The game between “minimal creature” and the immune system, the secret behind the mycoplasma pneumoniae pneumonia

Humans have discovered diseases caused by mycoplasma very early. In 1693, a German doctor first reported a local case of bovine contagious pleuropneumonia. This disease has been a major enemy of European animal husbandry for a long time. In the early 19th century, bovine infectious pleura pneumonia became popular throughout Europe. In 1843, the United States also reported the first local case. At the time, although there was a lot of research on this disease, no one had ever discovered what the pathogenic microorganisms that caused the disease were.

In the latter part of the 17th century, the Dutchman Antony van Leeuwenhoek invented the microscope, which also made him the first person to observe the microorganism such as bacteria. More than 200 years have passed, although the observation of bacteria has been relatively mature, scientists at that time still could not find the causative agent of bovine infectious pleura pneumonia. This has to make people wonder. Behind this disease is It is not that another pathogenic microorganism is lurking. In 1889, two doctors working at the Pasteur Institute in France were the first to isolate a microorganism from the lesions of bovine infectious pleural pneumonia. After culturing, they found that the colony of this microorganism was very small, and the bacterial body was not easy to stain, and it was difficult to identify the morphology. Therefore, at that time, it could only be generally referred to as pleural pneumonia microorganism (PPO). Since then, people have continuously found similar microorganisms in the lesions of animals such as sheep, dogs, rats, and poultry, so scholars collectively refer to this type of microorganism as pleura pneumonia-like microorganisms (PPLO).

In 1937, the second year that humans first observed the virus directly under an electron microscope, this pleurisy pneumonia-like microorganism was discovered in the human body. In the 1950s, this microorganism was first isolated from cells cultured in vitro. Subsequently, this microorganism was increasingly isolated in various organisms. When these pleuropneumonia-like microorganisms grow in liquid medium, most of them appear branched filamentous, so some people call it “mycoplasma”. In 1956, the International Bacteriological Organization officially named this type of microorganism as mycoplasma.

Before and after the Second World War, some experts discovered that there was a small-scale epidemic of pneumonia in the military and schools. The patients were mainly children and young people. This pneumonia is different from the well-known Streptococcus pneumoniae pneumonia. There is no large leaf-shaped shadow on the X-ray film, and no streptococcus pneumoniae can be cultured in the sputum. At that time, people could not find out the cause of the pneumonia, so they called it atypical pneumonia. Later, a scientist named Eaton isolated a pathogen from the sputum of a patient. He found that the pathogen had certain characteristics of the virus, but it was not completely like a virus. It could infect chicken embryos, but could not infect cells. . This pathogen between bacteria and viruses was called “Eaton factor” at the time, and this atypical pneumonia was also thought to be caused by “Eaton factor”.

Until the 1960s, when Dr. Hayflick developed a new experimental medium that could provide cholesterol and protein for mycoplasma replication, this “Eton factor” was isolated from egg yolk samples, confirming its mycoplasma Identity.

This mycoplasma, which can cause pneumonia and many other diseases, is actually the smallest cell-like organism found so far that can survive independently and replicate itself. Its diameter is generally 0.1 ~ 0.3 microns, only 1/10 of bacteria. Most filters for bacteria can pass through, so we also say that mycoplasma is a microorganism between bacteria and viruses.

In addition to causing infectious diseases, mycoplasma also contaminates a large number of cultured cells in vitro. Because it is ubiquitous, it can replicate itself and survive independently. For a long period of time, 57% to 92% of in vitro cultured cells have been contaminated with mycoplasma. Mycoplasma can not only pass through the filter device designed for the size of bacteria, but also resist the commonly used antibiotics, which has caused great problems for biological cytology research.

Unique characteristics, unique pathogenic process
In addition to its small size, Mycoplasma is characterized by its absence of cell walls, which is different from bacteria. At present, it is believed that Mycoplasma is evolved from Gram-positive bacteria with cell walls. During the process of evolution, the cell wall mutation is lost for some reason. There is no cell wall at all, which also explains many unique properties of Mycoplasma. For example, it is very sensitive to osmotic pressure and disinfectants. It is “immune” to penicillin antibiotics, and it also forms unique “omelette-like colonies.”

