Inter- and intra-bacterial strain diversity remains the "elephant in the (living) room".

Acinetobacter baumannii is an opportunistic Gram-negative bacterial pathogen responsible for severe nosocomial infections worldwide. Resistance to last-resort antibiotics causes A. baumannii to be ranked as a top priority for the research and development of new antibiotics by the WHO and an urgent threat to public health by the CDC. It is also a member of the ESKAPE group comprising the most problematic antibiotic-resistant nosocomial pathogens. Resistance towards desiccation, disinfectants, reactive oxygen species, and the host immune system helps A. baumannii thrive in hospital settings and infect individuals compromised by lines, tubes, and indwelling devices. A. baumannii displays extensive genomic heterogeneity, yet recent studies show that this level of plasticity is also prevalent in lab strains widely used to study A. baumannii biology. Successive subculturing of widely used strains and spontaneous genetic variations results in significantly altered genotypes and phenotypes, often not recognized by the scientific community. In addition, the current strain designation methods do not allow efficient communication about such differences. Even presumably identical strains from established culture collections have been found to demonstrate genetic heterogeneity. The "elephant in the (living) room" refers to the risk but also the potential of the bi-partite problem concerning the high diversity amongst A. baumannii isolates (inter-strain variability), and the universal issue of microevolution (intra-strain variability). This is generally ignored as it is not referenced adequately in scientific publications. We aim to raise awareness about the current issues and the problematic consequences generated by intra- and inter-strain diversity based on modern examples of A. baumannii isolates. Therefore, this review provides cases of broadly used A. baumannii strains and their genetic and phenotypic differences.