# The innate immune protein calprotectin ablates the bactericidal activity of β-lactam antibiotics

## TL;DR

The immune protein calprotectin reduces the effectiveness of beta-lactam antibiotics by binding zinc, which is needed for bacterial cell death.

## Contribution

This study identifies calprotectin as a host factor that causes antibiotic tolerance by sequestering zinc and inhibiting bacterial autolysins.

## Key findings

- Calprotectin induces antibiotic tolerance in Staphylococcus aureus by chelating zinc and inactivating autolysins.
- CP-deficient mice showed improved oxacillin efficacy, confirming the role of calprotectin in vivo.
- Zinc availability at infection sites is a critical determinant of beta-lactam antibiotic effectiveness.

## Abstract

Antibiotic failure is a major clinical problem that cannot always be explained by traditional resistance mechanisms. This study reveals that the host immune protein calprotectin, which is abundant at infection sites, can interfere with β-lactam antibiotics by sequestering zinc and inactivating bacterial autolysins that mediate cell lysis. This finding uncovers a mechanism of antibiotic tolerance that arises from the host response itself, rather than from the pathogen. Understanding how immune factors like calprotectin alter antibiotic activity opens the door to host-targeted strategies that enhance treatment outcomes. These findings suggest that local zinc availability during infection could be a critical, and previously underappreciated, determinant of β-lactam efficacy.

β-lactam antibiotics are among the most widely used treatments for bacterial infections, yet therapeutic failure is common even when no genetic resistance is detected. Understanding how host factors influence antibiotic efficacy is critical for improving outcomes. Here, we identify a host-derived mechanism of antibiotic tolerance mediated by calprotectin (CP), a zinc-binding protein released in large quantities by neutrophils during infection. We show that CP induces tolerance to β-lactam antibiotics in Staphylococcus aureus by chelating zinc and inactivating autolysins, zinc-dependent enzymes required for cell wall degradation and bacterial lysis following β-lactam treatment. This protective effect was specific to β-lactam antibiotics at concentrations of CP showing minimal impact on bacterial growth or metabolic state. Mechanistic studies revealed that CP inhibits the autolytic activity of Atl, the major S. aureus autolysin, by depriving the enzyme of its zinc cofactor. In a murine infection model, the efficacy of oxacillin was significantly enhanced in CP-deficient mice, demonstrating that CP impairs β-lactam activity in vivo. These findings reveal a form of immune-mediated antibiotic tolerance driven by metal sequestration and suggest that zinc availability at infection sites plays a critical role in shaping treatment outcomes.