Ocular Microbiology PK Mukherjee, Preeti Bandyopadya
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General ConsiderationsCHAPTER 1

 
2INTRODUCTION
Infection is an interaction between the host and the micro-organism. The host tries to minimize the effect of the microbe by developing a mechanism of resistance called immunity. Mere presence and multiplication of organism on the host is called infection that may not result in infectious disease infectious disease is a state where the invading organism develops an upper handover the defense mechanism of the host. Some of the terms used to designate infections are
Primary infection: First infection by a microorganism.
Reinfection: Subsequent infection by the same organism.
Reactivation: The organism lies dormant in a target area, to become active later under changed condition. The common examples are herpes zoster, herpes simplex, toxoplasmosis and histoplasmosis.
Latent period is the period lapsed between the primary infection and reactivation and the infection is called latent infection, i.e. herpes simplex and herpes zoster.
Secondary infection: An infection superimposed on a primary infection that may be by commensal or a pathogen. The ocular examples are superimposed bacterial infection in viral keratitis. The reverse is not reported. More common is fungal infection set-up on bacterial or viral infections.
Cross-infection is a second infection developing in an already diseased host from either a diseased person or a carrier.
Nosocomial infection is an infection acquired during hospital stay due to cross-infection from the environment, instruments, prosthesis, etc.3
Iatrogenic infection is a physician induced infection due to therapy, a common example is hepatitis B infection following blood transfusion.
Carriers are persons who harbor pathogenic organism without suffering any adverse effect brought about either due to partial treatment or immune response.
 
Various Types of Carriers
  1. Temporary carriers, who are capable of infecting for a short period, i.e. less than six months. The convalescent carrier is an example of this status.
  2. Convalescent carriers are those who have recovered from the disease and are capable of transmitting disease to others.
  3. Chronic carriers: This status lasts for months to years or even for rest of life.
  4. Paradoxical carriers are those carriers who acquire the disease from another carrier.
Based on the relationship between the microorganism and the host, the microorganisms have been divided into two broad groups, i.e.
  1. Saprophytes
  2. Parasites
The saprophytes are free-living organism that grow on dead or decaying organic matters. They generally are incapable of multiplying on living tissue. They are incapable of producing disease in the host because multiplication of the micro-organism is one of the essentials of infectious disease.
The position of saprophytes as causative organism for infectious disease has shown a dramatic change in recent years due to emergence of immunocompromise in large number of persons. The disease thus produced is called opportunistic 4infection. B. subtilis and B. cereus are emerging as causative organisms in devitalized tissues. In eyes, they are being reported frequently to cause traumatic endophthalmitis.
Free-living organisms are those organisms that thrive in soil and water. Most of the organisms met within nature are free-living and autotropic, i.e. they are capable of synthesizing necessary nutrient from water, atmosphere and dead organic matter. The common examples are P. aeruginosa, B. subtilis, clostridia, Actinomyces, acanthameba, all of which can cause systemic as well as ocular infection.
The parasites are those organisms which live on living host and derive nutrition from the host without any benefit to the host. Generally, parasites cannot exist independently. They may be harmless or harmful to the host. The former is called commensal while the later is called pathogen.
The commensals live in utmost harmony with the host which is larger than the commensals, without any harm to the host. The commensals derive nutrition from the host and are in many ways sheltered by the host. The host does not derive any benefit from the commensals. The commensals constitute the normal microbial flora of the body. The examples are Staphylococcus epidermidis on the skin, E. coli of gastrointestinal tract and C. xerosis in the eyes. The commensals survive on secretion and waste products of the host.
Pathogen: A microorganism capable of producing disease is called pathogen. The pathogens are heterotropic, i.e. they are not self-sustaining, they require complex organic substances for nutrition.
As per habitat the parasites can be:
  1. Ectoparasite, that lives on the surface of the host, i.e. scabies or phthiriasis. The disease produced by them are called infestation.
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  2. Endoparasites that live inside the body of the host, i.e. malarial parasites. The parasites that can not survive without a host are called obligate parasites, i.e. toxoplasma.
