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Updated: Aug 26, 2025
Alveolar Macrophage Phagocytosis and Bacteria Clearance in Mice
Published on: March 2, 2019
Mucoid Acinetobacter baumannii enhances anti-phagocytosis through reducing C3b deposition
Xiaoxia Gong1, Qian Zhao1,2, Yifan Wu1
1National Center for Veterinary Drug Safety Evaluation, College of Veterinary Medicine, China Agricultural University, Beijing, China.
Background:
Multidrug resistant (MDR) Acinetobacter baumannii causes serious infections in intensive care units and is hard to be eradicated by antibiotics. Many A. baumannii isolates are identified as the mucoid type recently, but the biological characteristics of mucoid A. baumannii and their interactions with host cells remains unclear.
Methods:
The mucoid phenotype, antimicrobial susceptibility, biofilm-forming ability, acid resistance ability, peroxide tolerance, and in vivo toxicity of clinical ICUs derived A. baumannii isolates were first investigated. Secondly, the phagocytic resistance and invasive capacity of A. baumannii 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 A. baumannii was investigated. Last, the relationship between C3b deposition and the abundance of capsule in A. baumannii isolates were analyzed.
Results:
These A. baumannii 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 Galleria mellonella 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 tuf gene (associated with the hydrolysis of C3b), the HM strain effectively inhibits C3b deposition on bacterial surface, resulting in the low-opsonization phenotype.
Conclusion:
Capsular characteristics facilitate the anti-phagocytic activity in hyper mucoid A. baumannii through the reduction of C3b deposition. Mucoid A. baumannii exhibits high phagocytosis resistance to both macrophages and epithelial cells.
Insights
Multidrug-resistant Acinetobacter baumannii with a mucoid phenotype causes severe infections. Hyper mucoid strains resist phagocytosis by inhibiting complement C3b deposition, aiding bacterial survival.
Area of Science:
- Microbiology
- Infectious Diseases
- Bacterial Pathogenesis
Background:
- Multidrug-resistant (MDR) Acinetobacter baumannii is a significant threat in intensive care units.
- The increasing prevalence of mucoid Acinetobacter baumannii necessitates understanding its characteristics and host interactions.
Purpose of the Study:
- To investigate the biological characteristics of mucoid Acinetobacter baumannii isolates.
- To analyze the interaction of mucoid Acinetobacter baumannii with host cells, including macrophages and epithelial cells.
- To determine the role of capsular components and complement C3b deposition in bacterial virulence.
Main Methods:
- Phenotypic characterization of mucoid Acinetobacter baumannii isolates (mucoid type, antimicrobial susceptibility, biofilm formation, acid/peroxide tolerance).
- Assessment of phagocytic resistance and invasive capacity against macrophages and epithelial cells.
- Quantification of complement factor C3b deposition on bacterial surfaces and analysis of its relationship with capsule abundance.
Main Results:
- Mucoid Acinetobacter baumannii strains exhibited MDR and high tolerance to acid and hydrogen peroxide.
- Hyper mucoid (HM) strains showed increased virulence in Galleria mellonella models, reduced biofilm formation, higher capsule molecular weight, and enhanced anti-phagocytic activity.
- HM strains demonstrated significantly reduced C3b deposition on their surface, correlating with increased capsule abundance and high tuf gene expression, leading to a low-opsonization phenotype.
Conclusions:
- Capsular characteristics in hyper mucoid Acinetobacter baumannii are crucial for anti-phagocytic activity by reducing C3b deposition.
- Mucoid Acinetobacter baumannii demonstrates significant resistance to phagocytosis by both macrophages and epithelial cells, contributing to its pathogenicity.
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