Bacterial keratitis

Introduction to bacterial keratitis Bacterial keratitis (bacterial keratitis) is the most important infectious corneal disease in the 1960s. After the 1970s, viral keratitis, fungal keratitis, and Acanthamoeba keratitis increased rapidly, but bacterial keratitis is still Infectious keratopathy with the highest incidence and blindness rate. basic knowledge The proportion of illness: the incidence rate is about 2% - 5% Susceptible people: no specific population Mode of infection: non-infectious Complications: corneal staphylitis iridocyclitis

Cause of bacterial keratitis

(1) Causes of the disease

The pathogenic bacteria have undergone great changes with the changes of the times. In the 1950s, pneumococcal bacteria were the mainstay. Staphylococcus aureus dominated in the 1960s. In the 1970s, Pseudomonas aeruginosa was the dominant species. In the 1980s, due to the use of aminoglycoside antibiotics, Pseudomonas aeruginosa was relatively reduced, while penicillin-resistant Staphylococcus was relatively increased. Since the 1990s, other Gram-negative bacteria, such as non-fermenting Gram-negative bacilli, Serratia marcescens, and anaerobic bacteria, have gradually increased.

Among the most common pathogenic bacteria, there are four species, Gram-positive bacteria Streptococcus pneumoniae (S) and Staphylococcus aureus (S), and Pseudomonas aeruginosa in Gram-negative bacteria. (pseudomonas aeruginosa, P) and Moraxella (M) are referred to as SSPM infection. According to the statistics of 120 cases of bacterial keratitis in Yokohama University in Japan, SSPM infection accounts for 72.5% (87 cases). The pathogenic bacteria were 15% (18 cases), 11.7% (14 cases), 35.8% (43 cases), and 10% (12 cases). The United States and Canada also had the same statistical results. This tendency not only appeared in industrial development. The same is true for countries and developing countries. In the Philippines, the number of SS2 infections was 92.2% (1624), which were 4.7% (83 cases), 42.31% (745 cases), 26.6% (468 cases) and 18.6%. (328 cases).

Trauma is one of the most common risk factors for bacterial keratitis. In occupational anterior segment trauma, 6% of bacterial keratitis occurs. In rural areas, bacterial infection occurs in 5% of patients with corneal abrasions. Causes of corneal epithelial and matrix trauma, firstly expose the corneal stromal tissue to the normal flora of the conjunctival sac, which is easy to cause bacterial infection, corneal foreign body and splashing of water sources are important carriers of external bacteria entering the cornea, if the patient has chronic Bacterial blepharitis or dacryocystitis, bacteria in these areas can easily lead to corneal infection.

In developed countries, wearing contact lenses is the most common risk factor for bacterial keratitis. All types of contact lenses may cause bacterial infection of the cornea. Among them, soft contact lenses have the highest proportion of overnight wearers. Statistics show that The incidence of corneal ulcers per year for rigid contact lens wearers is 0.02%, that of breathable rigid contact lenses is 0.04%, that of daily wear soft contact lenses is 0.04%, and that for traditional soft contact lenses is 0.2 for day and night wearers. %, wearing contact lenses caused by corneal bacterial infection, and the contact lens itself on the structure and function of the corneal epithelium, lens and lens box bacterial contamination, wearing and nursing methods, as well as the wearer's life and hygiene habits, etc. close relationship.

Among ocular surface diseases, the abnormality of tear volume and tear component and the destruction of eyelid closure function are common factors associated with corneal bacterial infection. All lesions causing corneal epithelial destruction such as monocystic corneal epithelial lesions, long-term antibiotics or Epithelial cell poisoning caused by antiviral drugs, local long-term use of glucocorticoids, macrovesicular keratopathy caused by endothelial decompensation, and various degeneration and malnutrition involving the corneal epithelium may be secondary to bacterial infection.

(two) pathogenesis

The clinical manifestations of corneal bacterial infection are the result of a combination of bacterial and host responses.

