Long-Term Survival of Virulent Tularemia Pathogens outside a Host in Conditions That Mimic Natural Aquatic Environments

Water and mosquitoes as key components of the infective cycle of Francisella tularensis in Europe: a review

Francisella tularensis is the pathogen of tularemia, a zoonotic disease that have a broad range of hosts. Its epidemiology is related to aquatic environments, particularly in the subspecies holarctica. In this review, we explore the role of water and mosquitoes in the epidemiology of Francisella in Europe. F. tularensis epidemiology has been linked to natural waters, where its persistence has been associated with biofilm and amebas. In Sweden and Finland, the European countries where most human cases have been reported, mosquito bites are a main route of transmission. F. tularensis is present in other European countries, but to date positive mosquitoes have not been found. Biofilm and amebas are potential sources of Francisella for mosquito larvae, however, mosquito vector capacity has not been demonstrated experimentally, with the need to be studied using local species to uncover a potential transmission adaptation. Transstadial, for persistence through life stages, and mechanical transmission, suggesting contaminated media as a source for infection, have been studied experimentally for mosquitoes, but their natural occurrence needs to be evaluated. It is important to clear up the role of different local mosquito species in the epidemiology of F. tularensis and their importance in all areas where tularemia is present.

Phenotypic and transcriptional characterization of F. tularensis LVS during transition into a viable but non-culturable state

Francisella tularensis is a gram-negative, intracellular pathogen which can cause serious, potentially fatal, illness in humans. Species of F. tularensis are found across the Northern Hemisphere and can infect a broad range of host species, including humans. Factors affecting the persistence of F. tularensis in the environment and its epidemiology are not well understood, however, the ability of F. tularensis to enter a viable but non-culturable state (VBNC) may be important. A broad range of bacteria, including many pathogens, have been observed to enter the VBNC state in response to stressful environmental conditions, such as nutrient limitation, osmotic or oxidative stress or low temperature. To investigate the transition into the VBNC state for F. tularensis, we analyzed the attenuated live vaccine strain, F. tularensis LVS grown under standard laboratory conditions. We found that F. tularensis LVS rapidly and spontaneously enters a VBNC state in broth culture at 37°C and that this transition coincides with morphological differentiation of the cells. The VBNC bacteria retained an ability to interact with both murine macrophages and human erythrocytes in in vitro assays and were insensitive to treatment with gentamicin. Finally, we present the first transcriptomic analysis of VBNC F. tularensis, which revealed clear differences in gene expression, and we identify sets of differentially regulated genes which are specific to the VBNC state. Identification of these VBNC specific genes will pave the way for future research aimed at dissecting the molecular mechanisms driving entry into the VBNC state.

An Unusual Etiology of Fluorodeoxyglucose Avid Intrathoracic Lymph Nodes

A middle-aged man in his 50s, active smoker, presented to the pulmonary office for lung cancer evaluation. On a low-dose computed tomography for lung cancer screening, he was found to have an 8 mm endobronchial lesion in the right main stem bronchus. A PET-CT revealed no endobronchial lesion, but incidentally, fluorodeoxyglucose (FDG) avidity was present in the right hilar (SUV 13.2) and paratracheal lymph nodes (LNs). He underwent bronchoscopy and EBUS-TBNA of station 7 and 10 R LNs. The fine needle aspiration (FNA) revealed necrotizing epithelioid granuloma. The acid-fast bacilli (AFB) and Grocott methenamine silver (GMS) stains were negative. He had suffered from pneumonic tularemia 13 months ago and immunohistochemical staining for Francisella tularensis on FNA samples at Center for Disease Control and Prevention was negative. The intense positron emission tomography (PET) avidity was attributed to prior tularemic intrathoracic lymphadenitis without active tularemia, a rare occurrence. To the best of our knowledge, PET-positive intrathoracic lymph node beyond one year without evidence of active tularemia has not been previously reported.

