Lethal effect of complement and lysozyme on polymyxin-treated, serum-resistant gram-negative bacilli

Area of Science:

• Microbiology
• Immunology
• Pharmacology

Background:

• Gram-negative bacteria can develop resistance to serum bactericidal activity.
• Antibiotic resistance poses a significant threat to public health.
• The complement system is a key component of innate immunity against bacterial infections.

Purpose of the Study:

• To investigate the effect of polymyxin B on serum resistance in Gram-negative bacteria.
• To determine the mechanism by which polymyxin B confers serum sensitivity.
• To explore the synergy between polymyxin B and the complement system.

Main Methods:

• Exposure of serum-resistant Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae, Citrobacter freundii) to polymyxin B.
• Assessment of bacterial susceptibility to normal human and rabbit sera.
• Evaluation of complement pathway involvement using various deficient sera (lysozyme-depleted, C2-deficient, C6-deficient).
• Detection of complement components (C3, C3b) on bacterial surfaces.

Main Results:

• Polymyxin B rendered serum-resistant E. coli, K. pneumoniae, and C. freundii susceptible to serum killing.
• Beta-lactam and aminoglycoside antibiotics did not confer serum sensitivity.
• Synergy was observed between polymyxin B and serum bactericidal activity.
• Killing of polymyxin B-treated bacteria occurred via the alternative complement pathway, but complement activation beyond C3 was implicated.

Conclusions:

• Polymyxin B can restore serum susceptibility to resistant Gram-negative bacteria.
• The mechanism involves the alternative complement pathway and likely affects complement activation beyond C3.
• Polymyxin B may enhance bacterial killing by modulating the host immune response.