Curli-independent defense against <i>Bdellovibrio bacteriovorus</i> in <i>E. coli</i>

Predatory bacteria are a group of organisms that use diverse methods to access nutrients and grow by killing prey bacteria. The predator <i>Bdellovibrio bacteriovorus</i> is capable of preying on a wide range of gram-negative bacteria by invading the periplasmic space, killing, digesting, and ultimately lysing prey cells. <i>B. bacteriovorus</i>, like a phage, replicates at the expense of its host, yet unlike phage defense, there are few characterized mechanisms for bacteria to resist <i>B. bacteriovorus</i>. Previously, we discovered that an extracellular amyloid protein called curli protects <i>Escherichia coli</i> from <i>B. bacteriovorus</i>. Here, we searched for additional modes of <i>B. bacteriovorus</i> resistance and identified a strain within the <i>E. coli</i> 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 <i>B. bacteriovorus</i>. 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 <i>B. bacteriovorus</i> viability. Our work demonstrates that <i>E. coli</i> encode curli-independent mechanisms that restrict <i>B. bacteriovorus</i> and expand our understanding of the antipredatory bacteria arm of the bacterial immune system.IMPORTANCEUnderstanding host-pathogen interactions has the potential to illuminate fundamental aspects of biology. Here, we investigate an atypical host-pathogen system, the interaction between <i>Escherichia coli</i> and the predatory bacterium <i>Bdellovibrio bacteriovorus. B. bacteriovorus</i> 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 <i>B. bacteriovorus</i> 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.