mcr –negative E. coli isolated from samples that carried mcr- positive isolates, were also studied. All E. coli were highly diverse and belonged to varied sequence types (STs), of which five were novel. Study isolates showed relatedness with mcr -bearing isolates from Bangladesh, Vietnam, Singapore, Thailand and Laos. mcr-1.1 was mainly found in conjugative IncHI2 (~ 216-241kb), bracketed between IS Apl1 on Tn 6630. One isolate had mcr-1.1 in IncX4 (~ 33kb). Clonal spread of mcr -bearing bacteria between different mothers (inter-mother), and spread of identical mcr -bearing plasmid in same or different mothers was noted. Spread of identical mcr-1.1 -bearing IncHI2 plasmid in different mothers suggested a role of the plasmid in spread of this gene. Unlike global isolates, study isolates were susceptible to most antibiotics tested, but showed similar IncHI2 plasmid harboring mcr-1.1 . Conclusion- This is the first study to analyze presence of mcr-1.1 in pregnant mothers’ and neonates’ gut. This study highlighted transmission of mcr-1.1 within and between different healthy mothers, further underlining efficiency of mcr-1.1 plasmid in transmission. Such an event is concerning since it may enhance the chances of mother-to-neonate transmission.