Take a Closer Look at the Ocular Flora

Deepen your understanding of its significance with this review from two external disease specialists.

BY JESSE PELLETIER, MD AND JOSEPH CAPRIOTTI, MD

The composition of the normal ocular flora plays an essential role in the healthy functioning of the eye, the maintenance of surface homeostasis, and in both prevention and causation of ocular infection.

A clear delineation of the interactions between normal flora and the ocular surface is of great benefit in understanding ocular health and ocular pathology. A thorough treatment requires the identification of colonizing organisms and knowledge of their relationship with both the surface structural elements and host immune mechanisms. This complex system of surface integrity includes physical barriers, fluid mechanics, bacteriostatic and bacteriolytic peptides, and a variety of B- and T-lymphocyte responses.

Comprehensive understanding of the microbial, chemical and anatomic factors that influence the dynamics of colonization and pathogenesis on the ocular surface remains elusive, though some key interactions have been elucidated in the recent literature. This article will briefly update readers on trends in these areas of research.

To Catch a Predator

Investigation of ocular flora has often employed microbial culture methods on samples obtained by conjunctival swab and/or corneal spatula. A shortcoming of this method is the well-described limitation of growth factors present in a variety of media for growth of some bacterial species.

For example, Propionibacteria acnes is a well known commensal organism of the ocular surface and has been implicated in late-onset endophthalmitis. These fastidious, slow-growing anaerobes may be difficult to culture using traditional media.² More recently, PCR testing has been used in a variety of settings to identify colonizing organisms.^2,3 The increased sensitivity of this detection method has increased our knowledge of conjunctival bacterial diversity.³ Understanding how to interpret the presence of bacterial genetic material instead of classical culture positivity may begin to pose challenges for the clinician.

Ocular flora have historically been ascribed two opposing roles in overall ocular health — one as source organisms for ocular infections, the other as defense organisms for prevention of pathogen colonization on the surface. It has been understood for some time that infection in the wake of surgery, ocular injection, or minor trauma is likely linked to infectious agents resident on the normal ocular surface.

Numerous studies have linked minor ocular trauma and subsequent infection.^4,5 The landmark Bhaktapur Eye Study⁶ demonstrated that treatment of minor corneal abrasions with broad-spectrum antibiotics was able to nearly completely eliminate subsequent development of corneal ulcers. Though the Bhaktapur study was not designed to detect the percentage of resident ocular flora in subsequent infections, we know that in cases of post-operative endophthalmitis following cataract surgery that correlation may be as high as 82%.⁷

The ophthalmic literature has previously demonstrated that surface flora are the major source for postoperative endophthalmitis.⁷ Eliminating pre-surgical flora has consistently been the most effective intervention for reducing postop endophthalmitis following routine cataract surgery^8,9 Groundbreaking work presented by Isenberg¹⁰ and Speaker¹¹ led to routine disinfection of the ocular surface with povidone-iodine before all invasive ophthalmic procedures.

Conversely, a protective role of normal ocular flora has also been described. Normal flora can secrete antibiotics and other chemical mediators to maintain homeostasis or may simply outcompete pathogenic bacteria for available nutrients.^12,13 Other important, but not yet fully elucidated, commensal organism functions may include preservation of epithelial integrity, inhibition of apoptosis, anterior segment wound repair, and maintenance of immunoregulation.¹⁴

Know Your Enemy

Normal ocular flora remains relatively consistent among human populations. It may, however, be altered by a variety of factors that include age, immunosuppression, ocular inflammation, dry eye, contact lens wear, antibiotic use, surgery, external exposure, climate and geography. The most commonly reported bacteria include coagulase-negative Staphylococcus (CNS), Staphylococcus aureus, Streptococcus spp., Corynebacterium spp., and Proprionibacterium acnes.^15-17 However, the normal flora may also consist of more pathogenic organisms that remain passive colonizers rather than invasive causes of disease.

Commonly-isolated pathogenic organisms include gram negative rods such as Pseudomonas aeruginosa, Haemophilus influenzae and fungal species. It is difficult to determine the precise factors that render an organism able to colonize the ocular surface without causing infection. A recent publication by Miller et. al.¹⁴ noted that the interaction likely depends on an intact anatomic barrier (surface epithelium), as well as immune system regulation based on pathogen-associated molecular patterns. If this cooperative interaction is robust, tolerance of the surface bacteria is likely. Uncooperative interactions may conversely stimulate the immune system and result in pathogenesis. In either case, epithelial breakdown may lead to inflammation or frank infection whether the organism is considered pathogenic or not.

The composition of the normal ocular flora changes dynamically throughout our lives and does not remain fixed. Our first exposure to potential colonizing bacteria occurs at birth. Initial colonization patterns may differ in cases of Caesarian section versus normal spontaneous vaginal delivery. While coagulase-negative Staphylococcus still remains the most commonly isolated bacteria, the conjunctiva of infants born vaginally may bear flora more similar to vaginal flora.¹⁸

In contrast, Caesarian section neonates tend to harbor more skin-associated flora, and in multiple studies have been shown to grow a lower number of isolated conjunctival strains.^18,19 Among the pathogenic bacteria that can be isolated in the neonate, Neisseria gonorrhea and Chlamydia trachomatis remain the most feared. Both are responsible for ophthalmia neonatorum; however, the former may invade intact corneal epithelium and cause rapid perforation with systemic sequelae.

