Background: Acinetobacter baumannii  is a nosocomial pathogen of critical importance due to the increasing numbers of antibiotic-resistant isolates. Colonies can have a smooth or matt appearance, but also exhibit slimy, mucoid growth, with the latter being increasingly isolated in patients in recent years.
Methods: We isolated 60 A. baumannii  strains from altogether 56 patients and found that all patients were infected by mucoid strains, with four patients having also matt phenotypes in addition to the mucoid ones. The morphology of the colonies and capsules was observed. The antibiotics susceptibilities were tested, and the biofilm formation ability was determined by crystal violet staining. The whole-genome sequencing (WGS) was performed on all the strains, and then the core genome multilocus sequence typing (cgMLST) and drug resistance gene analysis were performed. Finally, a part of isolates were selected to test virulence in a Galleria mellonella  model.
Results: We observed much larger capsules in the mucoid strains compared to the matt isolates. But the mucoid phenotype did not correlate with the amount of biofilm produced by the strain. Almost all mucus-type A. baumannii  were multi-drug resistant isolates, containing various antibiotic resistance genes. The main ST types of mucoid-type A.baumannii  were ST191 and ST195, of which ST191 isolates were more virulence, while ST195 isolates were weaker.
Conclusion: The mucoid A. baumannii  had resistance to most antibiotics and some strains had high virulence, which should be paid attention in clinical.
Keywords: Acinetobacter baumannii , capsule, biofilm formation, mucoid phenotype