Tularemia - a re-emerging disease with growing concern

Tularemia caused by Gram-negative, coccobacillus bacterium, Francisella tularensis, is a highly infectious zoonotic disease. Human cases have been reported mainly from the United States, Nordic countries like Sweden and Finland, and some European and Asian countries. Naturally, the disease occurs in several vertebrates, particularly lagomorphs. Type A (subspecies tularensis) is more virulent and causes disease mainly in North America; type B (subspecies holarctica) is widespread, while subspecies mediasiatica is present in central Asia. F. tularensis is a possible bioweapon due to its lethality, low infectious dosage, and aerosol transmission. Small mammals like rabbits, hares, and muskrats are primary sources of human infections, but true reservoir of F. tularensis is unknown. Vector-borne tularemia primarily involves ticks and mosquitoes. The bacterial subspecies involved and mode of transmission determine the clinical picture. Early signs are flu-like illnesses that may evolve into different clinical forms of tularemia that may or may not include lymphadenopathy. Ulcero-glandular and glandular forms are acquired by arthropod bite or handling of infected animals, oculo-glandular form as a result of conjunctival infection, and oro-pharyngeal form by intake of contaminated food or water. Pulmonary form appears after inhalation of bacteria. Typhoidal form may occur after infection via different routes. Human-to-human transmission has not been known. Diagnosis can be achieved by serology, bacterial culture, and molecular methods. Treatment for tularemia typically entails use of quinolones, tetracyclines, or aminoglycosides. Preventive measures are necessary to avoid infection although difficult to implement. Research is underway for the development of effective live attenuated and subunit vaccines.

Francisella tularensis subsp. holarctica bacteraemia in an immunocompetent male

Tularemia is a rare zoonotic disease caused by the two predominant subspecies of Francisella tularensis, namely subspecies tularensis and subspecies holarctica. The latter is less virulent than the former, is endemic in Europe, and usually has a mild disease course, although respiratory involvement and bacteraemia can occur. Tularemia in Belgium is rare, but the incidence seems to be increasing. It therefore seems prudent to raise awareness among clinicians for this potentially severe disease. We report the first case of pneumonic tularemia with bacteraemia from Belgium, and want to recommend including Francisella tularensis in the differential diagnosis of pneumonia when an unfavorable evolution is seen with standard treatment.

Francisella tularensis subsp. holarctica wild-type is able to colonize natural aquatic ex vivo biofilms

Biofilms are a matrix-associated lifestyle of microbial communities, often enabling survivability and persistence of such bacteria. The objective of this study was to investigate the survival of the wild-type strain A-271 of Francisella tularensis subsp. holarctica (Fth) in a natural aquatic ex vivo biofilm. To that purpose, we allowed Fth A-271 to produce its own biofilm on solid surfaces but also to colonize naturally formed biofilms from aquatic habitats, which were infected with Francisella in the laboratory. The survival rates of the bacteria in biofilms were compared to those of planktonic bacteria as a function of the employed culture condition. It could be shown by light- and electron microscopy that Fth is able to form a complex, matrix-associated biofilm. The biofilm form of Francisella showed longer cultivability on agar plates in natural water when compared to planktonic (free-living) bacteria. Be it as a part of the existing ex vivo biofilm or free-floating above as planktonic bacteria, more than 80% of Francisella were not only able to survive under these conditions for 28 days, but even managed to establish microcolonies and areas with their own exclusive biofilm architecture within the ex vivo biofilm. Here, we can demonstrate for the first time that a Francisella tularensis wild-type strain (Type B) is able to successfully colonize an aquatic multi-species ex vivo biofilm. It is worthwhile to speculate that Fth might become more persistent in the environment when it forms its own biofilm or integrates in an existing one. Multi-species biofilms have been shown to be more resistant against stress compared to single-species biofilms. This may have an important impact on the long-term survival of Francisella in aquatic habitats and infection cycles in nature.