There are many kinds of bacteria causing keratitis, including Micrococcal (mainly Staphylococcus and Micrococcus), Streptococcus, Pseudomonas and Enterobacteriaceae, and about 87% of bacterial keratitis is Due to the above four types of bacteria, the distribution of bacteria is different due to factors such as geographical, environmental, living and sanitary conditions. The statistics of the Department of Ophthalmology, Beijing Institute of Ophthalmology, 1989-1998 show that the copper-green leave Monocytogenes, coagulase-negative staphylococci, pneumococcus, coryneform bacteria and staphylococcus aureus are the main pathogens of bacterial keratitis.

1. Staphylococcus aureus keratitis Staphylococcus aureus can produce plasma coagulase, which can produce a variety of pathogenic factors in infected tissues. According to its action, it can be divided into two types: factors related to infection spread and toxicity-related factor.

(1) Factors related to infection spread include: hyaluronidase, lipase, phospholipase, nuclease, gelatinase, plasmin, protease and lytic enzyme.

(2) Toxicity-related factors: endotoxin A, B, C, D and E, hemolytic toxins a, p, 7 and 8, toxic shock syndrome toxin-1, and epidermal release toxins A and B, gold The plasma coagulase of Staphylococcus aureus forms a fibrin membrane around the bacteria, which surrounds the lesion to form an abscess, which has the function of killing white blood cells.

2. Coagulase-negative staphylococcal keratitis The bacteria itself does not secrete strong toxins, generally belonging to attenuated bacteria or conditional pathogens, which cause a slower course of corneal diseases, and a biofilm is formed around the bacteria. It is composed of glycoprotein secreted by bacteria and covers the surface of the bacteria. The bacteria adhere to the surrounding bacteria to form bacterial spots. In this state, the resistance of the bacteria to antibiotics is obviously enhanced, making the drug difficult to take effect.

3. Pneumococcal keratitis The pneumococcal capsule can escape the phagocytosis of neutrophils, so it is easy to invade the corneal tissue and spread rapidly. The bacteria can secrete lysin, neuraminidase and hemolytic toxin, causing tissue damage. The immunoglobulin A-degrading enzyme produced by the bacterium can hydrolyze almost all secretory IgA involved in the local immune response, and the local non-specific immune function of the eye is inhibited.

4. Pseudomonas aeruginosa keratitis Pseudomonas aeruginosa infection is related to the virulence and invasiveness of bacteria, mainly related to the virulence and invasiveness of bacteria, which can produce exotoxin A, elastase, extracellular An important pathogenic substance such as enzyme s, under the action of Pseudomonas aeruginosa flagella and protease, bacteria are easily transferred to the site of tissue damage; mucopolysaccharide protein complex makes bacteria adhere to the surface of tissue cells; elastase, alkali Protease and cytotoxins and hemolytic toxins facilitate their invasion and reproduction and cause necrotic damage to the corneal stroma.

5. Moraxella keratitis Different strains of Moraxella are similar in pathogenicity in the eye, which can produce protease and endotoxin, decompose and destroy corneal tissue, and some strains of Moraxella can produce similar phospholipase , enzymes and toxins of hyaluronidase and hemolytic toxins.

6. Actinobacteria keratitis caused by keratitis in the proportion of infectious keratitis less than 1 / 100, Nocardia is an obligate intracellular growth of microorganisms, can be propagated in phagocytic cells, It does not produce exotoxin, and the reproduction rate is slow. The cell wall contains lipopeptides and lipopolysaccharide components. These components will be different in different strains and different growth stages. Iron ions are important factors for the growth of Nocardia in the cells. Nocardia infection is often secondary to minor damage, and both humoral and cellular immunity are involved in the defense mechanism.

7. Streptococcal keratitis Streptococcus can produce a variety of toxins, mainly hemolytic toxins and erythrotoxin, hemolytic toxins are cytotoxic; rash toxin is an exotoxin, streptococcus can also produce a series of enzymes, mainly There are streptokinase and hyaluronidase, the former can activate plasminogen to plasmin, so that fibrin dissolves; the latter decomposes the extracellular matrix, which is beneficial to the spread of bacteria.

8. S. serranosus serranosus is difficult to invade normal corneal tissue. Only when the corneal epithelial barrier is destroyed, bacteria can invade and reproduce in the corneal stroma. Experiments confirmed that Salmonella caused The degree of keratitis is positively correlated with the amount of proteolytic enzymes secreted by the bacteria. The amount of proteolytic enzyme produced by the virulent strain is large, and the corneal lysis and necrosis reaction is obvious. On the contrary, the attenuated strain has less proteolytic enzyme secretion and tissue destruction. Also light.

