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How Bacteria Spread In The Blood And Why It Matters

Bacteremia, also known as blood poisoning, happens when bacteria enter the bloodstream and spread through the body. Normally, the immune system fights off bacteria before they cause serious harm. But sometimes, bacteria overcome the body's defenses, leading to dangerous infections.

If left untreated, bacteremia can turn into sepsis, a life-threatening condition that causes widespread inflammation and organ failure. Sepsis is responsible for more than one-third of hospital deaths every year.

Many people come into contact with bacteria daily without getting sick, so scientists are trying to understand why some infections turn deadly while others do not. If they can figure out how bacteria move through the body and enter the bloodstream, doctors may be able to stop these infections before they become life-threatening.

Dr. Michael Bachman and Dr. Caitlyn Holmes from the University of Michigan set out to answer an important question: How do bacteria travel from an infection site, like the lungs, into the bloodstream? Their research focused on a type of bacteria called Klebsiella pneumoniae, which often causes pneumonia and can lead to bacteremia.

Previous studies have shown that bacterial infections happen in three main stages. First, bacteria infect a specific area, such as the lungs. Then, they enter the bloodstream.

Finally, they spread and multiply while avoiding the body's natural filters, like the liver and spleen, which try to remove harmful invaders from the blood. Scientists have ways to measure the first and third stages, but the second stage—how bacteria escape into the blood—has been much harder to study.

To solve this mystery, the researchers used a new tracking method developed with colleagues at Harvard University. They labeled bacteria with unique DNA markers and then used computer analysis to follow their movement inside infected mice.

Before starting the experiment, the team thought bacteria would remain in the lungs until they multiplied to large numbers. Then, once the infection overwhelmed the lung's defenses, the bacteria would spill into the bloodstream in large amounts. This process, called "metastatic dissemination," had been the widely accepted explanation for how bacteremia begins.

However, their study revealed something unexpected. While about half of the mice showed this predicted pattern, the other half had bacteria escaping into the bloodstream in a completely different way. In these cases, individual bacteria managed to break free and enter the blood without needing to multiply first. The team called this "direct dissemination."

This discovery suggests that Klebsiella pneumoniae has two ways of spreading in the body. One way, metastatic dissemination, causes a stronger and more severe infection.

The other way, direct dissemination, allows bacteria to enter the bloodstream without first reaching high numbers in the lungs. Over time, infections seemed to shift toward the metastatic pattern, making them harder to treat.

Understanding these different pathways is important for treating bacterial infections more effectively. Doctors often follow a rule in infectious disease treatment: "Find and treat the source."

This means that if bacteria are escaping the lungs in a low and steady trickle, they might be setting up hidden reservoirs in other parts of the body. These small reservoirs could be targeted with treatments before they grow into a full-blown infection.

The researchers also tested bacteria and mice with genetic mutations to see if certain genes affected how bacteria spread. Their findings suggested that the way bacteria interact with the immune system plays a big role in determining which route they take to enter the blood.

Dr. Holmes explained that their study started with a simple but important question: How do bacteria leave the lungs? Now, they have found a major clue. This research closes a gap in scientists' understanding of bacterial infections and may help improve treatments for life-threatening bloodstream infections in the future.

Study Review and Analysis

This study provides key insights into how bacteria enter the bloodstream, challenging the previous assumption that they must first grow in large numbers. The discovery of two separate pathways—metastatic and direct dissemination—suggests that doctors may need different treatment strategies depending on how an infection spreads.

One of the most important findings is that some bacteria can enter the bloodstream early, even when an infection seems mild. This could explain why some patients develop bacteremia even if their lung infection does not appear severe.

It also raises new questions about whether bacteria hide in small numbers in certain areas of the body before causing a major infection later.

Another important takeaway is the role of the immune system. Since bacterial movement depends on interactions between the host and the bacteria, treatments that strengthen the immune response or target bacterial escape mechanisms could be valuable.

More research is needed to understand exactly how these interactions work and how they might differ in humans.

