Staphyloccocus aureus Rosenbach is an important pathogen of human beings, belonging to the genus Staphylococcus, which can cause a variety of serious infections. "Carnivorous bacteria" has another name.

Biological properties

Typical Staphylococcus aureus is spherical, with a diameter of about 0.8μm, and arranged in a grape string under the microscope. Staphylococcus aureus has no spores and flagella, most of them have no capsule, and Gram staining is positive. Staphylococcus aureus has low nutritional requirements, grows well on ordinary culture medium, is aerobic or facultative anaerobic, and the optimum growth temperature is 37℃ and the optimum growth pH is 7.4. The colony on the plate is thick, shiny and round, with a diameter of 1~2mm. Transparent hemolysis ring is formed around the colony of blood plate. Staphylococcus aureus has high salt tolerance and can grow in 10~ 15%NaCl broth. It can decompose glucose, maltose, lactose and sucrose, producing acid but not gas. Methyl red reaction is positive, VP reaction is weak positive. Many strains can decompose arginine, hydrolyze urea, reduce nitrate and liquefy gelatin. Staphylococcus aureus has strong resistance to sulfonamides and low sensitivity, but it is highly sensitive to penicillin and erythromycin.

epidemiology

Staphylococcus aureus is ubiquitous in nature and can be found in air, water, dust and human and animal excreta. Therefore, there are many opportunities for food pollution. In recent years, the US Centers for Disease Control reported that the infection caused by Staphylococcus aureus ranked second, second only to Escherichia coli. Staphylococcus aureus enterotoxin is a worldwide health problem. In the United States, food poisoning caused by Staphylococcus aureus enterotoxin accounts for 33% of the total bacterial food poisoning, while Canada accounts for more, accounting for 45%. There are many such poisoning incidents in China every year.

The epidemiology of Staphylococcus aureus generally has the following characteristics: seasonal distribution, mostly in spring and summer; There are many kinds of poisoned foods, such as milk, meat, eggs, fish and their products. In addition, poisoning incidents caused by leftovers, fried eggs, glutinous rice cakes and bean jelly are also reported from time to time. The nasal cavity carrier rate of patients with upper respiratory tract infection is 83%, so the purulent infection sites of people and animals often become pollution sources.

Generally speaking, Staphylococcus aureus can pollute food through the following ways: food processors, chefs or salespeople carry bacteria, causing food pollution; Food itself carries bacteria before processing, or is polluted during processing, producing enterotoxin, which causes food poisoning; The package of cooked food products is not sealed, and it is polluted during transportation; Pollution of other parts of the body when cows suffer from suppurative mastitis or local suppuration of livestock.

pathogenicity

Staphylococcus aureus is the most common pathogen in human purulent infection, which can cause local purulent infection, pneumonia, pseudomembranous enteritis, pericarditis and even sepsis, sepsis and other systemic infections. The pathogenicity of Staphylococcus aureus mainly depends on its toxins and invasive enzymes:

A. Hemolytic toxin: Exotoxin, which can be divided into four types: α, β, γ and δ, can damage platelets, destroy lysosomes and cause ischemia and necrosis of the body;

B. Leucocyte killing: it can destroy human leukocytes and macrophages;

C plasma coagulase: When Staphylococcus aureus invades human body, this enzyme makes fibrin in blood or plasma deposit or coagulate on the surface of bacteria, which hinders the phagocytosis of phagocytes. The infection caused by staphylococcus is easy to locate, which is related to this enzyme;

D. deoxyribonuclease: deoxyribonuclease produced by Staphylococcus aureus is resistant to high temperature and can be used as a basis for identifying Staphylococcus aureus;

E. Enterotoxin: Staphylococcus aureus can produce several kinds of protein enterotoxins that cause acute gastroenteritis, which can be divided into eight serotypes: A, B, C 1, C2, C3, D, E and F. The enterotoxins can be boiled at 100℃ for 30 minutes without being destroyed. The symptoms of its food poisoning are vomiting and diarrhea. In addition, Staphylococcus aureus also produces epidermin, gelatinase, protease, lipase, peptidase and so on.

