Microbial keratitis (bacterial, fungal)


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The commonest bacterial corneal pathogens are:

  • Pseudomonas sp. (Gram -ve)
  • Staphylococcus sp. (Gram +ve)
  • Streptococcus sp. (Gram +ve)
  • other Gram -ve organisms

Note: severe contact lens-related infections tend to be Gram -ve, particularly Pseudomonas species

Fungal infection causes a small proportion of infectious keratitis cases in temperate regions; however, in tropical climates it can cause up to 50% of cases. The most common fungal corneal pathogens are:

  • Candida sp. (yeast-like)
  • Fusarium sp. (filamentous)
  • Aspergillus sp (filamentous)

Predisposing factors

Bacterial keratitis is usually associated with one or more of the following:

  • contact lens wear, especially soft lenses worn overnight (incidence, soft daily wear: 2-4 per 10,000 per year, soft overnight wear: 20 per 10,000 per year ). The risk of MK with overnight corneal reshaping contact lenses (ortho-K) is similar to that for other overnight modalities. The anticipated reduction in contact lens-related microbial keratitis with silicone hydrogel lenses has not been observed. Other main risk factor for CL-related MK are:
    • increased days of wear
    • poor hand, lens and storage case hygiene
    • youth
    • male gender
    • smoking
    • internet purchase of lenses, particularly cosmetic lenses
  • ocular surface disease, including:
    • corneal exposure
    • corneal decompensation
    • chronic epithelial defect
    • neurotrophic keratopathy, e.g. secondary to HSK or diabetes
    • tear deficiency
  • ocular trauma or surgery, including loose or broken sutures
  • immune compromise
  • topical steroid use
  • lid margin infection (usually Staphyloccocal)

Fungal keratitis (filamentous) is usually secondary to trauma involving organic material; it can also be contact lens or solution related
Fungal keratitis (yeast-like) most usually complicates ocular surface disease or in immunocompromised patients

Symptoms of microbial keratitis (bacterial, fungal)

Pain, moderate to severe (usually acute onset, rapid progression)
Redness, photophobia (may be severe), discharge, blurred vision (especially if lesion on visual axis)
Awareness of white or yellow spot on cornea
Usually unilateral

Signs of microbial keratitis (bacterial, fungal)

Lid oedema
Discharge (mucopurulent or purulent)
Conjunctival hyperaemia and infiltration
Corneal lesion usually single (central or mid-peripheral)

  • excavation of epithelium, Bowman’s layer, stroma (tissue necrosis)
  • stromal infiltration beneath lesion
  • stromal oedema with folds in Descemet’s membrane
  • endothelial fibrin plaque beneath lesion
  • optical coherence tomography (OCT) may be helpful in determining depth of involvement

Anterior chamber activity (flare, cells, hypopyon or coagulum if severe)

Fungal keratitis produces similar signs to bacterial keratitis; however, it has been claimed that deep lesions, those having a feathery edge, the presence of satellite lesions and the presence of endothelial plaque are all features suggestive of a fungal as opposed to a bacterial infection. Fungal keratitis may develop more slowly (however Fusarium infection can progress rapidly and invasively)

Differential diagnosis

Corneal infiltrative lesions (contact lens related or marginal keratitis; see separate Clinical Management Guidelines)

  • peripheral, small (0.5-1.5 mm) with less anterior chamber response
  • not a marker for increased risk of bacterial infection

Acanthamoeba keratitis (see Clinical Management Guideline)

Management by optometrist

Practitioners should recognise their limitations and where necessary seek further advice or refer the patient elsewhere

GRADE* Level of evidence and strength of recommendation always relates to the statement(s) immediately above

Non pharmacological

Warn contact lens wearers not to discard their lenses or lens cases, but to retain them for culture
(GRADE*: Level of evidence=low, Strength of recommendation=strong)


Usually none. Beginning empirical antimicrobial therapy without laboratory evaluation may delay correct diagnosis and proper care if improvement does not promptly take place
(GRADE*: Level of evidence=low, Strength of recommendation=strong

Management category

A1: emergency referral to an ophthalmologist; no intervention. Severe sight-threatening condition. Emphasise to the patient the urgency of the condition and instruct him or her to attend the local hospital eye department or hospital A&E the same day, explaining that you will leave a message so that he or she is expected. Telephone the department to explain what you have done, preferably leaving your message with a doctor or other health care professional

Possible management by ophthalmologist

Corneal scrape (for culture and determination of antibiotic sensitivities) followed by initiation of intensive (round the clock) treatment with one or more antibiotics. Corneal isolate studies show changing patterns of bacterial pathogenesis and the development of resistant strains.

