# Curli-independent defense against Bdellovibrio bacteriovorus in E. coli

## TL;DR

This study identifies a new way Escherichia coli can resist the predatory bacterium Bdellovibrio bacteriovorus without using curli proteins.

## Contribution

The discovery of a curli-independent resistance mechanism in E. coli involving LPS-modifying enzymes expands understanding of bacterial defenses against predators.

## Key findings

- ECOR29 strain of E. coli resists B. bacteriovorus independently of curli proteins.
- LPS-modifying enzymes and outer membrane alterations are critical for resistance in ECOR29.
- No extracellular structures or secreted factors were found to mediate resistance in ECOR29.

## Abstract

Predatory bacteria are a group of organisms that use diverse methods to access nutrients and grow by killing prey bacteria. The predator Bdellovibrio bacteriovorus is capable of preying on a wide range of gram-negative bacteria by invading the periplasmic space, killing, digesting, and ultimately lysing prey cells. B. bacteriovorus, like a phage, replicates at the expense of its host, yet unlike phage defense, there are few characterized mechanisms for bacteria to resist B. bacteriovorus. Previously, we discovered that an extracellular amyloid protein called curli protects Escherichia coli from B. bacteriovorus. Here, we searched for additional modes of B. bacteriovorus resistance and identified a strain within the E. coli Reference (ECOR) collection, ECOR29, that uses a curli-independent mechanism that requires lipopolysaccharide (LPS)-modifying enzymes for defense. Over 30% of the ECOR collection is resistant to B. bacteriovorus. We successfully deleted the gene encoding the major curli subunit in many of these, and only ECOR29 remained resistant. We hypothesized that ECOR29 encoded an alternative resistance mechanism and identified determinants of defense using a forward genetic screen. Our screen revealed critical roles for enzymes that modify LPS, alter the outer membrane, and are homologous to plasmid partitioning systems. Examination of ECOR29 by electron microscopy did not identify overt phenotypes or visible alterations to extracellular structures. We also were unable to identify any secreted factors that impacted B. bacteriovorus viability. Our work demonstrates that E. coli encode curli-independent mechanisms that restrict B. bacteriovorus and expand our understanding of the antipredatory bacteria arm of the bacterial immune system.

Understanding host-pathogen interactions has the potential to illuminate fundamental aspects of biology. Here, we investigate an atypical host-pathogen system, the interaction between Escherichia coli and the predatory bacterium Bdellovibrio bacteriovorus. B. bacteriovorus has a unique predatory life cycle that requires intimate interactions with the outer membrane, periplasm, peptidoglycan, and inner membrane of prey cells. Accordingly, understanding mechanisms of B. bacteriovorus predation and resistance will help us to better understand the gram-negative cell envelope, an ideal target for novel antibacterial compounds. Predatory bacteria are abundant and ubiquitous threats to bacteria in a wide variety of environments. Further findings from experiments in this field will expand our understanding of some of the most basic aspects of the microbial world.