Strain Specific Variations in Acinetobacter baumannii Complement Sensitivity

Area of Science:

• Immunology
• Microbiology
• Infectious Diseases

Background:

• The complement system is crucial for innate immunity against bacterial infections, including those caused by Acinetobacter baumannii.
• Antibiotic-resistant A. baumannii poses a significant threat, and its susceptibility to complement-mediated killing varies among strains.
• Understanding the mechanisms behind this variation is essential for developing effective therapeutic strategies.

Purpose of the Study:

• To investigate the complement sensitivity phenotypes of nine clinical Acinetobacter baumannii strains.
• To identify factors contributing to the differential susceptibility of A. baumannii strains to complement-mediated attack, particularly the membrane attack complex (MAC).
• To correlate complement component binding and MAC formation with bacterial serum resistance and phagocytosis.

Main Methods:

• Flow cytometry assays were used to quantify opsonization with C3b/iC3b and MAC deposition on nine clinical A. baumannii strains.
• Neutrophil phagocytosis assays were performed to assess the impact of opsonization on bacterial clearance.
• Genomic analysis and RNA sequencing (RNAseq) were employed to identify genetic differences and gene regulation related to complement resistance.

Main Results:

• Significant variations in C3b/iC3b and MAC binding were observed among the A. baumannii strains, partially independent of capsule characteristics.
• Opsonization with C3b/iC3b enhanced neutrophil phagocytosis for most strains.
• While MAC formation correlated with C3b/iC3b binding, it did not directly correlate with overall serum resistance, suggesting additional resistance mechanisms.

Conclusions:

• Clinical A. baumannii strains exhibit diverse sensitivities to different components of the complement system.
• Bacterial capsule influences complement deposition, but other factors also modulate strain-specific complement resistance.
• Serum resistance in A. baumannii is multifactorial, involving mechanisms beyond the extent of MAC formation on the bacterial surface.