Fig. 4.

(A) The 6020 bp integron is derived from the p1AB5075 plasmid and provides spectinomycin and kanamycin resistance. The int1 gene encodes an integrase with embedded promoter and cmlA5 encodes a family chloramphenicol efflux MFS transporter. The remaining genes encode aminoglycoside inactivating proteins, where kanamycin resistance is encoded by ant(2″)-Ia and spectinomycin resistance is encoded by aadA3, aph(3″)-1b and aph(6)-Id based on the ontology from the Comprehensive Antibiotic Resistance Database (CARD) (Alcock et al. 2020). (B) Replicate kanamycin-resistant clones were taken from population B12 and passaged for 7 days with or without kanamycin. The rapid loss of kanamycin resistance in growth media without kanamycin suggests that the integron carrying kanamycin resistance confers a strong fitness cost. Notably, even when cultures were supplemented with kanamycin, only around 10% of clones tested were kanamycin-resistant, suggesting that the integron was not faithfully inherited during each cell division.