9. Acne Propionibacterium keratitis Because the normal human conjunctiva is relatively hypoxic, there are often various anaerobic bacteria, 40% to 85% of which are Propionibacterium acnes, which is a conditional pathogen, infection It is generally associated with mucosal barrier destruction and tissue hypoxia and necrosis. The polysaccharide released by this bacterium has leukocyte chemotaxis, and bacteria can multiply in cells. Propionibacterium acnes is often infected with other aerobic or anaerobic bacteria.

Most bacteria can invade the corneal stromal layer only when the corneal epithelium is damaged. Once the bacteria enters the cornea, polymophauclear leukocytes (PMN) become chemotaxis, and release of lytic enzymes leads to matrix necrosis, which is particularly toxic. In the case of Pseudomonas aeruginosa infection, in addition to the above reasons, proteolytic enzymes can also be produced during bacterial reproduction, so the condition is more serious and rapid, although the retrograde corneal membrane has a certain resistance to bacterial penetration. But eventually corneal perforation occurs.

Bacterial keratitis prevention

Measures should be taken to prevent corneal infections. Infections should be controlled early to control inflammation, so that it should be promoted to good aspects. For example, in rural areas and factories, it is necessary to actively promote and take measures to prevent the occurrence of eye injuries. For those who have been injured, they should be treated immediately to prevent them. Infection, in addition, it should also actively treat trachoma, correct trichiasis, cure conjunctivitis, blepharitis and dacryocystitis, correct eye disease such as valgus or hernia, and have positive significance in preventing bacterial keratitis.

Bacterial keratitis complications Complications, corneal staphylitis, iridocyclitis

During the development of bacterial keratitis, a series of complications and sequelae can occur, resulting in varying degrees of visual loss and even blindness.

1. The thinnest scar tissue of the cloud can only be seen when it is illuminated by hand or light.

2. The scar tissue with a slightly thick spotted plaque can be seen in the light of the room.

3. White thickest white dense scar tissue.

4. Corneal depression In the recovery process of corneal ulcer, the new connective tissue has not filled the bottom, and the epithelium has covered the long, leaving small depression of the cornea.

5. Adherent leucoma After the perforation of the corneal ulcer, the iris adheres to the perforation of the cornea with the overflow of the aqueous humor, and finally forms a scar, called a sticky leukoplakia.

6. Corneal staphyloma (corneal staphyloma) After the formation of large adhesion leukoplakia, due to the thinning of the cornea, it can not resist normal intraocular pressure, but gradually protrudes forward, called corneal staphyloma, such as scar tissue only accounts for part of the cornea The cornea is swollen. If the scar tissue accounts for all the cornea, it is called corneal swelling.

7. Corneal bulging cornea due to inflammation or ulceration, weakened during the formation of healing scars, can not resist the normal pressure in the eye and protrude forward, called corneal bulging.

8. Post-elastic membrane bulging (descemetocole) Corneal ulcers develop to the deeper layer. When perforation is about to occur, a thin layer of transparent tissue may appear on the base of the ulcer and bulge forward, which is called posterior elastic membrane bulging.

9. Corneal perforation When the cornea is perforated, the patient feels severe pain and "hot tears" (aqueous humor). After the perforation, the anterior chamber becomes shallow and disappears, and the original symptoms disappear. If the perforation occurs in the peripheral part, it is often accompanied by The iris is released.

10. Irregular corneal inflammation or corneal ulcer with iridocyclitis, its toxin entering the anterior chamber can stimulate the irido-ciliary body inflammation, iris ciliary body congestion, edema, vascular permeability changes, cells infiltrate into the aqueous humor, light The room water is turbid, or there is a corneal metaphyseal; in severe cases, there is a large amount of purulent exudate, deposited in the anterior chamber of the anterior chamber, known as anterior chamber pyogenic corneal ulcer, if not treated in time can produce post-iris adhesions.

11. The formation of corneal vasospasm is mostly reticular, occurs in the limbus near the lesion, and deep ulcers can sometimes develop deep brush-like blood vessels.

