Ocular and Lens Cultures and Its Bioburden

Introduction

Bioburden is known as the presence of microorganisms in a potential sterile medium. The eye is considered a possible sterile surface because of the antimicrobial nature of the tear film and the enzyme present on the tear film. Similarly, the lens surface and solution must be free from microorganisms. But in reality, microorganisms can be detected on the lens surface and eye. The presence of specific microorganisms is beneficial to maintaining normal cellular functions. But certain microorganisms can be a potential threat to ocular health. Ocular and lens bioburden assessments are necessary to identify harmful pathogens.

What Is Ocular Bioburden?

The eye is considered a potentially sterile organ. The antimicrobial action of the tear film and lysozyme present on the tear film prevents the growth of microorganisms. But certain microorganisms are present in the conjunctiva and the cornea. These microorganisms help maintain the normal cellular function of the cornea. The most commonly found microorganisms in the eyes are Actinobacteria, Proteobacteria, and Firmicutes species. Pseudomonas, Bradyrhizobium, Acinetobacter, Brevundimonas, Aquabacterium, and Sphingomonas species are also found in the conjunctival swabs. Fungi such as Candida, Ascomycota, and Basidiomycota species can also be found in healthy conjunctiva.

The factors which regulate the diversity and nature of the microorganisms are:

1. Ethnicity.

2. Age.

3. Seasonality.

4. Condition of the eye and tear film: Dry eye disease (disease of the eye with reduction of tear formation) and Sjögren’s syndrome (an autoimmune disorder).

5. Systemic disorders: Diabetes and Stevens-Johnson syndrome (a type of skin disorder).

What Is Lens Bioburden?

Lenses are sterile medical devices that are used for the treatment of moderate to severe ocular problems. Ideally, they should be free of microorganisms, but sometimes lenses are contaminated. The reasons are:

1. Improper Lens Hygiene:

Touching the lens with contaminated hands or keeping the lenses on contaminated surfaces.

1. Using the Lens During Swimming or Dust Exposure on the Lens Surface:

Water exposure during swimming and exposure to chlorinated pool water causes bacterial colonization into the lens surface.

1. Contaminated Lens Solution:

Contamination of lens case and lens care solution is one of the major causes of contamination. The lens care solution contains hydrogen peroxide, which can kill microorganisms, biofilm, and cells. But hydrogen peroxide of the old solutions can be broken down by the action of the catalase enzyme of the microorganisms. The contamination of the microorganisms occurs mainly from the tip of the bottle containing lens care solution. Also, contamination may occur from the biofilm of the lens case and lack of potency in the old solution.

1. Prolonged Use of the Lens:

Prolonged use of the lenses and nightwear of the lenses increases tissue injury and decreases the upregulation of corneal epithelium. Ortho-k lenses used for corneal reshaping may cause corneal tissue damage.

1. Improper Lens Selection:

• Hard or rigid lenses are less oxygen permeable. As a result, these lenses develop a hypoxic condition favorable for the growth of microorganisms.

• Lenses of polymethyl methacrylate (PMMA) and polyvinyl alcohol (PVA) are less oxygen permeable. Hydroxyethyl methacrylate (HEMA) hydrogel and silicone hydrogel lenses are more oxygen permeable and do not cause hypoxia.

Though the contamination occurs in various ways. The growth of the bacteria is taking place due to the altered pathophysiological condition of the lens surface. These conditions are:

1. Rigid and polymethyl methacrylate (PMMA) is less permeable to oxygen. As a result, tissue hypoxia (reduced oxygen supply) develops, which causes degradation and tinning of the corneal cells. Not only that, this hypoxic environment favors the growth of gram-negative organisms.

2. The tear film is necessary to provide proper protection of the ocular structures. Lenses alter the tear film's biochemistry and alter its functional ability. Tight-fitting lenses disrupt the tear flow and cause the accumulation of inflammatory substances. Also, it disrupts the tear exchange between the outer and inner surfaces of the lens.

3. Protein deposition on the lens surface is helpful as this provides transparency to the lens surface. But sometimes proteins with lipids and calcium are deposited onto the lens surface and form lens calculi. These deposits are mainly seen in patients using the same lens for a longer duration or using hydrogel lenses. These calculi may cause irritation, inflammation, and infection. Hydroxy ethyl methacrylate (HEMA) lenses are prone to tear film proteins deposition due to an electrostatic attraction between the material and positively charged lysozyme.

What Are the Microorganisms?

The nature of the pathogens varies a lot with different causative factors. The pathogens present on the lens surface are:

• The most commonly present pathogens are Staphylococcus aureus, Pseudomonas aeruginosa, Streptococcus pneumonia, and Serratia species. Pseudomonas aeruginosa is the most predominant pathogen present on the lens surface. Klebsiella pneumoniae, Neisseria gonorrhea, Neisseria gonorrhea, and Bacillus Cereus are other organisms that can also be found.

• Fungi on the lens surfaces are Fusarium, Aspergillus, Phaeohyphomycetes, Curvularia, Paecilomyces, and Scedosporium apiospermum.

• Acanthamoeba trophozoite is an opportunistic parasite and may also be found on the lens surface.

The adhesion mechanism on the lens surface facilitates the growth of microorganisms. Different scientists have proposed different methods for adhesion. These are:

1. Intermolecular forces, like Van der Waals, act between the lens surface and microbes.

2. The roughness increases the deposition of protein and lipids, which causes increased bacterial adhesion on the lens surface.

3. Microorganisms adhered to the rigid lens can easily be moved or dislodged. On the other hand, largely immobilized bacterial collection can be seen on the soft lens surfaces.

4. Lenses made up of hydrophobic materials accumulate more bacteria than lenses made up of hydrophilic materials.

5. Flagella and pilus are present on the surface of Pseudomonas aeruginosa, and Caulobacter crescentus, which helps in the adhesion process. Also, outer membrane protein and exopolymeric matrix help in the adhesion process.

6. Adhesive mono proteins produced by the fungi and Acanthamoeba are responsible for the adhesion of the microorganisms.

7. Adhesion of Staphylococcus aureus is facilitated by slime production. This slime helps to form biofilm and causes adhesion of the bacteria to the lens surface.

How to Reduce the Bioburden of Lens Cultures?

The accumulation of microorganisms can be prevented by following methods:

• Avoid prolonged use of lenses and use of the lenses during sleeping must be avoided.

• The lens should not be touched with contaminated hands and the sterility of the lens must be maintained.

• The lens solution should be changed at regular interval.

• Avoid using the lens while swimming and avoid entrapment of foreign materials in lens surface.

• The old, distorted lens must not be used.

• The hard and rigid lens should be avoided.

Conclusion:

Bioburden of the eyes and lenses is an important factor for ocular disease. An increase in the number of bacteria is responsible for eye diseases. Bacterial accumulation in the lens surface ulcers and corneal inflammation. That is why maintenance of lens hygiene, use of proper lens case, and lens care solutions are needed to prevent bacterial accumulation.