AUTHOR=Gong Xiaoxia , Zhao Qian , Wu Yifan , Zhou Hongwei , Ding Shuangyang , Zhu Kui 

TITLE=Mucoid Acinetobacter baumannii enhances anti-phagocytosis through reducing C3b deposition

JOURNAL=Frontiers in Medicine

VOLUME=9

YEAR=2022

URL=https://www.frontiersin.org/journals/medicine/articles/10.3389/fmed.2022.879361

DOI=10.3389/fmed.2022.879361

ISSN=2296-858X

ABSTRACT=<sec><title>Background</title><p>Multidrug resistant (MDR) <italic>Acinetobacter baumannii</italic> causes serious infections in intensive care units and is hard to be eradicated by antibiotics. Many <italic>A. baumannii</italic> isolates are identified as the mucoid type recently, but the biological characteristics of mucoid <italic>A. baumannii</italic> and their interactions with host cells remains unclear.</p></sec><sec><title>Methods</title><p>The mucoid phenotype, antimicrobial susceptibility, biofilm-forming ability, acid resistance ability, peroxide tolerance, and <italic>in vivo</italic> toxicity of clinical ICUs derived <italic>A. baumannii</italic> isolates were first investigated. Secondly, the phagocytic resistance and invasive capacity of <italic>A. baumannii</italic> isolates to macrophages (MH-S, RAW264.7) and epithelial cells (A549) were analyzed. Furthermore, the abundance of C3b (complement factor C3 degradation product) deposition on the surface of <italic>A. baumannii</italic> was investigated. Last, the relationship between C3b deposition and the abundance of capsule in <italic>A. baumannii</italic> isolates were analyzed.</p></sec><sec><title>Results</title><p>These <italic>A. baumannii</italic> strains showed different mucoid phenotypes including hyper mucoid (HM), medium mucoid (MM), and low mucoid (LM). All tested strains were MDR with high tolerance to either acid or hydrogen peroxide exposure. Notably, these mucoid strains showed the increase of mortality in the <italic>Galleria mellonella</italic> infection models. Besides, the HM strain exhibited less biofilm abundance, higher molecular weight (MW) of capsule, and greater anti-phagocytic activity to macrophages than the LM strain. Together with the increased abundance of capsule, high expression of <italic>tuf</italic> gene (associated with the hydrolysis of C3b), the HM strain effectively inhibits C3b deposition on bacterial surface, resulting in the low-opsonization phenotype.</p></sec><sec><title>Conclusion</title><p>Capsular characteristics facilitate the anti-phagocytic activity in hyper mucoid <italic>A. baumannii</italic> through the reduction of C3b deposition. Mucoid <italic>A. baumannii</italic> exhibits high phagocytosis resistance to both macrophages and epithelial cells.</p></sec>