A facultative parasite may exist without a host.
Opportunistic pathogens are commensals and saprophytes that can produce disease when the body resistance is below optimal level.
 
Source of Infection
  1. Normal flora
    Normal flora may become pathogen due to changed immunity that may be:
    1. Systemic as in AIDS
    2. Biochemical change, i.e. diabetes
    3. Indiscriminate use of antibiotic, steroid
    4. Chemotherapy and radiation.
    5. Local ocular conditions like abnormal tear status, ocular surface defect, contact lens, prolonged uses of antibiotics or steroids are some of the examples.
  2. Human beings are the commonest source of infection to others, both systemic as well as ocular.
    The mode of transmission among humans can be:
    1. Inhalation: This method of transmission is commonest mode in respiratory tract infection. The organisms may remain suspended in the air and inhaled or may fall on the ground and inhaled along with dust.
    2. Ingestion: This is the most frequent mode of transmission of gastrointestinal diseases, may be water borne or food borne.
    3. Inoculation: The organisms are virtually implanted in the tissue following breach in the skin. The examples are 6tetanus and rabies. The organism can be inoculated in-advertently by infected needles, syringes, i.e. hepatitis B, HIV or through blood transfusion. It is a common mode of transmission in drug addicts.
    4. Autoinoculation: When localized infection is transmitted to noninfectious part of the same person. The best example is vaccinia.
    5. Contact: This can either be direct contact as sexually transmitted diseases or indirect through fomites.
      Fomites are inanimate objects like clothes, instruments, toys and cosmetics. The best ocular example of contact transmission by fomites is trachoma.
    6. Transplacental: These are congenital in nature. The developing fetus gets the disease from the infected mother. The examples are rubella, cytomegalovirus, syphilis, and toxoplasmosis.
    7. Iatrogenic: The infection is transmitted either during therapy or diagnostic procedure, some of which can be nosocomial. The examples are meningitis following lumber puncture, urinary tract infection following catheterization.
  3. Animals: Animals are capable of transmitting diseases, to humans. The transmitting animals may be symptomatic or asymptomatic or may act as a carriers. The examples are bite of dog leading to rabies, ingestion of ova or cyst of parasites or helminths. The diseases transmitted by animals are called zoonotic diseases or zoonoses. Some of the birds are also known to spread disease. These diseases are called ornithosis,i.e. psittacosis.
  4. Insects: The insects that transmit the disease are called vectors. Common vectors are mosquito, flies, ticks, mites and lice.
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    The transmission may be
    1. Mechanical transmission: The examples are dysentery and typhoid which are transmitted from infected stool, water, etc. to the host.
    2. Biological: The vector acquires the organism at various stages of development. The pathogens multiply in the body of the vector and the developmental cycle is completed in the host. The examples are Anopheles mosquito transmitting malaria, Aedes aegypti transmitting yellow fever, Simulium damnosum transmitting river blindness, Chrysops fly (Loa-Loa), Ixodes-tick (Lyme disease).
    Besides being vectors some of the insects act as reservoir of the organisms
  5. Soil: Some free-living pathogens survive in the soil for long-time and infect the host either by inoculation, i.e. tetanus or inhalation Histoplasma capsulatum and Nocardia. Some of the helminths also survive in the soil and cause infection, i.e. roundworm and hookworm.
  6. Food: Contaminated food material are important source of avoidable transmission of diseases. The diseases are typhoid, amebiasis, cysticercosis, toxocariasis.
  7. Water: Water transmitted diseases are public health problem all over the world. The water may be contami-nated by pathogen that on ingestion cause diseases, i.e. cholera, infective hepatitis. The water may contain a vector which harbor the pathogen, i.e. cyclops harboring third stage larva of guinea worm.
To cause infection, the microorganisms undergo through following stages:
  1. The microorganisms colonize on the surface, they intend to invade.
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  2. Adherence: The microorganisms cause diffusion of toxin and enzyme that leads to next stage, i.e. invasion and multiplication. The bacteria contain structures called capsules and pili, they help in adherence.
