Antibiotics won’t work this time？ Let’s look at the attack and defense of pathogens and human body

At present, it is popular in novel coronavirus. At the same time, we also hear the news that there is no specific drug at present. Some people may ask: Why can’t we treat it with antibiotics?

Because the pathogen this time is a virus, not a bacterium!

What are the similarities and differences between viruses and bacteria? Why can’t we kill the virus with antibiotics?

Image source: veer gallery

Are tiny creatures on the earth.

Bacteria and viruses are microorganisms. As the smallest life form on the earth, people always ignore their existence, but this tiny creature is always painted with a sense of existence, and people will be infected by them if they are not careful.

Bacteria are cells, which have cell walls, DNA and organelles. They can produce and metabolize needed enzymes by themselves, and can divide and reproduce by themselves. Viruses are much smaller than bacteria, and the main structure is protein capsid and internal genetic material (DNA or RNA), and viruses cannot replicate themselves. Viruses need to replicate their genetic material by infecting host cells, and then release more offspring viruses to infect other host cells.

Bacteria can be harmless or even beneficial to human health, and can live independently. The purpose of the virus is to replicate itself, so it has no meaning unless it infects the host. It can be described as the purest "selfish gene".

The size difference between them is about 1000 times.

Bacteria in a broad sense are prokaryotes. Although they have a cell structure, they are still very different from human cells. Their structures are simpler, and they have no nucleus, but only a DNA aggregation area-pseudonucleus. Bacteria are generally spherical, rod-shaped, spiral, etc., and people do not forget to add a description of their shapes when naming them, such as Escherichia coli, Lactobacillus, Staphylococcus aureus and so on (ps. Yeast is a fungus, so there is no shape in the name.

At present, the smallest known bacteria are only 0.2 microns long, so they can only be seen under a microscope; The largest bacteria in the world can be directly seen by naked eyes, and it is 0.2-0.6 mm in size. It is a kind of bacteria called Sulphidophilus Namibia.

Bacterial size

Virus is a noncellular form composed of a nucleic acid molecule (DNA or RNA) and protein. It lives by parasitism and is an organic species between living things and nonliving things. After it enters the cell, its metabolism such as DNA replication is indeed the characteristic of living things, while after it leaves the cell, it is just a lifeless crystal.

Novel coronavirus electron micrograph 　

Most viruses are between 10 and 300 nanometers (nm) in diameter. Some filamentous viruses can be as long as 1400nm, but their width is only about 80nm. Most viruses cannot be observed under optical microscope, and scanning or transmission electron microscope is the main tool to observe the morphology of virus particles. Compared with bacteria, viruses differ in size by about 1000 times.

There are also different ways to invade the human body.

There are many "good people" in bacteria, among which saprophytic bacteria are important decomposers in the ecosystem, which makes the carbon cycle go smoothly; Some bacteria will carry out nitrogen fixation, so that nitrogen can be converted into forms that can be used by biology. The production of cheese, yogurt and fermented grains, the manufacture of some antibiotics and the treatment of wastewater are all related to bacteria. In the field of biotechnology, bacteria are also widely used. Of course, the "bad guys" in bacteria are many pathogenic bacteria, including tuberculosis, anthrax, plague and other diseases are caused by bacteria.

The relationship between bacteria and human body is mainly parasitic. Let’s mainly talk about pathogenic bacteria. Because most bacteria have their own metabolic system, bacteria can be parasitic between normal cells of human body after invading human body. Bacteria take away the nutrients necessary for the body; The growth of bacteria produces various metabolites, which disrupts the physiological balance of the body; Even the size of bacteria has become a pathogenic factor, which interferes with and destroys the function of cells. Therefore, in some diseases, the proliferation of bacteria alone has fatal consequences.

Parasitic diagram of staphylococcus aureus

The virus may have invaded the human body from a sneeze or a physical contact. Viruses need to parasitize in living host cells, and rely on host cells to provide the raw material system, energy and place needed for virus replication. When a virus is ready to infect a host cell, it needs the following six steps to complete its proliferation activities, namely adsorption, invasion, hulling, biosynthesis, assembly and release.

