Bacteria's Secret Weapon: How SapA Protein Helps Haemophilus influenzae Evade Immune Attack

Non-typeable Haemophilus influenzae (NTHi) is a major bacterial pathogen causing respiratory tract infections like otitis media and bronchitis. The human immune system deploys antimicrobial peptides (AMPs) as a first line of defense to kill invading bacteria. However, the exact mechanisms by which NTHi evades these crucial host defenses, particularly the role of specific bacterial proteins, have remained incompletely understood.

The study revealed that NTHi strains expressing SapA exhibited significantly enhanced survival rates when challenged with human AMPs. Specifically, wild-type NTHi showed a 3.5-fold higher survival rate compared to the ΔSapA mutant when exposed to LL-37 at 5 µg/mL (p<0.001). This indicates SapA provides robust protection against immune system attacks. The most critical finding was that purified SapA directly binds to human AMPs such as LL-37 and HBD-1 with high affinity, demonstrating a dissociation constant (K_D) of approximately 150 nM for LL-37. Furthermore, the ΔSapA mutant displayed increased membrane permeability, indicated by a 43% higher uptake of propidium iodide compared to the wild-type strain (p<0.01), suggesting SapA protects bacterial cell integrity. This protective effect was observed across multiple AMPs, confirming SapA's broad role in NTHi defense.