The recent appearance of the fantastic HBO series “The Last of Us”, telling about humanity overwhelmed by a pathogenic fungus, aroused public interest: could such an apocalyptic scenario one day become a reality? Soon, the case of the plant fungus that first infected a person, which circled the world’s media, did not greatly surprise scientists, but drew attention to the need for more intensive research in this area.
Opportunistic bacteria, which are exceptionally well adapted to survive in the medical environment, are also studied by scientists of the Life Sciences Center of Vilnius University. One of them is Dr. JŪRATĖ SKERNIŠKYTĖ – tells what mechanisms these bacteria use to survive in unfavorable media for them and whether truly opportunistic pathogens could take over humanity in the future.
Exclusivity – high antibiotic resistance
Opportunistic microorganisms – whether bacteria, fungi or viruses – are a common part of our environment and the human skin microbiota (Figure 1). It is the name “opportunistic” that means that in itself that microorganism is not necessarily pathogenic. In order for the opportunistic microorganism to become pathogenic, special conditions are required, for example, weakened human immunity. This is also how one of the best-known Herpes virus works – the host can carry the virus without feeling any symptoms, as long as provoking factors such as stress, weakened immunity activate the virus.
“The opportunistic pathogen is looking for niches where there are the most vulnerable people, and these are usually hospitals, intensive care units. Infections detected in hospitals – pneumonia, sepsis, wound contagion – are called nosocomial (hospitalized). According to the latest data of the European Center for Disease Prevention and Control, such infections affect about 6-7 per cent of all hospitalized patients,” says dr. J. Skerniškytė.
According to the scientist, although the number of such cases seems small, it is these infections that can be fatal for a person, since the opportunistic bacteria that cause them are extremely resistant to antibiotics, and their resistance is gradually increasing.
“The resistance to antibiotics (carbapenems) of the Acinetobacter baumannii type of bacteria, which we are also studying in our laboratories, is about 82% in Europe. As a result, even several states have reported outbreaks of nosocomial infections caused by opportunistic pathogens in intensive care units of COVID-19 patients during the pandemic years. On the other hand, in some countries, a decrease in infections has been recorded due to exceptionally high requirements for disinfection,” says the researcher.
Extremely dangerous for some groups of people
Although intensive care units appeared relatively recently – only after World War II , opportunistic bacteria have already been able to adapt to survive in such an environment of increased care. According to dr. J. Skerniškytė, these bacteria are able to attach to the surface of clinical equipment – intubation tubes, catheters – and form special structures, the so-called biofilms, which allow them to multiply by protecting themselves from antimicrobial agents used for disinfection (Figure 2).
“The bacteria then separate from the mature biofilm and travel directly to the bloodstream, lungs, or other internal organs through that catheter or intubation tube. Since the immunity of people in the intensive care unit is usually weakened, bacteria are able to attach and colonize organs, and being resistant to some antibiotics leads to difficult treatment and ultimately to the death of a person,” says the researcher.
According to Dr. J. Skerniškytė, these bacteria will not harm a person with strong immunity, but they are dangerous for several critical categories of people: the elderly, patients after organ transplantation, after chemotherapy and persons infected with the HIV virus. In order to avoid negative consequences for these and other patients in intensive care units, doctors are constantly resorting to newer detection tools in order to detect infections as early as possible and prescribe those antibiotics to which bacteria are not resistant.
“At the Life Sciences Center of Vilnius University, we study species of opportunistic bacteria that are naturally found only in habitats such as soil or water, but are not part of the human microbiota. And although the cause of nosocomial infections by these bacteria is relatively small so far, they are characterized by exceptionally high antibiotic resistance. Our data shows that bacteria can bring antibiotic resistance mechanisms from the environment,” explains Dr. J. Skerniškytė.
Bacteria use universal techniques for self-protection
According to scientists, the conditions in which bacteria live in the soil can also help to adapt in hospitals – bacteria in the soil are exposed to billions of other microorganisms with which they compete for nutrients and survival.