There are a variety of factors that may predispose the eye to changes in conjunctival flora composition — the first of which is the simple process of aging. Thiel et. al.¹⁷ described this trend in a study of normal 3- to 90-year-old subjects in Germany. The study found that found the spectrum of aerobic bacteria colonizing the conjunctiva diversifies and increases in density with age. Moreover, among anaerobic bacterial colonizers Propionibacteria are the most common in younger individuals and subsequently decrease with age. It was speculated that a shift in floral composition may be related to the interaction of aqueous tear deficiency with age, goblet cell changes, and lipid dysregulatory states that accumulate over time.

The ophthalmic literature has defined ocular surface flora changes in a variety of conditions. This is of paramount importance when evaluating more severe ocular infections where cultures are difficult or impossible to obtain.

For example, we recently published a study of the normal ocular flora in patients from a rural area of Sierra Leone.²⁰ In that study population, we noted a high proportion of ocular colonization by both fungi and gram-negative rods. A large number of the study participants were palm-tree tappers and subsistence farmers who reported frequent job-related ocular exposures. Interestingly, other studies in both the developed and developing world have also demonstrated the presence of fungi as a normal colonizer of the ocular surface.^15,21 We then hypothesized that these colonizing organisms may play a role in corneal infection after minor corneal trauma in rural Sierra Leone.²² The results of that study indicated that the most commonly implicated organisms in corneal ulceration cases were gram-negative bacteria, with gram-positive bacteria and fungi rates essentially equivalent.

Ocular flora changes in disease states and with ocular risk factors are also well described.^3,16,23,24 In some studies, contact lens wear has been found to disrupt the synergistic relationship between Staphylococcus species and diphtheroids.²³ While this does not necessarily result in infection, it may suppress normal immune mechanisms which serve to clear foreign pathogens.¹⁶

At the other end of the spectrum, severely burned patients have been shown to undergo colonization shifts with the replacement of Staphylococcus epidermidis and Corynebacterium populations in favor of gram-negative bacilli and Staphylococcus aureus.²⁴ A PCR-based dry eye study by Graham et. al. identified bacteria such as Rhodococcus, Klebsiella and Erwinia — organisms not typically associated with the normal ocular flora.³ Moreover, many of these bacteria have been linked to infections such as endophthalmitis and keratitis.

Clinical Implications

It is difficult to determine the significance of isolated rare, possibly pathogenic bacteria from an eye that is uninflamed. Questions to be answered include:

• Are these bacteria more likely to cause ocular infection in the absence of a precipitating event?
• Do we all harbor such atypical bacterial strains, and has the higher sensitivity of PCR has only illuminated this fact in tested subjects?
• What factors might cause rare ocular flora to overwhelm the ocular surface and cause infection?

There is some evidence that quorum-sensing mechanisms may activate when pathogenic bacterial strain populations reach a certain size.³ In effect, when a large concentration of bacteria is achieved, cell-to-cell signaling may increase virulence factors and overwhelm the immune system.² This has been demonstrated to occur with bacterial strains of S. aureus and P. aeruginosa.^3,25

Even our most commonly isolated normal flora undergo change and may develop resistance. Of particular concern has been the development of Methicillin-resistant Staphlococcus aureus (MRSA). As previously noted, this bacteria is a frequent colonizer of the ocular surface and surrounding mucous membranes. It is likely that selection for MRSA isolates was borne in hospital patients and health-care workers, but recent studies have demonstrated the increasing contribution of the community setting for its acquisition.²⁶ It has been implicated in a variety of infectious settings including conjunctivitis, corneal ulceration, endophthalmitis, and more recently acute, early post-refractive surgery infections.²⁷

There has been a steadily increasing rate of resistance to fourth-generation quinolones since their introduction. Treating these infections successfully has required elevated suspicion of the part of the treating physician coupled with the utilization of the microbiology lab and agents such as vancomycin, polymyxin B/trimethoprim, or bacitracin.

Our understanding of the ocular surface has been evolving to appreciate the contributions of mechanical, chemical and cellular mediators of health and disease. Our ability to detect surface organisms with ever-greater precision has also matured as we have developed PCR-based techniques for the study of ocular microbiology. We are poised to uncover more of the relationships that define colonization and pathogenesis as we are challenged to extract meaningful clinical direction from ever more sophisticated laboratory results. OM

References

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	Jesse Pelletier, MD is a founding partner and director of cornea, cataract and refractive surgery at the Ocean Ophthalmology Group in Miami, voluntary assistant professor of ophthalmology at Bascom Palmer Eye Institute, and an attending ophthalmologist at the Miami VA.
	Anthony Capriotti, MD is an ophthalmologist and research scientist with an interest in ocular microbiology, infection and inflammation. He is associate research director of the Ocean Ophthalmology Group and an adjunct scientist in the department of chemistry at Columbia University.