  3. The toxins liberated by bacteria can either be endotoxins or exotoxins.
    Endotoxins are produced by gram-negative bacteria following lysis or destruction of the cell wall. The action of endotoxin is similar in all organisms.
    Exotoxin: The toxins are liberated extracellular by intact bacteria. The toxin spreads by bloodstream, may produce distant or local effect. Exotoxins of different organisms have different action and invade different tissues. They are active in minute quantity and are highly antigenic. The exotoxin when treated with formaldehyde give toxoid which is non-toxic and is use to immunize persons. The examples are tetanus and diptheria. They are generally produced by gram-positive organisms but some gram-negative organisms also produce exotoxin, they are shigella, vibrio and E. coli.
  4. Enzymes: The common enzymes produced by various bacteria are Coagulase, collagenase, hyaluronidase, strepto kinase. Out of these coagulase and collagenase have significant ocular action.
  5. Besides toxins and enzymes, the organisms liberate other chemicals also. Some of them are hemolysin, fibrinolysin, leukocidin. The enzymes, toxins and lysins play an important role in invasiveness of the organism.
  6. The term invasiveness means property of organism to invade the host, multiply locally, spread via lymphatics and bloodstream. The invasiveness is enhanced by virulence of the microorganism besides enzymes and toxins.
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  7. The virulence of a microorganism depends basically on invasiveness and toxigenicity of the organism which are constantly opposed by the defense mechanism of the host.
    The important factors governing virulence are
    1. Infective dose
    2. Presence of capsules and spores in the organism
    3. Ability to produce toxin and enzymes
    4. Communicability of the microorganism.
 
MICROBIOLOGY IN RELATION TO THE EYES
The principle involved in ocular microbiology is the same as general clinical microbiology with few differences.
  1. There are only a few organisms that cause exclusively ocular diseases. They are Moraxella lacunata, Haemophilus aegyptius and Corynebacterium xerosis.
  2. Most of the organisms have systemic involvement.
    1. The ocular manifestation may develop after the systemic disease is well-entrenched.
    2. The ocular disease may cause systemic disease, i.e. meningococci, gonococci.
  3. The eyes being small organs, the specimens removed from them to isolate the organisms are very small that makes recovery and identification of organisms difficult.
  4. The specimens do not withstand transport time.
  5. Some of them require immediate staining or microscopic examination.
 
Normal Microflora of the Eyes
The eye being exposed to the atmosphere throughout the waking hours and their continuity with skin, nasopharynx make them vulnerable to contamination and keep them contaminated.10
The eyes are sterile only for few hours after birth and then get invaded by microorganisms.
The eyes may be infected during the birth if the labor is prolonged.
The organisms are found on the skin of the lids, the lid margins and conjunctiva in abundance. The cornea rarely yields microorganism unless diseased.
The microorganisms can either be saprophytes or commensals, both the group of organisms do not produce disease till the immunity of the host is perfect. Both the groups have tendency to cause opportunistic infection whenever the conditions are in favor of the organism and against the host. Microfloras are generally similar in both eyes at onset, course and type. Thus, if any organism has been detected in one eye, the other eye is also expected to harbor the same organism. This is true only for non-pathogens.
The normal ocular flora is divided in two broad groups—Resident flora and transient flora.
The resident floras are
  1. The same organism grows on repeated culture.
  2. They represent true colonization. The organisms are S. epidermidis, S. aureus, lactobacilli, diphtheroids. Propioni bacterium is being reported frequently to cause post-IOL endophthalmitis.
Transient ocular floras are those organismssame that are not found consistently when serial cultures are taken under the same condition from the same eye. All the cultures do not yield same microorganism.
