Type A Francisella tularensis acid phosphatases contribute to pathogenesis

Different Francisella spp. produce five or six predicted acid phosphatases (AcpA, AcpB, AcpC, AcpD, HapA and HapB). The genes encoding the histidine acid phosphatases (hapA, hapB) and acpD of F. tularensis subsp. Schu S4 strain are truncated or disrupted. However, deletion of HapA (FTT1064) in F. tularensis Schu S4 resulted in a 33% reduction in acid phosphatase activity and loss of the four functional acid phosphatases in F. tularensis Schu S4 (ΔABCH) resulted in a>99% reduction in acid phosphatase activity compared to the wild type strain. All single, double and triple mutants tested, demonstrated a moderate decrease in mouse virulence and survival and growth within human and murine phagocytes, whereas the ΔABCH mutant showed >3.5-fold decrease in intramacrophage survival and 100% attenuation of virulence in mouse. While the Schu S4 ΔABCH strain was attenuated in the mouse model, it showed only limited protection against wild type challenge. F. tularensis Schu S4 failed to stimulate reactive oxygen species production in phagocytes, whereas infection by the ΔABCH strain stimulated 5- and 56-fold increase in reactive oxygen species production in neutrophils and human monocyte-derived macrophages, respectively. The ΔABCH mutant but not the wild type strain strongly co-localized with p47^phox and replicated in macrophages isolated from p47^phox knockout mice. Thus, F. tularensis Schu S4 acid phosphatases, including the truncated HapA, play a major role in intramacrophage survival and virulence of this human pathogen.

Regulation of francisella tularensis virulence

Francisella tularensis is one of the most virulent bacteria known and a Centers for Disease Control and Prevention Category A select agent. It is able to infect a variety of animals and insects and can persist in the environment, thus Francisella spp. must be able to survive in diverse environmental niches. However, F. tularensis has a surprising dearth of sensory and regulatory factors. Recent advancements in the field have identified new functions of encoded transcription factors and greatly expanded our understanding of virulence gene regulation. Here we review the current knowledge of environmental adaptation by F. tularensis, its transcriptional regulators and their relationship to animal virulence.

Francisella tularensis blue-gray phase variation involves structural modifications of lipopolysaccharide o-antigen, core and lipid a and affects intramacrophage survival and vaccine efficacy

Francisella tularensis is a CDC Category A biological agent and a potential bioterrorist threat. There is no licensed vaccine against tularemia in the United States. A long-standing issue with potential Francisella vaccines is strain phase variation to a gray form that lacks protective capability in animal models. Comparisons of the parental strain (LVS) and a gray variant (LVSG) have identified lipopolysaccharide (LPS) alterations as a primary change. The LPS of the F. tularensis variant strain gains reactivity to F. novicida anti-LPS antibodies, suggesting structural alterations to the O-antigen. However, biochemical and structural analysis of the F. tularensis LVSG and LVS LPS demonstrated that LVSG has less O-antigen but no major O-antigen structural alterations. Additionally, LVSG possesses structural differences in both the core and lipid A regions, the latter being decreased galactosamine modification. Recent work has identified two genes important in adding galactosamine (flmF2 and flmK) to the lipid A. Quantitative real-time PCR showed reduced transcripts of both of these genes in the gray variant when compared to LVS. Loss of flmF2 or flmK caused less frequent phase conversion but did not alter intramacrophage survival or colony morphology. The LVSG strain demonstrated an intramacrophage survival defect in human and rat but not mouse macrophages. Consistent with this result, the LVSG variant demonstrated little change in LD₅₀ in the mouse model of infection. Furthermore, the LVSG strain lacks the protective capacity of F. tularensis LVS against virulent Type A challenge. These data suggest that the LPS of the F. tularensis LVSG phase variant is dramatically altered. Understanding the mechanism of blue to gray phase variation may lead to a way to inhibit this variation, thus making future F. tularensis vaccines more stable and efficacious.