Why vary what's working? Phase variation and biofilm formation in Francisella tularensis

The notoriety of high-consequence human pathogens has increased in recent years and, rightfully, research efforts have focused on understanding host-pathogen interactions. Francisella tularensis has been detected in an impressively broad range of vertebrate hosts as well as numerous arthropod vectors and single-celled organisms. Two clinically important subspecies, F. tularensis subsp. tularensis (Type A) and F. tularensis subsp. holarctica (Type B), are responsible for the majority of tularemia cases in humans. The success of this bacterium in mammalian hosts can be at least partly attributed to a unique LPS molecule that allows the bacterium to avoid detection by the host immune system. Curiously, phase variation of the O-antigen incorporated into LPS has been documented in these subspecies of F. tularensis, and these variants often display some level of attenuation in infection models. While the role of phase variation in F. tularensis biology is unclear, it has been suggested that this phenomenon can aid in environmental survival and persistence. Biofilms have been established as the predominant lifestyle of many bacteria in the environment, though, it was previously thought that Type A and B isolates of F. tularensis typically form poor biofilms. Recent studies question this ideology as it was shown that alteration of the O-antigen allows robust biofilm formation in both Type A and B isolates. This review aims to explore the link between phase variation of the O-antigen, biofilm formation, and environmental persistence with an emphasis on clinically relevant subspecies and how understanding these poorly studied mechanisms could lead to new medical countermeasures to combat tularemia.

Development and evaluation of a rapid RPA/CRISPR-based detection of Francisella tularensis

Francisella tularensis is a dangerous pathogen that causes an extremely contagious zoonosis in humans named tularemia. Given its low-dose morbidity, the potential to be fatal, and aerosol spread, it is regarded as a severe threat to public health. The US Centers for Disease Control and Prevention (CDC) has classified it as a category A potential agent for bioterrorism and a Tier 1 Select Agent. Herein, we combined recombinase polymerase amplification (RPA) with CRISPR/Cas12a system to select the F. tularensis target gene (TUL4), creating a two-pronged rapid and ultrasensitive diagnostic method for detecting F. tularensis. The real-time RPA (RT-RPA) assay detected F. tularensis within 10 min at a sensitivity of 5 copies/reaction, F. tularensis genomic DNA of 5 fg, and F. tularensis of 2 × 102 CFU/ml; the RPA-CRISPR/Cas12a assay detects F. tularensis within 40 min at a sensitivity of 0.5 copies/reaction, F. tularensis genomic DNA of 1 fg, and F. tularensis of 2 CFU/ml. Furthermore, the evaluation of specificity showed that both assays were highly specific to F. tularensis. More importantly, in a test of prepared simulated blood and sewage samples, the RT-RPA assay results were consistent with RT-PCR assay results, and the RPA-CRISPR/Cas12a assay could detect a minute amount of F. tularensis genomic DNA (2.5 fg). There was no nonspecific detection with blood samples and sewage samples, giving the tests a high practical application value. For example, in on-site and epidemic areas, the RT-RPA was used for rapid screening and the RPA-CRISPR/Cas12a assay was used for more accurate diagnosis.

Francisella novicida can utilize Paramecium bursaria as its potential host

Francisella novicida is a facultative intracellular pathogen and the causative agent of tularemia. Although cases of infection caused by exposure to contaminated water have been reported, its natural host and ecology in the environment remain unclear. In this study, we investigated in vitro the possibility that Paramecium bursaria may be a useful tool as a protist host model of F. novicida. Experimental infection with F. novicida resulted in a stable intracellular relationship within P. bursaria. This symbiotic intracellular relationship was not observed in experimental infections with other Francisella species and Legionella pneumophila. We found that F. novicida showed similar behaviour to that of the eukaryotic endosymbiont of P. bursaria, the green algae Chlorella, in the internalization process. In addition, stable intracellular localization of F. novicida was possible only when Chlorella was not present. Although we investigated the type VI secretion system of F. novicida as a candidate for the bacterial factor, we found that it was not involved in the establishment of an intracellular relationship with P. bursaria. These results suggested that P. bursaria is potentially a protist host model for F. novicida and may be a useful tool for understanding the relationship between protist hosts and their symbionts.