The organisms are
  1. S. aureus and other species of staphylococci
  2. Pneumococci
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  3. Bacillus species
  4. Branhamella catarrhalis
  5. E. coli
  6. Klebsiella
  7. Enterobacter
  8. Pseudomonas
  9. Peptococci
  10. Clostridia
  11. Many fungi are also found in the conjunctival sac of persons who live in villages.
Some important observations are:
  1. Haemophilus, pneumococci and streptococci are found more frequently in children.
  2. Xerosis is commonest saprophyte.
  3. Pseudomonas is recovered more frequently from hospitalized patients.
  4. Keratoconjunctivitis is predisposed to infection by Proteus and S. aureus.
  5. Prolonged use of antibiotics changes the microflora.
  6. Viruses are neither resident nor transient flora of the eyes.
Routes of ocular infectioncan can be exogenous or endogenous.
  1. Exogenous:
    1. Air, water, body fluid, infected drops, instruments, contactlens, foreign bodies and fomites.
    2. Continuity: Lids, lacrimal sac, conjunctiva to cornea, meninges, walls of paranasal sinuses.
  2. Inoculation: Penetrating injuries, surgical wounds, retained foreign body, viscoelastic, IOL, irrigating fluids, perforating corneal ulcers.
  3. Endogenous: Blood borne
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PROTECTIVE MECHANISM OF THE EYES
Eyes are highly sensitive sensory organs. They have been given adequate anatomical and physiological protection
The anatomical protection is offered by
  1. Walls of orbit: The boney orbital walls surrounds the eyeball from all sides. The wall is shorter on the lateral side, this makes the eyes vulnerable to trauma and infection from the lateral side.
  2. Lids: The combination of tarsal plate and orbicularis form a formidable curtain that can be activated both voluntarily as well as reflexly. This is the commonest form of protection from exogenous invasion by microbes.
  3. The lashes: The lashes entangle the microorganism and prevent them from entering the conjunctival sac.
  4. The orbital septum prevents infection spreading from pre-septal area to retroseptal space and vice versa.
  5. The conjunctival epithelium itself is resistant to many organisms. It had good phagotic action as well.
  6. The corneal epithelium is resistant to all organisms except Gonococcus, Meningococcus, Diphtheria, Haemphilus aegyptius and Listeria.
  7. The Descemet´s membrane also restrains organisms to go deeper but can not stop toxins from passing through it.
  8. The tear film has multiple protective functions —
    1. Mechanical flushing of the conjunctiva and the cornea. The tear production is increased manifolds as soon as an organism finds its way on the conjunctiva or cornea.
    2. Antimicrobial action: The lysozyme and lactoferrin present in tear have a microstatic action on many organisms. If the level of these two fall below the critical level, there is immediate rise of bacterial flora in the conjunctival sac.
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    3. The immuglobulins: The tear contains three immuno-globulins, they are IgA, IgG and IgM. The IgA and IgM constitute the first line of defense against the microbial invasion. They help to maintain a certain level of saprophytes, which in turn prevent the pathological flora from colonizing on the ocular surface. They also reduce adherence of bacteria to the epithelium. The immunoglobulins are thought to neutralize toxins.
The physiological protection are reflex in nature. They are
  1. Blink reflex that closes the eye automatically on approach of any foreign body.
  2. Bell´s phenomenon: As the lids close, the eyeball rolls up, thus more than 2/3 of the cornea goes under the upper lid. Even in absence of blinking as in lagophthalmos, the Bell´s phenomenon gives a fairly good protection.
  3. Mechanical action of lid movement keeps the conjunctiva moist and well-lubricated.
  4. Reflex tearing increases the tear production to flush out the microorganism.
Failure of any of the above defense mechanisms predis-pose ocular infections.
 
Factors that Predispose Ocular Infections
Rich blood supply of the eye except the cornea and lens makes the eye predisposed to hematogenous spread of the infection. The meninges and cerebrospinal fluid (CSF) transport intracranial infection to the eye. The organism finds the paper thin walls of paranasal sinuses easy to pass through. The retained intraocular and intraorbital foreign bodies are a constant and common source of ocular infection.14
The organisms that are known to cause ocular infections are
  1. Bacteria
  2. Chlamydiae
  3. Fungi
  4. Virus
  5. Parasites
    1. Protozoa
    2. Metazoa
      1. Cestodes
      2. Nematodes
  6. Arthropods
  7. Larvae