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- Forslund, Anna-Lena, 1964-, et al.
(författare)
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The type IV pilin, PilA, is required for full virulence of Francisella tularensis subspecies tularensis
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Annan publikation (populärvet., debatt m.m.)abstract
- Background: All four Francisella tularensis subspecies possess gene clusters with potential to express type IV pili (Tfp). These clusters include putative pilin genes, as well as pilB, pilC and pilQ, required for secretion and assembly of Tfp. A hallmark of Tfp is the ability to retract the pilus upon surface contact, a property mediated by the ATPase PilT. Interestingly, out of the two major human pathogenic subspecies only the highly virulent type A strains have a functional pilT gene.Results: In a previous study, we were able to show that one pilin gene, pilA, was essential for virulence of a type B strain in a mouse infection model. In this work we have examined the role of several pilin genes in the virulence of the pathogenic type A strain SCHU S4. pilA, pilC, pilQ, and pilT were mutated by in-frame deletion mutagenesis. Interestingly, when mice were infected with a mixture of each mutant strain and the wild-type strain, the pilA, pilC and pilQ mutants were out-competed, while the pilT mutant was equally competitive as the wild-type.Conclusions: This suggests that expression and surface localisation of PilA contribute to virulence in the highly virulent type A strain, while PilT was dispensable for virulence in the mouse infection model.
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- Honn, Marie, 1982-, et al.
(författare)
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OxyR : an important regulator of the oxidative stress response of Francisella tularensis LVS
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- An essential part of the oxidative stress response in Gram-negative bacteria is the H2O2-activated transcriptional regulator OxyR. Although it is much studied in common bacteria such as Escherichia coli, little is known about it about its role in the facultative intracellular bacterium Francisella tularensis. Here, we studied the role of OxyR in the strain F. tularensis LVS. We studied the effects of ROS on the LVS, ΔoxyR, ΔkatG and ΔoxyR/ΔkatG. The latter mutants lack expression of catalase, the function of which is important for degradation of reactive oxygen species, especially H2O2. The in vitro response of these mutants to defined ROS was assessed using H2O2, the O2- generating agent paraquat, and the ONOO- generator SIN-1. ΔoxyR was more susceptible to all ROS than LVS, as was ΔkatG, with the exception of O2-. Strikingly, ΔoxyR/ΔkatG was significantly more susceptible to all ROS tested compared to either single deletion mutant. Also the catalase activity was assessed and whereas LVS significantly upregulated the enzymatic activity in response to H2O2, this did not occur in the ΔoxyR mutant. Gene expression by ΔoxyR was compared to LVS and it was found that there was down-regulation of fur, katG, sodB, sodC, furA, and in particular of ahpC, in the mutant. LVS, ΔoxyR and ΔkatG replicated efficiently in bone marrow-derived macrophages, whereas ΔoxyR/ΔkatG showed no replication. In mice, the ΔoxyR mutant displayed impaired replication in liver but intact replication vs. LVS in spleen. Collectively, our results demonstrate an important role of OxyR in the oxidative stress response and virulence of F. tularensis. The combined mutation of ΔoxyR/ΔkatG led to severely impaired handling of oxidative stress.
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- Kelk, Peyman, 1976-, et al.
(författare)
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Inflammatory cell death of human macrophages in response to Aggregatibacter actinomycetemcomitans leukotoxin
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Aggregatibacter (Actinobacillus) actinomycetemcomitans is a facultative anaerobic gram-negative bacterium associated with severe forms of periodontitis. A leukotoxin, which belongs to the Repeats in Toxin (RTX) family, is believed to be one of its virulence factors and to play an important role in the bacterium's pathogenicity. This toxin selectively kills human leukocytes by inducing apoptosis and lysis. Here we report that leukotoxin-induced cell death of macrophages proceeded through a process that differs from the classical characteristics of apoptosis and necrosis. Interestingly, this process resembled pyroptosis, and resulted in an extensive leukotoxin-induced interleukin-1β (IL-1β) secretion. This activation was mainly mediated by caspase-1 activation, while the levels of mRNA for IL-1β were not affected by the leukotoxin. A similar pattern was seen for IL-18, but the level of that cytokine was about 30 times lower. Both of these cytokines are synthesized as biologically inactive precursors and need active caspase-1 for their activation and secretion. In conclusion, A. actinomycetemcomitans leukotoxin induces a pyroptosis-like cell death in human macrophages and that leads to a specific and excessive pro-inflammatory response. This novel virulence mechanism of the leukotoxin may play an important role in the pathogenic potential of this bacterium.
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- Lindgren, Marie, et al.
(författare)
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Importance of PdpC, IglC, IglI, and IglG for modulation of a host cell death pathway induced by Francisella tularensis LVS
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Modulation of host cell death pathways appears to be a prerequisite for the successful life styles of many intracellular pathogens. The facultative intracellular bacterium Francisella tularensis is highly pathogenic and effective proliferation in the macrophage cytosol leading to host cell death is a requirement for its virulence. To better understand how this is achieved, macrophages were infected with the F. tularensis live vaccine strain (LVS) and the effects were compared to those resulting from infections with deletion mutants lacking expression of either of the pdpC, iglC, iglG, or iglI genes. All of these genes encode components that together with a dozen other proteins form the Francisella pathogenicity island (FPI), a type VI secretion system. Within 12 h, a majority of the J774 cells infected with the LVS strain showed production of mitochondrial superoxide and after 24 h, marked signs of mitochondrial damage, caspase-9 and caspase-3 activation, phosphatidylserine expression, nucleosome formation, and membrane leakage. In contrast, neither of these events occurred after infection with the ∆iglI or ∆iglC mutant, although the former strain replicated. The ∆iglG mutant replicated effectively but induced only marginal cytopathogenic effects after 24 h and intermediate effects after 48 h. In contrast, the ∆pdpC mutant showed no replication, but induced marked mitochondrial superoxide production and mitochondrial damage, caspase-3 activation, nucleosome formation, and phosphatidylserine expression, although the effects were delayed compared to LVS. The unique phenotypes of the mutants provide novel insights regarding the roles of individual FPI components for the modulation of the cytopathogenic effects resulting from the F. tularensis infection.
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