Overall, this study adds valuable knowledge to the field of infectious diseases and could lead to better ways to prevent and treat bloodstream infections.

The research findings can be found in Nature Communications.

Copyright © 2025 Knowridge Science Report. All rights reserved.

Scientists Seek To Tempt Aliens Out Of Hiding With Chemical In Blood

Astrobiologists in Germany are developing a new testing device that could help tease dormant alien microbes into revealing themselves — and its key ingredient is a common amino acid that's found in abundance inside human blood.

"L-serine, this particular amino acid that we used, [...] we can build it in our bodies, ourselves," researcher Max Riekeles, who is helping to develop the alien-hunting device, told Mashable.

The compound is also prevalent across Earth's oceans and even down near the dark and otherworldly ecosystems that surround deep sea hydrothermal vents, where life evolved far away from anywhere it could feed itself via photosynthesis. NASA investigators too have found L-serine and similar "proteinogenic" amino acids — which are vital to many organisms' ability to synthesize their own proteins — buried within meteorites. These and other discoveries have left scientists wondering if any off-world amino acids might have once helped life evolve elsewhere out in the cosmos.

"It could be a simple way to look for life on future Mars missions," according to Riekeles, who trained as an aerospace engineer at the Technical University of Berlin, where he now works on extraterrestrial biosignature research. 

"But, it's always, of course, the basic question: 'Was there ever life there?'"

Riekeles and his team's device benefits from a phenomena called "chemotaxis," the mechanism whereby microbes, including many species of bacteria as well as another whole domain of microscopic organisms called archaea, migrate in response to nearby chemicals.  

Years of research has shown that many tiny organisms have a strong preference for "moving up the L-serine gradient" towards higher L-serine concentrations. This fact led Riekeles and his colleagues to develop their test kit with two chambers divided by a thin, semi-porous membrane: The first chamber would take in a sample from another world, while the second video-monitored chamber would hold a tantalizing concentration of L-serine in water.

"But, it's always, of course, the basic question: 'Was there ever life there?'"

Granted, the idea of studying single-celled organisms just by watching them move around goes all the way back to the earliest days of microbiology, when Antonie van Leeuwenhoek submitted the first paper on these little beings to London's Royal Society in 1676. 

Mashable Light Speed

"Advances in hardware and software the last few years really bring up the really old fashioned way of doing experiments with visual observations," Riekeles said, "especially when you combine it with big data, machine learning and so on."

A graphic of Mars' Valles Marineris, where robotic missions could seek out potential microbes in briny environments. Credit: NASA / JPL / Arizona State University

For their latest experiments, recently published in the journal Frontiers in Astronomy and Space Sciences, Riekeles and his co-researchers focused on three "extremophile" species capable of surviving and thriving in some of Earth's harshest conditions. Each candidate was selected to approximate the kinds of tiny alien lifeforms that might really live on an inhospitable outer space world — like Mars' cosmic ray-blasted, desert surface or Jupiter's icy, watery moons: Europa, Ganymede and Callisto.

"The bacteria Pseudoalteromonas haloplanktis, P. Halo, it survives in really cold temperatures, for example," Riekeles told Mashable, "and it's also tolerant of salty environments."

"And the salty environment, when it comes to Mars, is interesting because there are presumed to be a lot of salts on the Martian surface," he added.

In addition to the microbe P. Halo, which was harvested from the oceans off Antarctica and can grow happily at below-freezing temperatures as low as 27.5 degrees Fahrenheit (-2.5 degrees Celsius), the team also tested the bacterial spore Bacillus subtilis and archaeon Haloferax volcanii. A form of gut bacteria found across animal species, B. Subtilis develops a protective shell capable of enduring temperatures up to 212 F (100 C). And H. Volcanii, found in the Dead Sea and other heavily salted areas, can withstand aggressive radiation exposures, drawing frequent comparisons between it and hypothetical Martian microbes.

"It's not only salt tolerant," Riekeles noted. "If you don't put it into an environment where there is salt, it won't survive."