Investigation of Staphylococcus aureus

① Inspection of Staphylococcus aureus

Sample treatment: Aseptically take 25g or 25mL food samples, put them into 225mL sterilized physiological saline, and homogenize them to make 10- 1 diluent.

Enrichment culture: inoculate 10- 1 diluent into 7.5%NaCl broth or tryptone broth, and culture at 37℃ for 24 hours.

Isolation and culture: streaking blood plate and Baird-Parker plate with the above diluent or culture solution respectively, and culturing at 37℃ for 24-48 hours. Staphylococcus aureus is a golden or white colony on the blood plate, which is large and convex with smooth surface and hemolysis circle around it. On Baird-Parker plate, the colony is round, 2 ~ 3 mm in diameter, gray or black in color, surrounded by turbid zone.

Staining observation: suspicious colonies were selected from the plate for Gram staining, and Staphylococcus aureus was Gram-positive. Microscopically, it is arranged like a grape, without spores and capsules, with a diameter of 0.5 ~1μ m.

Plasma coagulase test: 0.5mL rabbit plasma and 0.5mL Staphylococcus aureus meat paste broth were sucked for 24 hours, cultured at 36 65438 0℃, and observed every half hour for 6 hours. Coagulation occurred, that is, when the small test tube was tilted or inverted, the contents did not flow, and it was judged as positive. Make a contrast between yin and yang at the same time.

Url] Thermonuclease test: After 24-hour broth culture was treated with boiling water bath for 65438 05 minutes, it was inoculated on toluidine blue -DNA plate with inoculation ring and cultured at 36 65438 0℃ for 24 hours. Those with light pink around the seed line are positive. Staphylococcus aureus is positive in this test.

② Detection of staphylococcal enterotoxin

The detection of staphylococcal enterotoxin mainly includes animal test, serological test, immunofluorescence test and enzyme-linked immunosorbent assay, so I won't go into details here.

Control of Staphylococcus aureus

① Prevent Staphylococcus aureus from contaminating food.

Prevent all kinds of food from being contaminated by carriers: Conduct regular health check-ups for production and processing personnel. Those who suffer from local suppurative infections (such as scabies and finger suppuration) and upper respiratory tract infections (such as sinusitis, suppurative pneumonia and oral diseases) should temporarily stop working or change their posts.

In meat processing plants, the diseased parts of poultry and livestock carcasses infected by local suppuration should be cut off and treated by high temperature or other appropriate means.

② Prevent the production of staphylococcal enterotoxin.

Food should be kept at low temperature and well ventilated to prevent the formation of enterotoxin; In spring and summer when the temperature is high, food should not be stored in a cool and ventilated place for more than 6 hours, and should be thoroughly heated before eating.

Staphylococcus aureus infection can be treated with erythromycin, penicillin, gentamicin, vancomycin or cephalosporin VI.

Staphylococcus aureus enteritis

The disease is caused by Staphylococcus aureus, mostly due to the imbalance of intestinal flora caused by long-term use of antibiotics in the primary disease. Antibiotic-sensitive strains were inhibited, and drug-resistant Staphylococcus aureus strains took the opportunity to reproduce. Staphylococcus aureus is an invasive bacterium, which can produce toxins and is destructive to the intestine. Therefore, the onset of Staphylococcus aureus enteritis is urgent, and the poisoning symptoms are serious, mainly manifested as vomiting, fever and diarrhea. Vomiting often occurs before fever, which is very high. Mild stool frequency is slightly more, which is yellow-green paste stool; The frequency of severe stool can reach dozens of times a day. The stool is dark green, watery and looks like seawater, so it is called seawater-like stool. There is a lot of mucus and it smells like fish. Sometimes the sheet-like pseudofilm can be discharged. The false membrane is put into physiological water, and the exfoliated intestinal mucosa floats on the water surface, which is very helpful for diagnosis. Children with fluid loss, dehydration, electrolyte disorder and severe acidosis may have shock. Select the mucus part of stool smear, and a large number of pus cells can be seen under the microscope. For example, after gram staining, a large number of gram-positive cocci can be seen under the microscope. When Staphylococcus aureus grows in fecal culture, it can be diagnosed.