Francisella acid phosphatases inactivate the NADPH oxidase in human phagocytes

Francisella tularensis contains four putative acid phosphatases that are conserved in Francisella novicida. An F. novicida quadruple mutant (AcpA, AcpB, AcpC, and Hap [DeltaABCH]) is unable to escape the phagosome or survive in macrophages and is attenuated in the mouse model. We explored whether reduced survival of the DeltaABCH mutant within phagocytes is related to the oxidative response by human neutrophils and macrophages. F. novicida and F. tularensis subspecies failed to stimulate reactive oxygen species production in the phagocytes, whereas the F. novicida DeltaABCH strain stimulated a significant level of reactive oxygen species. The DeltaABCH mutant, but not the wild-type strain, strongly colocalized with p47(phox) and replicated in phagocytes only in the presence of an NADPH oxidase inhibitor or within macrophages isolated from p47(phox) knockout mice. Finally, purified AcpA strongly dephosphorylated p47(phox) and p40(phox), but not p67(phox), in vitro. Thus, Francisella acid phosphatases play a major role in intramacrophage survival and virulence by regulating the generation of the oxidative burst in human phagocytes.

Inhibition of AcpA phosphatase activity with ascorbate attenuates Francisella tularensis intramacrophage survival

Acid phosphatase activity in the highly infectious intracellular pathogen Francisella tularensis is directly related with the ability of these bacteria to survive inside host cells. Pharmacological inactivation of acid phosphatases could potentially help in the treatment of tularemia or even be utilized to neutralize the infection. In the present work, we report inhibitory compounds for three of the four major acid phosphatases produced by F. tularensis SCHU4: AcpA, AcpB, and AcpC. The inhibitors were identified using a catalytic screen from a library of chemicals approved for use in humans. The best results were obtained against AcpA. The two compounds identified, ascorbate (K(i) = 380 +/- 160 microM) and 2-phosphoascorbate (K(i) = 3.2 +/- 0.85 microM) inhibit AcpA in a noncompetitive, nonreversible fashion. A potential ascorbylation site in the proximity of the catalytic pocket of AcpA was identified using site-directed mutagenesis. The effects of the inhibitors identified in vitro were evaluated using bioassays determining the ability of F. tularensis to survive inside infected cells. The presence of ascorbate or 2-phosphoascorbate impaired the intramacrophage survival of F. tularensis in an AcpA-dependent manner as it was probed using knockout strains. The evidence presented herein indicated that ascorbate could be a good alternative to be used clinically to improve treatments against tularemia.

A Francisella tularensis Schu S4 purine auxotroph is highly attenuated in mice but offers limited protection against homologous intranasal challenge

Background

Francisella tularensis is a Gram-negative coccobacillus that causes the febrile illness tularemia. Subspecies that are pathogenic for humans include those comprising the type A (subspecies tularensis) or type B (subspecies holarctica) biovars. An attenuated live vaccine strain (LVS) developed from a type B isolate has previously been used to vaccinate at-risk individuals, but offers limited protection against high dose (>1000 CFUs) challenge with type A strains delivered by the respiratory route. Due to differences between type A and type B F. tularensis strains at the genetic level, it has been speculated that utilization of an attenuated type A strain as a live vaccine might offer better protection against homologous respiratory challenge compared with LVS. Here, we report the construction and characterization of an unmarked ΔpurMCD mutant in the highly virulent type A strain Schu S4.

Methodology/Principal Findings

Growth of Schu S4 ΔpurMCD was severely attenuated in primary human peripheral blood monocyte-derived macrophages and in the A549 human lung epithelial cell line. The Schu S4 ΔpurMCD mutant was also highly attenuated in mice when delivered via either the intranasal or intradermal infection route. Mice vaccinated intranasally with Schu S4 ΔpurMCD were well protected against high dose intradermal challenge with virulent type A or type B strains of F. tularensis. However, intranasal vaccination with Schu S4 ΔpurMCD induced tissue damage in the lungs, and conferred only limited protection against high dose Schu S4 challenge delivered by the same route. The level of protection observed was similar to that conferred following vaccination with wild-type LVS or the analogous LVS ΔpurMCD mutant.

Conclusions/Significance

Collectively, these results argue that development of the next generation live attenuated vaccine for Francisella should be based on use of the less pathogenic type B biovar rather than the more reactogenic type A biovar.

