Download Eye Infections

Microbiology: Eye Infections (Jackson)
BACTERIA CAUSING EYE INFECTIONS:
Staphylococcus Aureus:

Relevant Virulence Factors:
Alpha toxin (primary)
Teichoic acid (aids in colonization)
Antiphagocytic compounds

Etiology/Pathogenesis:
Basics: principle cause of eye infections due to high carriage rates in humans
Infections:
o Blepharitis: infection of eyelid margin or sebaceous gland (also called a sty)
o Dacrocystitis: inflammation of lacrimal sac
o Conjunctivitis: inflammation of conjunctiva (can spread to cornea, eyelids and sclera)
o Keratoconjunctivitis: conjunctiva and cornea
o Endophthalmitis: infection of the aqueous or vitreous humor; requires ulceration/penetrating injury to
compromise cornea and sclera

ID:
Shape/Staining: Gram positive cocci in clusters
Biochemical Tests:
o Catalase positive
o Coagulase positive
o Antimicrobial susceptibility testing
Streptococcus Pneumoniae:

Relevant Virulence Factors:
Polysaccharide capsule: interferes with complement pathway (84 different serotypes); primary virulence factor
Pneumolysin: membrane damaging cytolysin related to SLO
Cell wall: techoic acid and peptidoglycan contribute to inflammatory response

Etiology/Pathogenesis:
Basics: common in upper respiratory tract; may cause infection due to close proximity of eyes to this area
Infections:
o Dacrocystitis
o Conjunctivitis
o Keratoconjunctivitis

ID:
Shape/Staining: Gram positive diplococcus (pneumococcus; lancet shaped)
Classification: not part of Lancefield grouping
Biochemical Tests:
o Capsular serotyping
o Quelling reaction (capsular swelling due to adding anti-capsule Abs)
o Optochin (P disk) susceptibility test
o Bile solubility (differentiate from S. viridians)
Haemophilus Influenzae:

Relevant Virulence Factors:
Polysaccharide Capsule: antiphagocytic and antigenic change; most important VF
o 6 different serotypes (a-f)
o Serotype b (Hib): most virulent; composed of polyribitol phosphate (now vaccine for children to
prevent associated meningitis)

Etiology/Pathogenesis:
Basics: exclusively found in humans, and we have a high carriage rate in upper respiratory tract (normal flora
typically has no capsule)
Infections:
o Conjunctivitis
o Keratoconjunctivitis

ID:
Shape/Staining: Gram negative rod (pleiomorphic)
Satellite Growth Phenomenon: require blood products (hematin/X factor and NAD/V factor) for growth; will
grow only when supplied with these
Capsular serotyping: with anti-capsule Abs
Pseudomonas Aeruginosa:

Relevant Virulence Factors:
Exotoxin A:
o Cytotoxin that causes ADP-ribosylation of elongation factor 2, stopping protein synthesis and leading to
cell death
o Same mechanism as diphtheria toxin
o Promotes tissue invasion and evasion of immune system
Exotoxin S:
o ADP-ribsoylation of other proteins
Elastase:
o A cytolysin that acts as a protease for elastin, human IgA, IgG, complement and collagen
o Primary cause of corneal perforation during eye infection
Adhesin for colonization:
o Adhesion to cornea requires trauma expose receptors

Etiology/Pathogenesis:
Basics: found free-living in most environments, and is an oppotunisitc pathogen; causes eye infections by
contaminating water and contact lens solution, or via iatrogenic means (contaminated ophthalmologic
equipment)
Infections:
o Conjunctivitis
o Keratitis: infection of the cornea; requires trauma to expose receptors

Associated with extended wear contacts

Can rapidly destroy cornea in 1-2 days
o Endophthalmitis

ID:
Shape/Staining: Gram negative rod (motile in wet mount)
Growth Characteristics:
o Classified as aerobic because it never uses fermentation pathway, but without O2 it can use NO3
o Tolerates lots of temperatures and high salt content
o Fruity odor on solid media
o Blue-green fluorescence under UV light (pyoverdin)
Biochemical Tests:
o Oxidase positive*
Chlamydia Trachomatis:

Relevant Virulence Factors:
Life Cycle: replicates in reticulate body and the lyses out to release infectious elementary bodies; organism can
remain in dormant state while still eliciting inflammatory response