“For example, we know that bacteria secrete molecules in the soil called siderofores, which help to absorb the vital iron. There is also not much free iron in the human body, in the blood, and we see that bacteria use the same mechanisms to assimilate iron. Another case is protozoa that feed on bacteria in the soil, such as amoeba, which phagocyte (envelop and break down) bacteria in a similar way to macrophages (cells involved in the body’s defense against bacteria and other pathogens) in order to destroy that bacterium in the human body. Therefore, bacteria that are able to protect themselves from amoebs also protect themselves from phagocyte cells of the human immune system,” says dr. J. Skerniškytė. Thus, we can say that the environment becomes like a kind of school for opportunistic bacteria, where they can apply the acquired survival strategies to cause infection.
In addition to these universal techniques, bacteria are able to form polysaccharide capsules, which also contribute to protection. These capsules act as a shield, due to which macrophages in the human body do not recognize the bacterium and do not fight it. Another structure is the outer membrane vesicles of bacteria, which help protect against antibiotics (Figure 3). To destroy the bacterium in the human body, antibiotics must get inside it. However, if the bacterium secretes these vesicles, they, like special traps, make antibiotics harmless even before they enter the bacterium itself.
“Laurita Klimkaitė, a doctoral student working in our laboratory, studies the pathogenicity potential of Stenotrophomonas bacteria from the clinic and soil samples. The initial results show that clinical bacteria are more virulent, but when we compare the genetic profiles of clinical and natural bacteria, we see many similarities. Therefore, we are trying to find out whether bacteria isolated from the environment could cause infection,” says the scientist.
One of the indicators that a bacterium could infect a person is temperature. The temperature of the soil in which microorganisms multiply is much lower than that of the human body – 37 °C, which makes it difficult for a large part of soil bacteria to adapt to higher temperatures.
“However, in our research, we keep noticing one other soil bacterium that is able to function at higher-than-normal temperatures as well. Therefore, it is possible that we bring those bacteria to hospitals ourselves, and when conditions change, they can become pathogenic for patients. Interestingly, to date, the natural habitat of the Acinetobacter baumannii bacteria is not entirely clear. They are rarely found in the soil, most of them are found precisely in hospitals. It seems that by creating special conditions for the treatment of a group of vulnerable people, we have also created an environment for the evolution of microorganisms,” says dr. J. Skerniškytė.
The script of the film is hardly possible
The topic of climate change, which is currently being actively discussed, is causing a lot of debate among scientists, and precisely because of the hypothesis that a warming atmosphere will also stimulate the evolution of microorganisms, their adaptation to live at higher temperatures. This can make it easier for microbes to infect warm-blooded animals – a scenario that’s what’s being explored in the fantastic HBO series The Last of Us. The creators raise the idea that, in fact, the existing fungus of tropical forests could infect humans as well.
“In reality, the fungus Ophiocordyceps unilateralis is very specific and infects only certain insects, usually a specific type of ants – by affecting the central nervous system, it forces them to behave as it is useful for the fungus – to go to a specific part of the forest, where the lighting is most suitable, where the wind blows in a certain direction. The fungus takes the ant on a tree leaf and, when it dies, it grows, and then it can throw out the spores in the most suitable way for itself,” says dr. J. Skerniškytė.
After scientists explained that it was impossible for the plot of the fantastic series to turn into reality, the world’s media was surrounded by a story about a researcher of plant fungi living in India, who was first infected with a plant fungus in history. Although this fungus is also adapted to infect only low-temperature organisms – plants, doctors soon found that the infected botanist had weak immunity and, working with plants and large amounts of this fungus for a large part of his life, became an exceptional case in the history of the world.
“This fungus is opportunistic, so it adapted to infect a particular person because it had the right conditions for it. Finally, the researcher was cured, so there is nothing to fear. However, it is a good reminder that climate change is not just an economic problem,” says the scientist.
She points out that people should not fear the predominance of microorganisms on Earth, but rather take care of their health, check it regularly and not engage in self-medication, especially when taking antibiotics: “People tend to stop taking antibiotics or reduce the dose of drugs, barely feel better. The exact dose of antibiotics prescribed by doctors and the duration of administration is extremely important, because not all bacteria are killed when they are not followed, and the rest are given excellent conditions for adapting and acquiring antibiotic resistance.”