Microbial keratitis (bacterial, fungal)

The CMGs are guidelines on the diagnosis and management of a range of common and rare, but important, eye conditions that present with varying frequency in primary and first contact care.

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Contact Lens Keratitis







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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 keratitis is rare in the UK but common in some other parts of the world. 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 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

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

Fungal keratitis produces similar signs to bacterial keratitis, though the infection 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 of microbial keratitis (bacterial, fungal) 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 them to attend the local hospital eye department or hospital A & E the same day, explaining that you will leave a message so that they are 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

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

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

Lay 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 abnormal 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 11
Date of search 23.05.18
Date of revision 31.05.18
Date of publication 16.10.18
Date for review 22.05.20
© College of Optometrists 

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