A new study led by Yuqin Song and Jie Feng at the Institute of Microbiology, Chinese Academy of Sciences, has identified a previously unrecognized genetic mechanism that helps Acinetobacter baumannii adapt to its host, with implications for how clinicians understand and treat one of the most persistent pathogens in hospitals.
A. baumannii is a major nosocomial pathogen known for multidrug resistance and its ability to persist in clinical environments. The research shows that a mobile genetic element called ISAba1 drives amplification of the crp-osmC gene cluster, boosting bacterial growth, competitive fitness, serum survival and colonization in mouse models.
The researchers analyzed 582 complete A. baumannii genomes and found ISAba1 was the insertion sequence most frequently associated with gene amplification. Beyond well-known antimicrobial resistance genes, they identified a recurrently amplified crp-osmC cluster flanked by ISAba1 elements, a structure designated composite transposon Tn7691. Screening 7,490 genomes revealed a strong association between Tn7691 and crp-osmC amplification and the globally prevalent high-risk ST2 lineage and its single-locus variants.
Experimentally increasing the copy number of the cluster improved growth at later stages and gave the bacteria a clear competitive advantage in co-culture. Amplification also enhanced fitness in mouse pulmonary infection and intestinal colonization models, and improved survival and competitive fitness in mouse serum.
Transcriptomic analysis linked the amplification to increased expression of genes involved in phenylalanine metabolism, siderophore biosynthesis and sulfur metabolism, suggesting improved metabolic flexibility and nutrient acquisition under nutrient-limited host conditions. CRP retained its DNA-binding activity and directly bound the promoter of the siderophore-associated bas gene cluster, supporting its regulatory role in iron acquisition.
The adaptive advantage came with apparent costs. The amplified strain showed reduced expression of genes involved in pili and biofilm formation, decreased biofilm formation and, unexpectedly, increased susceptibility to multiple antibiotics. The findings suggest enhanced host-associated fitness may be accompanied by a trade-off in antibiotic resistance.
The work, published in mLife, expands the conventional view that insertion sequence-mediated gene amplification primarily drives antimicrobial resistance. It demonstrates that ISAba1-mediated amplification can also reshape host-associated fitness, offering new insight into the genomic plasticity, adaptive evolution and clinical success of A. baumannii.
For hospitals and drug developers, the trade-off is the most consequential finding. A pathogen that gains a foothold in patients may simultaneously lose some of its defenses, raising the possibility that treatment strategies could exploit this vulnerability. The identification of Tn7691 as a marker tied to the high-risk ST2 lineage also gives surveillance efforts a potential signal for tracking strains that are especially suited to host colonization.
