Tularemia is a highly infectious zoonotic disease, conferred to humans by bug bites, contaminated water, or contact with infected wildlife – hence its other name, “rabbit fever.” It’s much less common than it used to be, but the U.S. still has around 200 cases per year, and most infections occur between May and September: as summer approaches, now seems a relevant time to learn more about this fascinating pathogen.
Francisella tularensis under microscope. From the CDC.
Tularemia is caused by a bacterium called Francisella tularensis, which actually has several subspecies – only two of which cause disease in humans: F. tularensis tularensis (FTT), and F. tularensis holarctica. It doesn’t just show up in two forms, though – there are several ways that tularemia can present in humans, depending on how they got infected. All infections come with a fever, but more local symptoms can show up where the bacteria entered the body – for example, skin ulcers, sore throat, or eye irritation, and nearby swollen lymph nodes. The most dangerous form of tularemia, though, is a lung infection by FTT: either from directly breathing in bacteria suspended in the air, or from an untreated infection elsewhere in the body, as the bacteria can be carried to the lungs via the bloodstream. Once it has taken over the lungs, FTT is able to spread to other organs throughout the body, resulting in a deadly systemic infection.
When this bacterium enters the lungs, it suppresses the immune system from within the cells. FTT is what’s called a facultative intracellular parasite: it lives and replicates inside host cells, rather than in the extracellular environment. In particular, its host cell of choice is the macrophage – a cell whose whole job is to kill and eat invaders like FTT that would do the body harm. In order to survive and thrive in a host cell which should want to get rid of this invader, not feed it, FTT has to hijack several systems within the cell to feed itself and make sure the host cell’s built-in defenses don’t sound the alarm.
Interestingly, FTT combines these two tasks, manipulating the host cell’s metabolism in ways which both suit the bacterium’s nutritional needs and suppress the host immune response. Just last November, a study (Jessop et al., 2025) came out which investigated the ways an FTT bacterium can completely change the host cell’s whole metabolic pathway, cutting off its ability to make critical immune signals with products that would usually be generated as it turns sugar into energy – all just by passively existing within the cell.
Diagram of the layers surrounding a Gram negative bacterium with a capsule. From Pocket Dentistry.
FTT is a non-motile, Gram negative bacterium – this means it cannot move around in its environment, and that it has a “sandwich” type of casing: a cell membrane, wrapped in a cell wall, and another cell membrane outside that. FTT also has another layer outside this sandwich: a capsule made of polysaccharide molecules which are anchored to the wall. Basically, this is not an organism that’s going to be wiggling around, poking things, letting a lot of stuff in and out… for the most part, the molecules which make up its capsule are its primary means of interacting with the world. And the world, for an intracellular parasite, consists of the host cell cytoplasm and cellular machinery.
To investigate FTT’s particular effects on host cells, then, it makes sense to isolate what happens when you introduce just the molecules which make up its capsule. This allows us to answer the question: what happens when the outside of this unmoving fortress of a bacterium brushes up against the sensitive machinery that makes the host cell tick? And that’s exactly what Jessop et al. did.
This study compiles a lot of experiments, but most of them run basically like this: bother a macrophage, throw in some capsule, and see how it responds. They “bother” the cell with a molecule called R848, which interacts with specific receptors (Toll-like receptors, TLRs) to put the cell on high alert, driving production and release of inflammatory factors. In this way, they simulate what the cell would be doing if there were an actual invader detected. By introducing capsule, they can then investigate how FTT might alter or dampen this immune response to fly under the radar.
In general, we might expect that an immune-suppressing molecule was directly interacting with the molecules responsible for inflammation… but this wasn’t the case: pull-down assays did not reveal capsule molecules attached to the proteins responsible for conferring the “alarm” set off by R848-activated TLRs. In other words, the alarm is still sounded, but nobody’s home to respond to it: FTT is interfering with something further downstream.
We do know that FTT manipulates host cell metabolism, and that intermediate steps of metabolism produce important materials for synthesis of molecules used elsewhere in the cell – including pro-inflammatory cytokines, which would signal the cell to mount a bacteria-killing immune response. Glycolysis is the process by which cells break down sugar, producing a little bit of energy in the form of ATP and, more importantly, making products which can be used in the TCA cycle, which is the driving force that allows the mitochondria to make a lot more ATP. Glycolysis has a lot of steps, though, and before it produces pyruvate (the molecule it feeds into the TCA cycle), it produces a lot of other useful things – if FTT could reduce production of these essential metabolites, it could prevent the host from mounting an immune response. However, once again, capsule was not found to interact directly with any of the glycolytic machinery – but changes in glycolysis are observed. Somehow, FTT is indirectly controlling the glycolytic pathway. To understand this, we’re going to have to talk about lactic acid.
Humans are, generally, aerobic organisms: we have to breathe so that our cells can make energy from the food we eat. However, when our energy requirements outrun the oxygen we can provide our cells, they can take another path – fermentation.