“There are many types of mycoplasma, among which mycoplasma pneumoniae is one of the most common pathogens of respiratory infections in children.” Director Zhao introduced, “Mycoplasma pneumoniae has two suction cup-like structures, that is, P1 and P30 adhesion proteins on the top of mycoplasma pneumoniae filaments , It can firmly adhere to the surface of epithelial cells.” The surface of airway epithelial cells has a layer of structure that can swing autonomously-cilia. They have the ability to swing directionally and are very sensitive. They can expel sputum and pathogens that invade the respiratory tract. . When the cilia function is impaired, the airway sputum excretion capacity decreases, and the increased sputum in the airway will stimulate the airway, causing coughing and sputum production. After mycoplasma pneumoniae invades the respiratory tract, it will be located between the cilia by gliding movement, and it will firmly adhere to the surface receptors of epithelial cells with the help of two adhesion proteins to resist the removal of cilia and the phagocytosis of phagocytes. At the same time, adhesion proteins also have direct cytotoxicity, and the secretion of community-acquired respiratory distress syndrome toxin can cause airway mucosal damage such as vacuolation of epithelial cells and loss of cilia function. On the other hand, the body’s immune inflammatory response, the inflammatory cascade amplification effect produced by cytokines, and the body’s own immune mechanism all participate in the complex pathogenic mechanism of M. pneumoniae.

The specific process of mycoplasma pneumonia is also different from common bacterial pneumonia and viral pneumonia. “Bacterial infections are often invasive, can destroy tissues and cells, cause inflammation, exudation, necrosis and purulence of local tissues, and also invade the blood to cause sepsis; while viral infections invade the cells of the body, and the virus is abundant in the cells of the body Copying, causing cell death.” Director Zhao said that Mycoplasma pneumoniae causes inflammation and immune response when it hardly enters the cell. “If we understand our respiratory tract organization as our home, a bacterial infection is like the home breaking into a group of’big men, robbing the house and destroying the house, a mess; the virus infection is like a few elves I broke into the house, entered the wall, and turned upside down; and mycoplasma, like a few Kongwu powerful’reckless men, silently stood at the door of the house and the wall, surrounded the house, pushed and shook, released Toxins damage our home.”

In addition to local damage to the airway, Mycoplasma pneumoniae is also characterized by immune inflammation. Typical mycoplasma pneumoniae pneumonia is more common in older children, which is related to the body’s immune inflammatory response. When mycoplasma pneumoniae infects newborns and infants, due to their weak immune and inflammatory response, they often only have some local symptoms such as coughing and wheezing. Children over the age of five have gradually improved immune function. When Mycoplasma invades the respiratory tract and is adsorbed on the cilia surface, a large amount of Mycoplasma pneumoniae antibodies produced by the body will combine with the antigen of Mycoplasma pneumoniae, and the antigen-antibody binding will attract inflammatory cells and inflammation The medium infiltrates the interstitial lung. “So we often say that Mycoplasma pneumoniae pneumonia is an interstitial pneumonia, which damages the lung interstitial and damages the cilia of the respiratory tract, and rarely violates the alveoli in the early stage. The cilia damage of the respiratory tract causes difficulty in sputum excretion, and the patient will cough and expel sputum , And a strong immune inflammatory reaction will cause fever. So coughing and fever are the two most typical symptoms of Mycoplasma pneumoniae pneumonia.” And these two symptoms are common in respiratory diseases, especially respiratory infectious diseases. It is necessary to make a differential diagnosis with other upper respiratory tract infections and pneumonia, which is also a necessary step in the diagnosis of Mycoplasma pneumoniae pneumonia.

Fever caused by Mycoplasma pneumoniae pneumonia is also different from bacterial infection. Endotoxins—lipopolysaccharide molecules are often present in the cell walls of bacteria. The human body is extremely sensitive to it. Very small amounts (1 to 5 ng/kg body weight) of endotoxins can cause body temperature to rise, so bacterial infections often cause high fever, and children will Accompanied by obvious symptoms of infection and poisoning, he is depressed. Mycoplasma does not have such a cell wall containing endotoxin. Fever caused by mycoplasma infection is often related to excessive immune inflammatory response, so mild mycoplasma infection is often low fever, and severe fever of severe pneumonia pneumonia mycoplasma pneumonia often reacts with immune inflammation. Related, because there is no endotoxin, the child has no obvious symptoms of infection. When the body temperature drops to normal, the child usually has a good mental state.

Less alveolar exudation is also a clinical feature of Mycoplasma pneumoniae pneumonia. Most lung inflammations, especially those caused by bacteria, have more exudate in the alveoli. The doctor can hear the fine wet rales produced by the gas passing through the fluid in the alveoli through the stethoscope. However, mycoplasma pneumoniae pneumonia often does not invade the alveoli in the early stage, and the inflammation of the alveoli is less oozing, so the wet rales will not be heard on auscultation. “If the doctor is not experienced enough or negligent, it is easy to miss pneumonia. Obvious fever, cough, course of more than 3 days, even if the lungs can’t hear the fine wet rales, a chest radiograph should be taken for examination. Because M. pneumoniae pneumonia often does not hear wet rales, when clinically suspected M. pneumoniae pneumonia Chest radiographs should be taken in time to avoid missed diagnosis, and parents should fully understand.” Director Zhao said.