Intestinal morphology and microflora to Vibrio alginolyticus in pacific white shrimp (Litopenaeus vannamei)

In recent years, the shrimp farming industry encountered significant economic losses induced by Vibrio alginolyticus. In this study, the influence of Vibrio alginolyticus on intestinal histomorphology and microbiome composition in Litopenaeus vannamei were studied. The results showed that the intestinal mucosal epithelial cells of Vibrio group (VA group) injected only with Vibrio alginolyticus showed large area exfoliation at 12 h, and the tissue morphology of intestine recovered at 48 h. Compared with the control group (CK group), the abundance of Proteobacteria was significantly higher (P < 0.05), while the abundance of Actinobacteria was significantly lower after infection with Vibrio alginolyticus. The abundance of Shewanella in intestinal microbiome of Litopenaeus vannamei was significantly higher at 12 h (P < 0.05), but the abundance of Candidatus_Bacilloplasma was significantly lower at 48 h after infection (P < 0.05). In VA group, the diversity of intestinal microbiome was significantly lower at 12 h, which could be caused by the proliferation of Candidatus_Bacilloplasma and Shewanella. All above findings suggested that the stability of the dynamic balance of microbiome in the intestine helped Litopenaeus vannamei to resist pathogen colonization.

Phase Variation of LPS and Capsule Is Responsible for Stochastic Biofilm Formation in Francisella tularensis

Biofilms have been established as an important lifestyle for bacteria in nature as these structured communities often enable survivability and persistence in a multitude of environments. Francisella tularensis is a facultative intracellular Gram-negative bacterium found throughout much of the northern hemisphere. However, biofilm formation remains understudied and poorly understood in F. tularensis as non-substantial biofilms are typically observed in vitro by the clinically relevant subspecies F. tularensis subsp. tularensis and F. tularensis subsp. holarctica (Type A and B, respectively). Herein, we report conditions under which robust biofilm development was observed in a stochastic, but reproducible manner in Type A and B isolates. The frequency at which biofilm was observed increased temporally and appeared switch-like as progeny from the initial biofilm quickly formed biofilm in a predictable manner regardless of time or propagation with fresh media. The Type B isolates used for this study were found to more readily switch on biofilm formation than Type A isolates. Additionally, pH was found to function as an environmental checkpoint for biofilm initiation independently of the heritable cellular switch. Multiple colony morphologies were observed in biofilm positive cultures leading to the identification of a particular subset of grey variants that constitutively produce biofilm. Further, we found that constitutive biofilm forming isolates delay the onset of a viable non-culturable state. In this study, we demonstrate that a robust biofilm can be developed by clinically relevant F. tularensis isolates, provide a mechanism for biofilm initiation and examine the potential role of biofilm formation.

Presence of Francisella tularensis subsp. holarctica DNA in the Aquatic Environment in France

In 2018, the incidence of tularemia increased twofold in the west of France, with many pneumonic forms, suggesting environmental sources of infection. We investigated the presence of Francisellatularensis subsp. holarctica and other Francisella species DNA in the natural aquatic environment of this geographic area. Two sampling campaigns, in July 2019 and January 2020, allowed the collection of 87 water samples. Using a combination of real-time PCR assays, we tested the presence of either Francisella sp., F. tularensis/F. novicida, and F. tularensis subsp. holarctica, the latter being the only tularemia agent in Europe. Among 57 water samples of the first campaign, 15 (26.3%) were positive for Francisella sp., nine (15.8%) for F. tularensis and/or F. novicida, and four (7.0%) for F. tularensis subsp. holarctica. Ratios were 25/30 (83.3%), 24/30 (80.0%), and 4/30 (13.3%) for the second campaign. Among the thirty sites sampled during the two campaigns, nine were positive both times for Francisella sp., seven for F. tularensis and/or F. novicida, and one for F. tularensis subsp. holarctica. Altogether, our study reveals a high prevalence of Francisella sp. DNA (including the tularemia agent) in the studied aquatic environment. This aquatic environment could therefore participate in the endemicity of tularemia in the west of France.