A culture of Haloferax volcanii bacteria. Credit: Granitehead1 / Wikimedia Commons

All three microbes in the study moved from the sample chamber into the test chamber with the L-serine at a fast clip. Within an hour, each produced a "cell density" of roughly 200 percent more microbes in the test chambers that contained about 1.5 grams of L-sirene per liter of water. What's more, B. Subtilis climbed to 400 percent more bacteria during tests that doubled the concentration of L-serine molecules.

"We tried, also, other substances, like glucose and ribose," Riekeles added, "but L-serine was, for these three organisms, the most potent."

However, Dirk Schulze-Makuch — a professor of planetary habitability at the Technical University in Berlin, who worked with Riekeles on this project — cautioned that challenges still remain before a device like this can touch down on the Martian surface.

"One big problem," Schulze-Makuch wrote for the website Big Think, "is finding a spot that's accessible to a lander but where liquid water might also exist." 

"The Southern Highlands of Mars would meet these conditions," he said. Another possibility would be low-altitude spots on Mars like the floor of the expansive canyon Valles Marineris or inside caves, where "atmospheric pressures are sufficient to support liquid (salty) water."

Scientists Track Pneumonia-causing Bacteria As They Infect The Blood Stream

Bacteremia, or blood poisoning, occurs when bacteria overcome the body's immune defenses.

Bacteremia can worsen into sepsis, a condition that accounts for more than 1 in 3 hospital deaths per year.

Yet people are routinely exposed to and fight off bacteria from the environment without this deadly series of events occurring.

Scientists are trying to figure out exactly how bacteria spread throughout the body to cause systemic infection in the hopes of eventually stopping this process in its tracks.

Michael Bachman, M.D., Ph.D., clinical associate professor of pathology and microbiology and immunology at U-M Medical School and former postdoc Caitlyn Holmes, Ph.D., have tried to solve this mystery, focusing on gram negative bacteria like Klebsiella pneumoniae, a common source of pneumonia-initiated bacteremia.

In previous work, they determined that bacteria spread in three phases: infection of an initial site, such as the lungs; entrance into the bloodstream; and finally, replication and avoidance of filtration by the liver and spleen.

Traditionally, analyzing a bacterial infection is done by culturing tissue and counting the number of resulting bacteria.

"Experimentally, we can measure the first phase pretty easily in terms of how the bacteria infect the lungs and we can measure the third phase pretty easily in terms of how the bacteria survive in these blood-filtering organs and whether they replicate or not. But that transition out of the lungs and into the bloodstream has traditionally been difficult to measure," said Bachman.

Using an innovative barcoding-style system developed with colleagues at Harvard University, Bachman, Holmes and their team were able to label bacteria with short snippets of DNA in mouse models and use computer analysis to track the movement of K. Pneumoniae throughout the body.

They expected that the bacteria would replicate in the lungs until such a point that their clones overwhelmed the lung's defenses, spilling out into the blood stream, says Bachman.

And while they did see this type of spread -- which they called metastatic dissemination -- there was evidence of another type as well.

Unexpectedly, "about half of the mice had the metastatic pattern, and the other half contained bacteria that escaped on their own into the bloodstream without the need to replicate to large numbers first," Bachman explained about this second mode, which they called direct dissemination.

Overall, metastatic pathway correlated with a stronger infection than the direct route.

Furthermore, over time, infection progressed to more of the metastatic pattern.

"We need to understand the biology of each of these routes in order to figure out the best treatments," said Bachman.

"There's a mantra in infectious disease that is to find and treat the source to stop the bacteremia."

Uncovering the existence of the direct route may mean that bacteria are setting up low level reservoirs in other parts of the body that could be better targeted to treat blood infections.

Additionally, Holmes created mutations in both the K. Pneumoniae and mice that affected the mode of dissemination hinting that the interaction between the bacteria and the host's immune system may determine the outcome of the infection.

"The project began with a very basic question -- how does bacteria leave the lungs -- that we have now provided some insight into, closing a significant gap in our knowledge," said Holmes.

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