Reader and Industry Forum

Bacterial Environment and the Eye

Reader and Industry Forum

Bacterial Environment and the Eye


The “normal bacterial flora” of the eye consists simply of the bacteria living in and on our bodies on a semi-permanent basis without causing any disease. The human conjunctiva and eyelid are home to a number of microorganisms (Table 1); many of these are pathogenic and virulent, yet these microorganisms have an important role in secreting certain antibiotics that protect us from infections of the eye. They also prevent pathogens by competing with the harmful bacteria for nutrients. A host of factors cause variations in the growth of the microorganisms, including age, sex, genetic makeup, contact lenses, surgery, climate, and geography (Levinson and Jawetz, 1989).

TABLE 1 Classification of common eye pathogens (Khan et al, 1982)

Gram Positive
Cocci Bacilli
Staphylococcus aureus and epidermidis Bacillus cereus and four other species
Micrococcus species Corynebacterium diphtheriae (WO phage B)
Streptococcus pneumoniae (diplococcus) Propionibacterium acnes (acne)
Streptococcus pyogenese and agalactiae agalactiae Clostridium perfringens (gangrene)
Streptococcus viridans and equisimilus Listeria monocytogenes
  Bacillus cereus and four other species
Gram Negative
Cocci Bacilli
Neisseria gonorrhoeae (diplococcus) Pseudomonas aeruginosa
Neisseria meningitidis Haemophilus influenzae and Haemophilus aegyptius (Koch-Weeks Bacilli)
Moraxella (Branhamella) catarrhalis Moraxella lacunata (diplobacillus)
  Proteus species (especially Proteus mirabilis)
  Serratia marcescens
  Escherichia coli, Klebsiella species
  Azotobacter species, Fusobacterium species
  Pasteurella species, Yersinia species
  Bacteroides fragilis

Common Flora

Among the studies conducted on the eye, conjunctiva, and normal flora, significant cases have been reported by Rajvanshi (1968), Khorazo and Thompson (1935), Fernández-Rubio et al (2009), and Ansari et al (2008). These researchers have found Staphylococcus, CoNS (coagulase negative Staphylococcus), followed by diptheroids, to be the predominant organisms prevailing in the eye.

Another group of authors including Chang (1957), Matuura (1971), and Locatcher-Khorazo and Seegal (1972) have even reported that a varied percentage of eyes have no normal flora, also called “sterile eyes.”

Both groups of authors agree that a higher density of bacteria is isolated from the eyelid as compared to the eye. The normal flora of the eyelid is similar to that of the skin. Studies by Weiss et al (1993), Groden et al (1991), Au et al (1993), and Taylor et al (1988) have reported that eyelids also have a predominance of CoNS followed by diptheroids. A small percentage of the total organisms of the eyelids also consist of Staphylococcus aureus and Acinetobacter species.

Staphylococcus There are 32 species of Staphylococcus, out of which only Staph. aureus, Staph. intermedius, and Staph. schleiferi are coagulase positive. CoNS species are a major constituent of the normal human skin flora but, increasingly, are the cause of major infections associated with medical devices such as catheters, cardiac valves, and prosthetic devices; they also particularly affect immunocompromised patients (

Diptheroids This term is used to describe any non-pathogenic bacteria of the diphtheria genus (Corynebacterium). Microscopic evaluation of the bacteria depicts different morphologies, such as clubbing, branching, coccoid forms, and palisading. The primary Corynebacterium species isolated from the eye is Pseudodiphtheria (C. hofmanni). C. xerosis, C. renale, and C. mycetoides also are present in significant amounts in the eye (Linoli and Marconi, 1976). Studies of eyelids have also detected an obligate anaerobe, P. acnes (Groden et al, 1991).

