Intrinsic determinants of susceptibility to a lytic interbacterial toxin
University of California- San Francisco
(2016-2023)

In the wild, most microbes live in communities composed of many different species living together (e.g. the gut microbiome, microbial mats). These can be harsh environments where organisms compete for limited space and nutrition. To aid their survival, microbes have developed many methods to discriminate between neighboring cells to figure out who is their competitor and who is their friend. One such method, used by nearly 25% of Gram-negative bacteria, is the type VI secretion system (T6SS).

The T6SS is a spring-loaded macro-molecular machine that transports protein toxins (called "effectors") from a donor cell directly into a recipient cell. Upon extended physical contact, a donor cell internally assembles a syringe-like secretion apparatus that can be as long as the entire cell (!). Then, effectors are either inserted onto or inside the secretion apparatus. In a single contractile step, the T6SS is injected into the recipient cell, where the effectors are released and can wreak deadly havoc via a diverse array of toxic activities.

A major question in this field concerns what types of organisms can be successfully targeted by T6SS attack. Some recipient bacteria are completely resistant to T6SS attacks, primarily via the protective benefits of antitoxins which block the activity of T6SS effectors. However, across microbes there exists a spectrum of susceptibility that cannot be fully explained by the presence or absence of antitoxins.

The goal of my graduate dissertation was to understand the basis of T6SS susceptibility outside of toxin:antitoxin immunity. I studied this through the lens of a cell wall-degrading effector, Tae1, that has a dominant role in the toxicity of a T6SS encoded by Pseudomonas aeruginosa (Pae). Using cutting-edge functional genomics, cell biology, and biochemistry, I found that the susceptibility of Escherichia coli (Eco) to Tae1 is a consequence of its hard-wired, hyperactive response to cell wall damage by the toxin. We found that throttling this hyperactive response (by manipulation of an indirect, but related pathway in cell envelope metabolism) offered novel protection to Tae1 despite extensive cell wall damage. This work suggests that the hard-wired (and potentially co-dependent) activites of essential pathways in Eco participate in T6SS susceptiblity in vivo. The dynamic, responsive nature of living bacterial cells to their stressors offers an additional avenue for understanding the principles for T6SS toxicity beyond direct toxin:substrate and toxin:antitoxin interactions.

Related Publications

Trotta K (2023). Physiological requirements for lysis by a peptidoglycan-degrading interbacterial toxin. UCSF.

Dissertation: https://escholarship.org/uc/item/0q21g54f

Trotta KL, Hayes BM, Schneider JP, Wang J, Todor H, Grimes PR, Zhao Z, Hatleberg

WL, Silvis MR, Kim R, Koo BM, Basler M, Chou S (2023). Lipopolysaccharide transport regulates bacterial sensitivity to a cell wall-degrading intermicrobial toxin. PLOS Pathogens.

DOI: 10.1371/journal.ppat.1011454 (PLOS Pathogens)

DOI: 10.1101/2023.01.20.524922 (Biorxiv)

Mitchell G, Silvis MR, Talkington KC, Budzik JM, Dodd CE, Paluba JM, Oki EA, Trotta KL, Licht DJ, Jimenez-Morales D,Chou S, Savage PB, Gross CA, Marletta MA, Cox JS (2022). Ceragenins and antimicrobial peptides kill bacteria through distinct mechanisms. mBio.

DOI: 10.1128/mbio.02726-21 (mBio)

DOI: 10.1101/2020.10.20.346411 (Biorxiv)

Intrinsic determinants of susceptibility to a lytic interbacterial toxin University of California- San Francisco (2016-2023)

Intrinsic determinants of susceptibility to a lytic interbacterial toxin
University of California- San Francisco
(2016-2023)