Adsorption: The virus "targets" the target cell by recognizing the specific receptor protein molecules on the surface of the host cell membrane (for example, novel coronavirus virus recognizes angiotensin converting enzyme 2 -ACE2)； on the surface of human respiratory tract and lung cells); Invasion: then the virus either enters the host cell in some way (such as membrane fusion) or directly injects genetic material into the host cell; Shelling: Immediately after the viral infectious nucleic acid is released from the capsid; Biosynthesis: "non-stop" biosynthesis-synthesizing virus nucleic acid and protein according to gene instructions and with the help of raw materials, energy and places provided by host cells; Assembly: The newly synthesized virus nucleic acid and protein will be assembled into progeny virus; Release: the progeny virus is released outside the host cell.

Schematic diagram of virus replication

Human body’s defense and counterattack

Both bacteria and viruses have to break through the human body’s defenses to complete the invasion, but the human body will eventually find their existence, and the human immune system has its own protective measures and early warning mechanism.

The first line of defense: skin and mucous membrane. The first way for human beings to protect themselves is defense, which makes the human body form a relatively closed system through the skin and mucosa. When harmful substances are about to invade the human body, the skin and mucosa will block the external pathogenic factors out of the body.

The second line of defense: bactericidal substances and phagocytes. There are always bactericidal substances and phagocytes patrolling the mucosal surface and inside the human body to prevent the invasion of pathogens. Take lysozyme as an example, it can destroy the cell wall of bacteria, cause the contents of the cell wall to escape and dissolve the bacteria, and can also directly combine with negatively charged virus proteins to form double salts with DNA, RNA and apoprotein, thus inactivating the virus. Therefore, the enzyme has antibacterial, anti-inflammatory and antiviral effects.

The third line of defense: specific immunity. When the powerful pathogen broke through the first two lines of defense, the human body’s counterattack just began. Through the phagocytosis of phagocytes and the analysis of special immune cells (T cells), the human body has produced antibodies that can specifically recognize invaders. The antibodies can make pathogens stick together and no longer be invasive, and finally the pathogens exposed between cells will be killed. But is the virus that invaded the cell safe? No! The human body can subtly identify which cells are infected by the virus, and then send a "killer" to kill the infected cells and release the virus inside the cells for antigen elimination.

However, it takes time for the human body to fight back, and pathogens will invade and occupy the human body through such a time gap!

Auxiliary appearance: drug therapy

Bacterial infection once became the greatest enemy of mankind. For example, the plague was called the "Black Death" in Europe, which reduced the population of Europe by one third in three years. At present, plague and cholera are classified as Class A infectious diseases in China. It was not until the discovery and popularization of antibiotics that human beings controlled the outbreak of bacterial infection.

Antibiotics mainly kill bacteria by destroying the structure of bacterial cells, such as cell wall, cell membrane, changing internal metabolism, hindering the synthesis of nucleic acid and protein, etc.

Chemical structural formula of penicillin

However, antibiotics are ineffective against viruses! Because bacteria and viruses have completely different structures!

Because bacteria have cell walls, as well as their own nucleic acid replication machines and ribosomes, antibiotics can kill bacteria with little side effects on humans as long as they are designed for these targets. However, the virus has no cell wall, its own nuclease and ribosome, and all its functions depend on the host cell. So antibiotics can’t kill viruses (we can’t make an "antibiotic" that kills host cells).

The ideal antiviral drugs can not only act on one or several links of the virus proliferation cycle, but also interfere or block it without affecting the normal metabolism of host cells. For example, ribavirin, a common drug, provides a large number of nucleotide analogues, which replace normal nucleotides, making the virus lose its replication ability and play a role in inhibiting virus amplification (but it also has great side effects on human body). In addition, the anti-influenza drug oseltamivir works by blocking the release of the offspring virus.

In view of this pneumonia epidemic, researchers from Wuhan Institute of Virology, China Academy of Sciences and Academy of Military Medical Sciences have also preliminarily screened out ramidivvir (GS-5734, phase II clinical anti-Ebola drug) and Chloroquine (Sigma-C6628, antimalarial drug) which have good inhibitory effects on novel coronavirus (2019-nCoV) at the cellular level.

But we have to say that in the face of virus treatment, human beings have never found a universal specific drug like antibiotics. Active treatment is to mobilize the human body’s own immune ability to fight against viruses, because only organisms themselves really know how to fight against organisms.