Transmission Infections in the eye are quite common, and the channel through which an eye can become infected is either from an external route or by microbial encroachment through intraocular pathways carried via the blood stream. External bacterial eye infections can easily spread to the surrounding area, such as the cornea, inner eye, eyelid, eyelashes, and the brain. Common bacterial infections are blepharitis, conjunctivitis, internal and external hordeolum, microbial scleritis, canaliculitis, keratitis, dacryocystitis, preseptal cellulitis, orbital cellulitis, endophthalmitis, and panophthalmitis.

Studies conducted by Ramesh et al (2010) have shown that most of the bacterial infections are caused by Staphylococcus aureus, followed by Streptococcus pneumonia and CoNS. Margins of the eyelid and their glands are infected predominantly by S. aureus and S. epidermidis. Strep pneumoniae usually causes infections of the tear duct inside the eye (Ramesh et al, 2010; Alvarez and Tabbara, 1996; Galin, 1962; McCulley et al, 1982).

Contact Lenses and Bacteria

Wearing contact lenses might change the equation of the bacteria present in the eye. A study conducted by Fleiszig and Efron (1992) stated that there is no significant difference between the eye floras of the people who wear contact lenses as opposed to those who don’t. However, previous use of contact lenses increased the incidence of bacteria in the eye. Also, it was reported that the use of contact lens changed the proportion of diptheroids and CoNS in the eye. The presence of contact lenses favors the growth of CoNS, while decreasing the growth of diptheroids.

Studies by researchers of Langone Medical Center in New York City recently have shown that there are a number of changes in the bacterial flora once frequent use of contact lenses begins. Proportion of Methylobacterium, Lactobacillus, Acinetobacter, and Pseudomonas bacteria in the eyes of the contact lens wearers increased compared to those who didn’t wear contact lenses (Press Trust India, 2015).

Another study conducted on the residents of Nigeria to research the effects of extended wear and disposable contact lenses on the incidence of bacteria in the eye revealed that a vast majority of the lenses increased chances of contamination in the eye. The study showed that Streptococcus spp. were present in the extended wear contact lenses (4.23%) while the Klebsiella spp. were found more in the disposable lenses. Also, there were significant differences in the bacterial growth of the disposable and extended wear contact lenses (p < 0.05) (Emina and Idu, 2011).

Another study of 21 patients with samples taken before and after lens wear showed that gram positive culture growth decreased after lens wear over a six-month period (McBride, 1979). In this particular study, Staphylococcus epidermidis was the most frequently isolated organism, followed by Micrococcus spp., diphtheroids, and Bacillus spp. Nonfermentative, gram-negative rods and fungi were also isolated sporadically.

Changes in the normal flora found in the eye and conjunctiva due to lens wear do occur. Most have reported that the main reason for this is because of the bacterial contamination added directly from the lens to the ocular surface. This contamination may be due to improper handling of the lens or the lens storage cases (Szczotka-Flynn et al, 2010). Elderly subjects using thick contact lenses for extended wear showed the presence of more potential pathogens, which contribute to higher risk of eye infection associated with contact lens wear (Fleiszig and Efron, 1992).


It is important that eyecare providers understand that the introduction of contact lenses to the eye on a regular basis changes the dynamics of the normal flora that would otherwise maintain the proper homeostasis of the eye. Patients, likewise, need to be properly educated in the handling and cleaning of their contact lenses as well as be made aware of possible risks of infection that contact lenses may pose. CLS

To obtain references, please visit and click on document #243.

Dr. Gonzalez is a graduate from Inter American University School of Optometry in Puerto Rico (IAUPR) and has been in practice in Texas since 1995. He is a recognized industry expert in adoption and usage of ophthalmic medications by optometrists and is a board certified Clinical Medical Optometrist who has lectured extensively including AOA’s Optometry’s Meeting and the American Academy of Optometry. Dr. Holt is in private practice at West Point Eye Center in West Point, Utah. He graduated from the IAUPR and then completed a residency at the University of Utah and VA Medical Center. He then worked for more than seven years at Mount Ogden Eye Center. He can be reached at