Glycolysis, as mentioned above, makes a small amount of ATP, but its main purpose is to feed into mitochondrial respiration: the TCA cycle and oxidative phosphorylation. It does this by shuffling electrons and protons around to specific carrier molecules – specifically, reducing NAD+ to NADH. NADH will, in turn, donate its newly-gained proton and electrons to the Electron Transport Chain (ETC), which ends in O2 being converted into H2O, and a lot of ATP being produced. When there’s not enough oxygen around, though, the ETC stalls out, NADH builds up, and no more energy is produced – bad news for a cell. This is where fermentation comes in.
Diagram of NAD+ / NADH through glycolysis (glucose-6-P to pyruvate), feeding the tri-carboxylic acid (TCA) cycle in well-oxygenated circumstances, or producing lactate via lactate dehydrogenase (LDH) in poorly-oxygenated circumstances. From Enzymes (Second Edition), via ScienceDirect.
If the TCA cycle isn’t running, the cell has to rely solely on glycolysis for energy. If NADH isn’t being converted back to NAD+, though, glycolysis can’t run, either. To solve this, the extra proton and electrons on NADH are discarded onto pyruvate – the end product of glycolysis. NAD+ thus restored, glycolysis can proceed. The molecule that used to be pyruvate, now reduced, has a new identity: lactate, or lactic acid. This is a toxic waste molecule – it’s what builds up to make your muscles burn with hard cardio.
How is this relevant to FTT, though? Well, if glycolysis is the usual way to supply pyruvate and NADH to the TCA cycle (necessary to feed the host cell and its parasite), but glycolysis also produces necessary elements for the immune response, it would be nice for FTT if some other way to make pyruvate and NADH was possible.
The enzyme which catalyzes the conversion of NADH and pyruvate to NAD+ and lactate (LDH) is actually multiple enzymes in one: the LDHA subunit prefers to bind pyruvate, and so performs the fermentation described above. However, to make the system respond better to energy needs of the cell & allow equilibrium to be established, a subunit with the exact opposite function is also attached! LDHB, then, can produce pyruvate from lactate (and NAD+ from NADH). So, while fermentation may be the “usual” direction to go, here, the system could be “convinced” to run the opposite way, if a sufficient amount of lactate were introduced. And that is exactly what FTT does.
Because lactic acid is toxic – not generally something you want hanging around – your cells have membrane proteins whose job is to transport it from the cytoplasm to the extracellular space. Meet monocarboxylate transporter 4 (MCT4) – usually a great little lactate exporter, but under the influence of capsule, it completely reverses activity and pulls lactate into the cell, which can then be converted into pyruvate… totally circumventing the need for glycolysis.
Selections from Figure 3: (B) Concentrations of glycolytic intermediate metabolites in control cells (“Mock”) versus cells treated with R848, capsule, or both. Significant “rebalancing” of GAPDH (note: typo in figure labels this intersection as “GADPH”) associated metabolites – fructose-1-6-biphosphate, glyceraldehyde-3-phosphate, dihyroxyacetone phosphate, and glycerol-3-phosphate – are seen as the capsule plus R848 (yellow) levels much more closely match baseline (blue) than do R848 alone (pink). (E) Ratio of NADH/NAD+ in control cells (“Mock”) versus cells treated with R848, capsule, or both. Cells treated with capsule and R848 have a significantly lower ratio as compared to solely R848-treated, bringing them back to normal levels.
Of course, glycolysis doesn’t just stop – what does happen, though, is inhibition of the feedback loops which R848 would normally upregulate to produce more of those inflammatory cytokine-feeding intermediate glycolysis metabolites. By manipulating the ratios of NADH to NAD+ to bring them back to “normal” from R848-associated elevations (Fig. 3E) and pulling products towards increased TCA cycle productivity, metabolite levels which would have been elevated are brought back into normal ranges (Fig. 3B). In other words, TCA productivity goes up – producing lots of energy to keep the host cell happy and feed the invading bacteria – and, through the same process, products which could be used to make “alarm” molecules are kept at low levels.
In this way, Francisella tularensis tularensis exhibits amazing efficiency and ingenuity. It makes its home in the most dangerous cells of its host – the macrophages which are specifically built to destroy parasites like FTT. It uses host cell machinery to generate excess energy to feed its own function and replication, and in the same move, negates the host cell’s immune response. This allows it not only to take over these cells, but to easily spread to the rest of the body without being attacked – after all, it’s already living inside the security team.
This study was an interesting read and covers a lot of ground, demonstrating a complex relationship between FTT and host cell metabolism and immune response – however, much remains unknown! Targeting lactic acid metabolism could be a fruitful mechanism for treating this lethal infection in the future, but there is still a long way to go in understanding exactly how FTT’s capsule is triggering these changes, its effects on metabolic processes outside the mainline of glycolysis and the TCA cycle, and even how similar immune-suppression pathways might be used by other microbes.
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