Etiology/Pathogenesis:
Transmission: primarily STI, but also role in eye infections
o Vertical: mother to newborn (inclusion conjunctivitis)
o Horizontal: person to person (trachoma)
Trachoma:
o Chronic follicular conjunctivitis causing trachiasis (inward growth of eyelashes that scrape cornea)
o Recurrent infections due to fingers and contaminated objects
o Chronic inflammation of eyelids lead to corneal scarring and blindness
Inclusion Conjunctivitis (Opthalmia Neonatorum):
o Associated with genital serotypes (passed from mother to child during birth via contact with cervical
secretions)
o Eye discharge 2-25 days after delivery

ID:
Shape/Staining: Gram negative outer membrane with no peptidoglycan (does not gram stain); coccobacilli
o Note: no peptidoglycan also means you cannot treat with b-lactams
Detection: intracellular parasite (several methods to detect)
FUNGI CAUSING EYE INFECTIONS:
Candida Albicans:

Relevant Virulence Factors:
Adhesins and invasive hyphae (bind fibronectin, collagen and laminin)
Proteases and elastases (invasion process)

Etiology/Pathogenesis:
Basics: commensal of oral cavity and urogenital tract
Infections:
o Endophthalmitis: white cotton ball expanding on retina/floating in vitreous humor
o Chorioretinitis: in immunocompromised patients; manifestation of systemic diseases that can lead to
blindness

ID:
Blood culture (detection of disseminated disease)
KOH/Gram stain reveals budding round/oval yeast with hyphae
Histoplasma Capsulatum:

Relevant Virulence Factors:
Dimorphic growth phases (pathogenic form in tissue at body temperature)
Evasion of immune system (can be dormant; can grow in phagocytes)

Etiology/Pathogenesis:
Basics: associated with bird and bat droppings; mold grows in soil when its humid, conidia inhaled by host and
converts to pathogenic yeast form
Predominantly a pulmonary disease with possible dissemination  chorioretinitis
Coccidiodes Immitis:

Relevant Virulence Factors:
Mold form produces infectious arthroconidia
Dimorphic growth phases (spherule form, not yeast, invades tissue)

Etiology/Pathogenesis:
Basics: geographically limited to hot, dry regions of Southwestern US
Primarily a pulmonary infection that can disseminate to the eye and CNS chorioretinitis
o More likely in immunocompromised
Fusarium spp.:

Abundant soil fungi

Infections in humans: usually in immunocompromised
Keratomycosis: infection of cornea
Onychomycosis: infection of nails
PARASITES CAUSING EYE INFECTIONS:
Acnthamoeba spp:

Etiology/Pathogenesis:
Life Cycle:
o Trophozoites (Ameba): are free living
o Cysts: infectious stage; equivalent to spores
Basics: inhabit soil, fresh or brackish water
Infections:
o Ulcerative Keratitis:

Amebas invade ocular tissue through break in corneal epithelium

Infection follows mild corneal trauma (ie. improperly sterilize hard contact lenses)

Granulomatous inflammation occurs as a result of trophozoite and cyst penetration

Normal immune defenses insufficient
o Encephalitis: primarily in elderly and immunocompromised

ID:
Trophozoites and cysts can be seen in corneal biopsies
Toxoplasma Gondii:

Relevant Virulence Factors:
Often carried by cats: obligate, intracellular protozoan that requires warm-blooded mammal as host
o Sexual stage occurs in intestinal tract of cats and oocysts passed in feces
Humans accidentally ingest oocysts: after cleaning litter box or eating infected meat

Etiology/Pathogenesis:
Congenital Toxoplasmosis:
o ~50% of the population has been infected
o Can be passed in utero (“Don’t clean cat litter while pregnant”)
Chorioretinitis:
o Most common delayed manifestation of congenital toxoplasosis
o Reactivation of dormant tissue cysts, especially in immunosuppression
o Proliferates in the retina, leading to inflammation of the choroid
Toxocara Canis: Round Worm in Dogs**

Relevant Virulence Factors:
Often carried by canines: eggs ingested, eventually travel to lung where they are coughed up and then reingested (completes maturation in small bowel); larvae can enter pulmonary capillaries and go systemic
o Tons of eggs released by the worm into the dog’s feces; can live in the soil for years
Humans accidentally ingest eggs: larvae pass through pulmonary capillaries and enter systemic circulation; grow
in capillaries (penetrate wall/enter tissue)

Etiology/Pathogenesis:
Visceral Larva Migrans: causes eosinophilic granulomas and tissue necrosis of liver, heart, lung, brain and
skeletal muscle
Chorioretinitis: due to ocular larva migrans (larva migrating to eye)
Retinal Granulomas: cause unilateral strabismus and decreased visual acuity