On bacteria and radiation

About radiation and bacteria One means of getting rid of unwanted bacteria is lethal radiation, but there are bacteria that indirectly use that radiation as an energy source, and others that are resistant to "out of this world" levels of radiation

If you look through the glass window into a modern operating room, when it is empty of people, you will notice that it is illuminated with purple light. This light comes from lamps installed in the room, which also emit UV radiation (which itself is invisible).

These lamps are lit when the room is empty, in order to destroy all the microorganisms that disinfection by other means did not destroy. UV radiation facilities are also found in biological safety cabinets - special cells used to work with dangerous bacteria and viruses - where they are used to disinfect the facilities after work.

Radiation can be fatal to all creatures and not just to microorganisms. Although we use radiation to destroy unicellular organisms, they are less sensitive to radiation than multicellular organisms. A measure of this matter is LD50 (lethal dose 50), meaning the dose of radiation that kills 50% of the population. In bacteria, LD50 is usually 10 times higher than that which kills multicellular organisms (such as humans).

Outside of hospitals, gamma radiation is used to sterilize two types of products - medical supplies, such as surgical instruments, sewing materials, disposable laboratory tools, medicines, syringes, gloves and catheters; and food products, including meat and poultry products, mainly minced meat, vegetables and spices.

Right: Giemsa staining of Deinococcus radiodurans bacteria quartets through a light microscope. The chromosomes are colored black. Left: a section of a bacterial quartet in an electron microscope. Courtesy Michael J. Daly, Uniformed Services University, Bethesda, MD, USA

What is radiation? The types of electromagnetic radiation can be divided into two groups: non-ionizing radiation and ionizing radiation.

Non-ionizing radiation has relatively long wavelengths and includes radio and television waves, mobile phones, radar, microwaves, infrared radiation, visible light and ultraviolet (UV) radiation. This radiation damages the surface only, and does not penetrate the depth of materials.

Micro radiation is mainly manifested in warming. Ultraviolet radiation (with a wavelength of 300-220 nm) is absorbed by the hereditary material - the DNA - and causes the formation of dimers (pairs) of bases (mainly thymines) and distortions in the structure. Too many structural distortions in DNA (that the cell is not enough to repair) cause errors in DNA replication and cell death.

Ionizing radiation is lethal to living creatures, so it is necessary to use radiation protection measures when in an area with a high level of exposure. This is radiation with shorter wavelengths, which includes X-rays (X-rays), gamma rays (g) and cosmic rays. Such radiation causes changes in the electron composition of the molecules through which it passes, and induces the creation of ions, free radicals (such as hydroxyl radical, OH., and hydride radical, H.) and free electrons, which cause serious damage to many molecules in cells, and especially to DNA. Ionizing radiation penetrates into the irradiated bodies.

Gamma radiation is mainly used for industrial needs. The commercial sources of this radiation are the radioactive isotopes cobalt-60 (60Co) and cesium-137 (137Cs), which are by-products of nuclear reactor activity.

The useful units of radiation when working with bacteria are rad and gray:

Rad = 100 weaves per gram of irradiated material. (Arg = 10-7 joules)

1 gray = 100 rad

Data: Brocks Biology of Microorganisms

Radioactive radiation as an energy source for a unique habitat
Researchers from many universities, led by Thalis Onstott (Onstott) from Princeton University, found in South Africa, at a depth of three kilometers below the surface of the earth, a habitat that has not been disturbed by human activity until now, where several species of bacteria and archaeons live.

This habitat has no sunlight, so the food chain in it cannot be based on photosynthesis. This habitat has rocks that contain radioactive uranium and sulfur compounds. The research team was unable to isolate and grow bacteria and archaea from this habitat, but was able to verify their existence by identifying 16S-RNA sequences using PCR. The team found that the main bacteria (about 88%) in the habitat is a bacterium from the Firmicutes group, which is genetically close to thermophilic (heat-loving) sulfur-reducing bacteria.

After examining the metabolites in the environment, the researchers concluded that the bacterium indirectly absorbs the radioactive radiation energy emitted from the uranium, which causes the decomposition of water molecules and the emission of molecular hydrogen (H2). The bacterium oxidizes the molecular hydrogen using the sulfur, and the energy released in the process is what feeds it and the other bacteria and archaea in the system, which are probably fed by the bacterium itself or by products it secretes into the environment.

The radiation-resistant bacteria "out of this world"
In 1956, at a meat canning plant in Oregon, United States, food spoiled in several cans irradiated with gamma radiation. In the irradiated boxes, researchers Arthur Anderson and his colleagues from the Oregon Agricultural Research Station found a bacterium that is now known as Deinococcus radiodurans (in Greek, Deinos means "strange"). It turned out that the bacteria is resistant to radiation levels of 10,000 Gray.

For comparison, a radiation level of 5-6 Gy is lethal to humans and a radiation level of 1,000 Gy will sterilize a solution of Escherichia coli bacteria (for other examples, see table). Such high levels of radiation, claim researchers Kenneth Minton (Minton) and Michael Daly (Daly) from the Uniformed Services University in Maryland, have never existed on Earth, not even in its early days.