  • monotherapy: fluoroquinolones (e.g.levofloxacin, moxifloxacin) are adequate for most cases but not for resistant species of Staphylococcus aureus and Pseudomonas aeruginosa
  • dual therapy: the recommended fortified agents (a cephalosporin and an aminoglycoside) are not commercially available
  • may be combined with systemic antibiotics if lesion close to limbus

Corneal scrape will normally include inoculation of both bacterial and fungal culture media
Polymerase chain reaction [PCR] technique may be used to identify causative organisms
Possible admission to hospital when good compliance is unlikely, or for overnight treatment of severe infections (axial lesions, lesions 6mm or more in diameter, or with 50% or more stromal thinning)
Hypotensive agents for secondary glaucoma
Topical steroids (only when infection controlled) – not well supported by evidence base
Amphotericin B (as 0.15% eye drops) is the drug of choice in fungal keratitis caused by yeasts (e.g. Candida)
Fungal infections usually require combined topical (e.g. natamycin 5%, econazole 1% or voriconazole 1%) and oral (e.g. voriconazole) therapy. Clinical strategies continue to evolve

Evidence base

*GRADE: Grading of Recommendations Assessment, Development and Evaluation (www.gradeworkinggroup.org)

Sources of evidence

Carnt N, Samarawickrama C, White A, Stapleton F. The diagnosis and management of contact lens-related microbial keratitis. Clin Exp Optom 2017;100:482-493

Cheung N, Nagra P, Hammersmith K. Emerging trends in contact lens-related infections. Curr Opin Ophthalmol. 2016;27:327-332

Dahlgren MA, Lingappen A, Wilhelmus KR. The Clinical Diagnosis of Microbial Keratitis. Am J Ophthalmol. 2007;143(6):940–4

FlorCruz NV, Evans JR. Medical interventions for fungal keratitis. Cochrane Database Syst Rev. 2015;4:CD004241

Herretes S, Wang X, Reyes JM. Topical corticosteroids as adjunctive therapy for bacterial keratitis. Cochrane Database Syst Rev. 2014;10:CD005430

Jongkhajornpong P, Nimworaphan J, Lekhanont K, Chuckpaiwong V, Rattanasiri S. Predicting factors and prediction model for discriminating between fungal infection and bacterial infection in severe microbial keratitis. PLoS One. 2019;14(3):e0214076.

Lin A, Rhee MK, Akpek EK, Amescua G, Farid M, Garcia-Ferrer FJ, Varu DM, Musch DC, Dunn SP, Mah FS; American Academy of Ophthalmology Preferred Practice Pattern Cornea and External Disease Panel. Bacterial Keratitis Preferred Practice Pattern®. Ophthalmology. 2019;126(1):P1-P55

McDonald EM, Ram FS, Patel DV, McGhee CN. Topical antibiotics for the management of bacterial keratitis: an evidence-based review of high quality randomised controlled trials. Br J Ophthalmol. 2014;98(11):1470- 7

Shalchi Z, Gurbaxani A, Baker M, Nash J. Antibiotic resistance in microbial keratitis: ten-year experience of corneal scrapes in the United Kingdom. Ophthalmology. 2011;118(11):2161-5

Plain language summary

This is a serious condition in which the cornea (the clear window of the eye) becomes infected. The usual cause is contact lens wear, but infection of the cornea can also result from exposure of the eye (for example if the eyelids are not blinking normally), loss of sensation in the eye surface, injury or surgery, lack of tears (dry eye), and in people whose immune system is not functioning properly. The usual cause is bacterial (i.e. caused by a common germ) but some cases are due to fungal infection.

Microbial keratitis is a very serious condition. It usually begins suddenly with redness and pain in one eye. The eye waters and there may be a discharge. Light may hurt the eye, making it difficult to open. The vision of the eye may be blurred.

The optometrist will usually observe an area in the cornea where the clear tissue has been turned cloudy by infection. There may be an ulcer on the surface. The inflammation extends into the chamber at the front of the eye also.

Microbial keratitis is a sight-threatening emergency, so such patients should be referred immediately to the ophthalmologist. The ophthalmologist will take specimens from the ulcer and begin treatment with antibiotic eye drops. These will need to be put into the eye very frequently. The patient will most likely be admitted to hospital so that treatment can continue day and night. If the infection is caused by a fungus, the treatment will usually be with anti-fungal eye drops and tablets.

When the infection is controlled, steroid eye drops may be added. If the infection was contact lens related, the patient will be given advice on whether it is safe to wear lenses again.

Microbial keratitis (bacterial, fungal)
Version 12
Date of search 19.06.20
Date of revision 18.12.20
Date of publication 20.08.21
Date for review 18.06.22
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