Abstract
BackgroundBurkholderia pseudomallei and Burkholderia mallei are gram-negative pathogens responsible for the diseases melioidosis and glanders, respectively. Both species cause disease in humans and animals and have been designated as category B select agents by the Centers for Disease Control and Prevention (CDC). Burkholderia thailandensis is a closely related bacterium that is generally considered avirulent for humans. While it can cause disease in rodents, the B. thailandensis 50% lethal dose (LD50) is typically ≥ 104-fold higher than the B. pseudomallei and B. mallei LD50 in mammalian models of infection. Here we describe an alternative to mammalian hosts in the study of virulence and host-pathogen interactions of these Burkholderia species.ResultsMadagascar hissing cockroaches (MH cockroaches) possess a number of qualities that make them desirable for use as a surrogate host, including ease of breeding, ease of handling, a competent innate immune system, and the ability to survive at 37°C. MH cockroaches were highly susceptible to infection with B. pseudomallei, B. mallei and B. thailandensis and the LD50 was <10 colony-forming units (cfu) for all three species. In comparison, the LD50 for Escherichia coli in MH cockroaches was >105 cfu. B. pseudomallei, B. mallei, and B. thailandensis cluster 1 type VI secretion system (T6SS-1) mutants were all attenuated in MH cockroaches, which is consistent with previous virulence studies conducted in rodents. B. pseudomallei mutants deficient in the other five T6SS gene clusters, T6SS-2 through T6SS-6, were virulent in both MH cockroaches and hamsters. Hemocytes obtained from MH cockroaches infected with B. pseudomallei harbored numerous intracellular bacteria, suggesting that this facultative intracellular pathogen can survive and replicate inside of MH cockroach phagocytic cells. The hemolymph extracted from these MH cockroaches also contained multinuclear giant cells (MNGCs) with intracellular B. pseudomallei, which indicates that infected hemocytes can fuse while flowing through the insect’s open circulatory system in vivo.ConclusionsThe results demonstrate that MH cockroaches are an attractive alternative to mammals to study host-pathogen interactions and may allow the identification of new Burkholderia virulence determinants. The importance of T6SS-1 as a virulence factor in MH cockroaches and rodents suggests that the primary role of this secretion system is to target evasion of the innate immune system.
Highlights
Burkholderia pseudomallei and Burkholderia mallei are gram-negative pathogens responsible for the diseases melioidosis and glanders, respectively
B. mallei, B. pseudomallei, and B. thailandensis are able to survive and replicate inside phagocytic cells in a process that involves escape from the endocytic vacuole, replication in the cytosol, intra- and intercellular spread by actin polymerization, and fusion with uninfected cells to form multinucleated giant cells (MNGCs) [4]
B. pseudomallei is virulent in the MH cockroach and Ccluster type VI secretion system (T6SS-1) mutants exhibit attenuated virulence In an attempt to determine if the MH cockroach might serve as a surrogate host for B. pseudomallei, we challenged juvenile MH cockroaches (Figure 1) with K96243 and T6SS mutant derivatives
Summary
Burkholderia pseudomallei and Burkholderia mallei are gram-negative pathogens responsible for the diseases melioidosis and glanders, respectively. Both species cause disease in humans and animals and have been designated as category B select agents by the Centers for Disease Control and Prevention (CDC). Burkholderia thailandensis is a closely related bacterium that is generally considered avirulent for humans. While it can cause disease in rodents, the B. thailandensis 50% lethal dose (LD50) is typically ≥ 104-fold higher than the B. pseudomallei and B. mallei LD50 in mammalian models of infection. Gram-negative pathogens often use secretion systems to deliver virulence factors to the cytosol of host cells, where they modulate cell physiology to favor the microbe. T6SS-1, occasionally referred to as tss-5 [10], is important for host cell interactions and virulence in B. mallei and B. thailandensis [11,12]
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