From this arises an interesting evolutionary question. What is the origin of the bacteria's resistance to radiation? Is the appearance of resistance a side effect of drought resistance, for example? Or did these bacteria come from somewhere in outer space, from a place where radiation levels are extremely high, as claimed by the followers of panspermia, the theory that claims that the origin of life on Earth is in outer space.

The strange bacterium is resistant even to extremely dry and cold conditions. Its resistance to these three factors - radiation, cold and dryness - earned the bacterium the title of "the toughest bacterium in the world", and with this title it was included in the Guinness Book of Records in 1995.

The bacterium was harnessed for the purposes of cleaning and purification of areas contaminated with both chemical waste and radioactive waste, since the radioactive radiation kills the known bacteria capable of purifying and recycling the chemical waste, and the area remains contaminated.

Although, D. radiodurans does not break down organic solvents, such as toluene and phenol, but the introduction of the appropriate genes into the bacterium created a "superbug" capable of breaking down organic solvents in a radioactive environment. The introduction of genes for resistance and the recycling process of heavy metals, such as mercury (turning toxic mercury ions into metallic mercury - which is less toxic) created the "ultimate recycling bacteria".

The circular structure of the DNA of Deinococcus radiodurans Prof. Avraham Minsky's laboratory, Weizmann Institute of Science

Over resistance
The radiation damages the bacterium's genetic material, and the bacterium uses several enzyme systems to repair the radiation damage. But while a normal bacterium can repair up to about 5 simultaneous DNA damage, D. radiodurans can repair over two hundred. Are repair systems for radiation damage the cause of the hyper-resistance of this bacterium? - in a careful examination of the repair systems of D. radiodurans

And even replacing them with corresponding key enzymes from E. coli, it turned out that the repair systems are indeed necessary for resistance, but they are not responsible for the bacterium's super-resistance.

The solution to the mystery was found in 2003, in the laboratory of Prof. Avi Minsky from the Weizmann Institute. It turns out that the bacterial chromosome is packed in a tight ring-like structure, which does not allow the DNA fragments that are created due to radiation damage, and which are emitted from the chromosome, to reach the cytoplasm and disappear there (and see: Adeva Baruch, "Three preserved rings, one working", news section, "Galileo" 54 ). This allows the repair systems to repair the damage relatively easily - the broken pieces of DNA must be reconnected, not re-produced. This fact is what gives the bacterium such a high resistance to radiation.

And not only that: it turns out that the quartets observed under the microscope are not bacterial quartets, but one bacterium containing four copies of the hereditary material, each of which is in a separate cell compartment. At least two of the copies are always packed in the ring structure and are used as a backup, while one or both of the other copies are "released" from the ring structure and are used by the transcription and translation systems. If one of the copies is damaged, it is repaired by the repair enzymes, and immediately afterwards one of the ring structures in the adjacent section is "released" and the "released" chromosome passes through a special opening to the section where the "repaired" chromosome is located, for the purpose of comparing and reproducing the hereditary information.

In conclusion, Lethal radiation for the various issues is used by us to kill unwanted bacteria in medical products and food, by damaging their DNA. But there are bacteria and other creatures that owe their existence to radioactive radiation, as an (indirect) source of energy. Another bacterium is extremely resistant to ionizing radiation thanks to a special circular packaging of the DNA, and is able to recycle and purify toxic chemicals in polluted areas.

Dr. Dror Bar-Nir teaches microbiology and cell biology at the Open University

for further reading : The article in Science describing the special habitat in South Africa:
Long-Term Sustainability of a High-Energy, Low-Diversity Crustal Biome, Lin et al., Science 20, October 2006: 479-482.

The article in Science describing the resistance of Deinococcus radiodurans to radiation:
Resistance to Radiation, Daly and Minton, Science 24, November 1995: 1318.

Deinococcus - the most stable organism on earth, Pinchas Fox, Synthesis 13, 1996.

Deinococcus radiodurans - the radiation-resistant bacterium, Dror Bar-Nir, 2003.

Weizmann Institute scientists revealed the defense mechanisms of the world's most resistant creature to radioactive radiation, 2003.

frame:

Deinococcus radiodurans

It is an aerial, gram-positive, pink-colored bacterium. Belongs to the Deinococcales division.

In addition to its resistance to extreme conditions, this bacterium is also strange in that it is the only bacterium that stains positively in Gram staining, which does not belong to the two divisions that include all other Gram-positive bacteria (the Firmicutes and the Actinobacteria). A careful examination of the structure of its shell revealed that its basic structure is that of Gram-negative bacteria, but it is the addition of several layers that are unique to this bacterium that prevents the color from being washed off the bacterium.

For many years researchers have claimed that the presence of the dyes in bacteria in general helps them protect themselves from radiation. A (relatively) simple experiment - isolating colorless mutants of the bacterium and exposing them to high levels of radiation - disproved this claim. Colorless mutants are as resistant to radiation as the pink wild strain.