Abstract

Francisella tularensis, a category-A bioterrorism agent causes tularemia. F. tularensis suppresses the immune response of host cells and intracellularly proliferates. However, the detailed mechanisms of immune suppression and intracellular growth are largely unknown. Here we developed a transposon mutant library to identify novel pathogenic factors of F. tularensis. Among 750 transposon mutants of F. tularensis subsp. novicida (F. novicida), 11 were isolated as less cytotoxic strains, and the genes responsible for cytotoxicity were identified. Among them, the function of slt, which encodes soluble lytic transglycosylase (SLT) was investigated in detail. An slt deletion mutant (Δslt) was less toxic to the human monocyte cell line THP-1 vs the wild-type strain. Although the wild-type strain proliferated in THP-1 cells, the number of intracellular Δslt mutant decreased in comparison. The Δslt mutant escaped from phagosomes during the early stages of infection, but the mutant was detected within the autophagosome, followed by degradation in lysosomes. Moreover, the Δslt mutant induced host cells to produce high levels of cytokines such as tumor necrosis factor-α, interleukin (IL)-6, and IL-1β, compared with the wild-type strain. These results suggest that the SLT of F. novicida is required for immune suppression and escape from autophagy to allow its survival in host cells.

Highlights

  • Francisella tularensis, a gram-negative, facultative intracellular bacterium causes tularemia in humans and animals [1]

  • F. novicida is cytotoxic to the human monocyte cell line THP-1, and cells detach from the culture plate

  • We developed a transposon mutant library of F. novicida to isolate mutants that were less cytotoxic to THP-1 cells

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Summary

Introduction

Francisella tularensis, a gram-negative, facultative intracellular bacterium causes tularemia in humans and animals [1]. Its reservoirs are rabbits and rodents, and it is transmitted to humans via routes such as arthropod bites and direct contact with infected animals [2]. F. tularensis is aerosolized and causes disease in humans at only 10 colony-forming units (CFUs) [3]. F. tularensis is considered a potential biological weapon and, as such, is considered a category-A bioterrorism agent [4].F. tularensis comprises the subspecies tularensis ( called type A), holarctica (type B), mediasiatica, and novicida.

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