# Campylobacter jejuni resistance to human milk involves the acyl carrier protein AcpP

## TL;DR

This study shows how Campylobacter jejuni bacteria can evolve to resist the antimicrobial effects of human milk, potentially allowing them to thrive in breastfed infants.

## Contribution

The study identifies mutations in the acyl carrier protein AcpP as a novel mechanism for C. jejuni resistance to human milk.

## Key findings

- Evolving C. jejuni in human milk leads to mutations in AcpP and PorA.
- AcpP changes improve bacterial growth in human milk and alter membrane architecture.
- These changes protect bacteria from bacteriophage and polymyxin.

## Abstract

Campylobacter jejuni is a common foodborne pathogen worldwide that is associated with high rates of morbidity and mortality among infants in low- to middle-income countries (LMICs). Human milk provides infants with an important source of nutrients and contains antimicrobial components for protection against infection. However, recent studies, including our own, have found significantly higher levels of Campylobacter in diarrheal stool samples collected from breastfed infants compared to non-breastfed infants in LMICs. We hypothesized that C. jejuni has unique strategies to resist the antimicrobial properties of human milk. Transcriptional profiling found human milk exposure induces genes associated with ribosomal function, iron acquisition, and amino acid utilization in C. jejuni strains 81–176 and 11168. However, unidentified proteinaceous components of human milk prevent bacterial growth. Evolving both C. jejuni isolates to survive in human milk resulted in mutations in genes encoding the acyl carrier protein (AcpP) and the major outer membrane porin (PorA). Introduction of the PorA/AcpP amino acid changes into the parental backgrounds followed by electron microscopy showed distinct membrane architectures, and the AcpP changes not only significantly improved growth in human milk, but also yielded cells surrounded with outer membrane vesicles. Analyses of the phospholipid and lipooligosaccharide (LOS) compositions suggest an imbalance in acyl chain distributions. For strain 11168, these changes protect both evolved and 11168∆acpPG33R strains from bacteriophage infection and polymyxin killing. Taken together, this study provides insights into how C. jejuni may evolve to resist the bactericidal activity of human milk and flourish in the hostile environment of the gastrointestinal tract.

In this study, we evolved C. jejuni strains which can grow in the presence of human milk and found that cell membrane alterations may be involved in resistance to the antimicrobial properties of human milk. These bacterial membrane changes are predominantly linked to amino acid substitutions within the acyl carrier protein, AcpP, although other bacterial components, including PorA, are likely involved. This study provides some insights into possible strategies for C. jejuni survival and propagation in the gastrointestinal tract of breastfed infants.