How is the detection of mycoplasma carried out?
At present, there are not many detection methods for Mycoplasma pneumoniae pneumonia. In clinical diagnosis and treatment, it is actually a more complicated matter to use comprehensive inspection methods to judge.

·Mycoplasma culture

Mycoplasma positive culture is the gold standard for judging mycoplasma infection, but mycoplasma grows slowly, has strict requirements on the culture environment, and has a long cultivation time. It is generally only used for clinical research.

There is a clinical mycoplasma rapid culture kit, which is based on high nutrition and rapid growth factor to quickly grow mycoplasma. The tester can judge the presence of mycoplasma by the color change of the culture medium. Director Zhao introduced that mycoplasma has no cell wall and can survive independently in vitro, so antibiotics can be added to the culture medium. These antibiotics mainly achieve the sterilization effect by defecting the cell wall of the bacteria. Mycoplasma has no cell wall, which is equivalent to being immune to the added antibiotic. After this operation, the bacteria in the petri dish are killed, and at the same time there are no live cells in the petri dish for the virus to grow. If it can grow in such an environment, there is a higher probability that it is mycoplasma.

Mycoplasma will form some acidic metabolites in the process of metabolism. When the color of the medium changes (pH value changes), it can be indirectly determined that the mycoplasma has grown in the culture dish. Director Zhao admitted that such a test is actually “rough”. “In the clinic, its positive rate is 50% to 60%, which is still relatively low.” In addition, the fungus is difficult to be killed by antibiotics. If the fungus contaminates the sample, the result will be inaccurate. “Because of low sensitivity and specificity, it is not recommended clinically.

·Mycoplasma antibody detection-IgG, IgM detection

“After being infected with Mycoplasma pneumoniae, the human body can produce specific IgM and IgG antibodies.” Director Zhao introduced, “Antibodies generally appear 1 week after infection and reach a peak in 3 to 4 weeks, and then gradually decrease. Due to the incubation period of Mycoplasma pneumoniae infection For 2 to 3 weeks, when the patient presents with symptoms, some patients have reached a very high level of IgM antibody, so IgM antibody positive can be used as a diagnostic indicator of acute phase infection. If IgM antibody is negative, it can not deny Mycoplasma pneumoniae infection, IgG antibodies must be detected. IgG appears later than IgM, and dynamic observation is required. If a significant increase is found, it indicates a recent infection, and a significant decrease indicates that it is in the late stage of infection. Simultaneous determination of IgG and IgM can improve the diagnosis rate, achieve guidance on medication, and improve efficacy Purpose.

It should be noted that antibodies to Mycoplasma pneumoniae can be present in the child’s body for 6 months or even more than 1 year, so there is no need to repeatedly review the Mycoplasma antibody after a short period of time because positive Mycoplasma antibody alone cannot be used as a diagnostic standard for Mycoplasma infection. Combined with clinical. In addition, it takes time for antibodies to be produced in the body. Infants and young children may have false negative or low titer antibodies due to imperfect immune function and low antibody production capacity. Therefore, early detection of mycoplasma in clinical is mainly based on antigen detection, that is, Direct detection of the presence of mycoplasma in the sample.

·Mycoplasma antigen detection

“Now DNA and RNA tests are commonly used in clinics. We directly test whether there is mycoplasma genetic material in the sample-DNA. If there is, then the mycoplasma must exist.” However, DNA determination can only determine the presence of mycoplasma, but cannot Tell whether it is alive. DNA is relatively stable. In many archaeological studies, we can sometimes even detect DNA from biological specimens left over thousands of years ago. “Mycoplasma DNA also has such characteristics. Even if mycoplasma has died in our body, as long as it is not excreted, it can still detect the DNA of mycoplasma.” Director Zhao told us that it is impossible to judge the past simply from the presence of DNA. Infection is still a current infection, and we need to consider the amount of DNA detected and clinical symptoms.

The detection of RNA is relatively more accurate. RNA is a protein secreted by mycoplasma. Only live mycoplasma can secrete RNA. So if mycoplasma RNA is detected, then we can think that there is live mycoplasma. “But this test is not perfect. It is more expensive. Not only is it expensive, but RNA is very fragile, it breaks down quickly and is easily damaged, and it is relatively difficult to preserve the sample. Often it is necessary to quickly accept the test after sampling, which is Laboratory conditions and specimen sampling requirements are higher.”

Now there are clinical second-generation sequencing technology to judge the identity of the pathogen from the gene sequence.

There are many clinical methods for detecting mycoplasma. Different methods have their own advantages and disadvantages, which require comprehensive evaluation by doctors. “So, we have always emphasized that doctors mainly look at the patient’s condition when diagnosing and treating the disease. The laboratory test results are for reference, and the doctor needs to make a comprehensive judgment based on the clinical condition.” Director Zhao emphasized.