Bacterial keratitis symptoms common symptoms keratitis conjunctival hyperemia continuous sputum corneal ulcers tears photophobia

1. The clinical manifestations of keratitis caused by different pathogen infections are not the same.

2. The symptoms are more acute, the symptoms are heavy, red eyes, pain, photophobia, tearing, decreased vision, tingling in the eyes, foreign body sensation, eyebrow bow pain, eyelids and secretions increased.

3. Signs

(1) signs outside the cornea: ciliary congestion, swelling of the eyelids, conjunctival congestion and edema, iris congestion (expressed as iris discoloration and pupil dilation).

(2) corneal signs: corneal infiltration, corneal ulcer, corneal edema, posterior elastic membrane bulging, corneal depression, anterior chamber empyema and corneal perforation, corneal ulcer caused by different bacteria, clinical manifestations are very different, 3 is the main difference between SSPM infectious corneal diseases.

Examination of bacterial keratitis

1. The lesion scraping test can be used for rapid diagnosis of the disease. The smear and conjunctival sac specimens can be taken with a sterile cotton swab or a smear, secretion or necrotic tissue can be taken from the bottom and edge of the ulcer with a Kimura or Lindner spatula, methanol or 95% ethanol is fixed for 5 to 10 minutes, then Gram and Giemsa staining. The former distinguishes Gram-positive bacteria (stained purple) or Gram-negative bacteria (red stained), all of which are dyed blue. It can clearly identify inflammatory cells and corneal epithelial cells. After staining with Gram-stained lesions, it can be quickly determined according to cell morphology under direct microscopic examination. It is also possible to determine which bacterial infection is caused by mycobacteria and Nocardia. Can be used for acid resistance.

2. Bacterial culture The final diagnosis of the disease must be determined by bacterial culture.

(1) Blood agar medium: It is the most commonly used solid medium and is suitable for the growth of most common eye bacteria.

(2) Enrichment medium: The liquid medium used is meat soak soup, meat cream soup, brain heart dip soup and the like.

(3) The anaerobic bacteria are cultured with thioglycolic acid broth, blood agar medium, etc., and cultured in an anaerobic bag or an anaerobic tank at an incubator temperature of 36 to 37 °C.

(4) In order to improve the positive rate of bacterial culture, it must be done:

1 Culturing before starting the application of antimicrobial therapy.

2 When the culture is used for treatment, the patient should stop using the antibacterial drug for 12 to 24 hours before culturing.

3 If necessary, repeat the material culture repeatedly, especially when the non-pathogenic bacteria are opportunistically infected, when the same bacteria are separated several times in the same part, the isolated bacteria can be determined as pathogenic bacteria.

3. Limulus test Based on the soluble lipopolysaccharide endotoxin in the cell wall of Gram-negative bacilli, the endotoxin is released into the infected tissue when the bacteria die, and the patina can be quickly detected within 1 h with the amoeba-like cell lysate. Gram-negative bacilli infections such as Pseudomonas, Moraxella, Serratia marcescens and Proteus.

4. Blood routine examination can determine the extent of infection.

Diagnosis and identification of bacterial keratitis

The diagnosis of bacterial keratitis can be determined based on the current medical history, risk factors, and the original ocular and systemic diseases. As mentioned above, various factors can change the clinical manifestations of bacterial keratitis, antibiotic treatment or antibiotics - Corticosteroid combination therapy can also affect and alter the typical clinical features of a visit, and laboratory diagnosis must be performed to determine bacterial corneal infection.

The clinical symptoms of corneal infection caused by different types of bacteria are different. The main identification points of different types of corneal infections are clinical manifestations and the results of corneal bacterial culture and identification. The disease should be differentiated from non-bacterial keratitis, for example: Long-term epithelial defects, especially after corneal transplantation, can develop into a matrix infiltration similar to infectious keratitis. Neurotrophic or exposed keratopathy can develop ulcer and stromal inflammation, herpes simplex keratitis, blepharitis Caused by non-infectious immune infiltration, infiltration caused by contact lenses, etc., should be distinguished from bacterial keratitis.

The material in this site is intended to be of general informational use and is not intended to constitute medical advice, probable diagnosis, or recommended treatments.