Identification of an orphan response regulator required for the virulence of Francisella spp. and transcription of pathogenicity island genes

Francisella tularensis is a category A agent of biowarfare/biodefense. Little is known about the regulation of virulence gene expression in Francisella spp. Comparatively few regulatory factors exist in Francisella, including those belonging to two-component systems (TCS). However, orphan members of typical TCS can be identified. To determine if orphan TCS members affect Francisella gene expression, a gene encoding a product with high similarity to the Salmonella PmrA response regulator (FTT1557c/FNU0663.2) was deleted in Francisella novicida (a model organism for F. tularensis). The F. novicida pmrA mutant was defective in survival/growth within human and murine macrophage cell lines and was 100% defective in virulence in mice at a dose of up to 10(8) CFU. In addition, the mutant strain demonstrated increased susceptibility to antimicrobial peptide killing, but no differences were observed between the lipid A of the mutant and the parental strain, as has been observed with pmrA mutants of other microbes. The F. novicida pmrA mutant was 100% protective as a single-dose vaccine when challenge was with 10(6) CFU of F. novicida but did not protect against type A Schu S4 wild-type challenge. DNA microarray analysis identified 65 genes regulated by PmrA. The majority of these genes were located in the region surrounding pmrA or within the Francisella pathogenicity island (FPI). These FPI genes are also regulated by MglA, but MglA does not regulate pmrA, nor does PmrA regulate MglA. Thus, the orphan response regulator PmrA is an important factor in controlling virulence in F. novicida, and a pmrA mutant strain is an effective vaccine against homologous challenge.

AcpA is a Francisella acid phosphatase that affects intramacrophage survival and virulence

AcpA of Francisella spp. is a respiratory-burst-inhibiting acid phosphatase that also exhibits phospholipase C activity. To better understand the molecular basis of AcpA in virulence, a deletion of acpA was constructed in Francisella novicida. The phosphatase and lipase activities were reduced 10-fold and 8-fold, respectively, in the acpA mutant compared to the wild type and were found mostly associated with the outer membrane. The acpA mutant was more susceptible to intracellular killing than the wild-type strain in the THP-1 human macrophage-like cell line. In addition, mice infected with the acpA mutant survived longer than the wild-type strain and were less fit than the wild-type strain in competition infection assays. Transmission electron microscopy showed that the acpA mutant was delayed in escape from macrophage phagosomes, as more than 75% of acpA mutant bacteria could still be found inside phagosomes after 12 h of infection in THP-1 cells and human monocyte-derived macrophages, whereas most of the wild-type bacteria had escaped from the phagosome by 6 h postinfection. Thus, AcpA affects intracellular trafficking and the fate of Francisella within host macrophages.

Macrophage pro-inflammatory response to Francisella novicida infection is regulated by SHIP

Francisella tularensis, a Gram-negative facultative intracellular pathogen infecting principally macrophages and monocytes, is the etiological agent of tularemia. Macrophage responses to F. tularensis infection include the production of pro-inflammatory cytokines such as interleukin (IL)-12, which is critical for immunity against infection. Molecular mechanisms regulating production of these inflammatory mediators are poorly understood. Herein we report that the SH2 domain-containing inositol phosphatase (SHIP) is phosphorylated upon infection of primary murine macrophages with the genetically related F. novicida, and negatively regulates F. novicida-induced cytokine production. Analyses of the molecular details revealed that in addition to activating the MAP kinases, F. novicida infection also activated the phosphatidylinositol 3-kinase (PI3K)/Akt pathway in these cells. Interestingly, SHIP-deficient macrophages displayed enhanced Akt activation upon F. novicida infection, suggesting elevated PI3K-dependent activation pathways in absence of SHIP. Inhibition of PI3K/Akt resulted in suppression of F. novicida-induced cytokine production through the inhibition of NFkappaB. Consistently, macrophages lacking SHIP displayed enhanced NFkappaB-driven gene transcription, whereas overexpression of SHIP led to decreased NFkappaB activation. Thus, we propose that SHIP negatively regulates F. novicida-induced inflammatory cytokine response by antagonizing the PI3K/Akt pathway and suppressing NFkappaB-mediated gene transcription. A detailed analysis of phosphoinositide signaling may provide valuable clues for better understanding the pathogenesis of tularemia.

Changing trends of Candida species in neonatal septicaemia in a tertiary North Indian hospital

Four hundred and fifty four blood samples of clinically diagnosed septicemic neonates were collected over a period of six months from the neonatal ICU of Kalawati Saran Children Hospital, New Delhi. 144 samples were culture positive; out of which 50 (34.7%) were Candida isolates. 92% isolates were Candida tropicalis, 4% were C. albicans and C. kefyr each. The study emphasises the changing pattern of Candida species and their importance in blood stream infections in neonates.