The whole-genome molecular epidemiology of sequential isolates of Acinetobacter baumannii colonizing the rectum of patients in an adult intensive care unit of a tertiary hospital.

is a nosocomial multidrug-resistant pathogen that specifically colonizes and infects patients in intensive care units (ICUs). Genome plasticity of may contribute to the rapid development of resistance during patient treatment. There is little clarity on the dynamics of colonization of in humans. We studied 269 serial isolates of colonizing the rectum of 32 adult patients in an ICU; 5 to 16 isolates were obtained from each patient over a period of weeks to months. The isolates were sequenced using the Illumina platform. Multi-locus sequence typing revealed genetic diversity with 13 sequence types (STs); the dominant ST2 accounted for 202 of the 269 isolates and was isolated from each patient. A core genome comparison of isolates was used to elucidate a detailed understanding of the relationship among isolates within an ST and the genomic relationship among STs. One to three STs were observed in the isolates from each patient. Instances of unchanged colonization, sequential lineage colonization, and colonization with multiple lineages of were seen. The number of resistance genes carried by the isolates varied from 2 to 17. The predominant genes corresponded to those encoding resistance to β-lactam and aminoglycoside antibiotics. Generally, there was a strong correlation between ST and a particular antimicrobial resistance gene profile. This study makes an important baseline contribution to information that will help us understand the role of colonization in the development of opportunistic infections in ICU patients. IMPORTANCE is a multidrug-resistant nosocomial pathogen that colonizes and infects debilitated patients in the ICU. There is very little information on the genomic characteristics of colonizing strains. This information is important to understand the evolution of lineages of that develop resistance while patients receive antibiotic treatment in the ICU. Our study demonstrated different patterns of colonization of the rectum of ICU patients with different STs of while one ST colonized all patients. Some STs carried more antibiotic resistance genes compared to others. However, there was a correlation between ST and a particular resistance gene profile. Our results further elucidate the dynamics of enteric colonization of this opportunistic pathogen.