What is a Corneal smear for Gram Stain?

Gram stain is a specialized method of staining to distinguish bacterial species into two groups. These groups are known as Gram-positive and Gram-negative bacteria.

Specific therapy of ocular infections often requires an etiological diagnosis that is a combined effect of observation of characteristic clinical features and microbiological investigations. The clinical impression is central to guiding the laboratory investigation, and the aim of laboratory investigation is to confirm or rule out the clinical diagnosis. However, clinical features may vary considerably, and no one clinical feature may be pathognomonic of a particular pathogen. In addition, there may be a racial, geographical, and climatic difference in the distribution and type of causative agents associated with infections. Ophthalmologists have at their disposal in vivo and in vitro methods of diagnosis of ocular infections. The expertise of the clinician and the microbiologist along with the facilities available, determine the success with an accurate diagnosis. A wide range of conventional and molecular techniques are available that not only provide a rapid diagnosis for known common infections but have the potential to bring to the fore unknown organisms that may be associated with ocular infections.

The external ocular surface harbors commensal organisms, such as Staphylococcus species, Corynebacterium species, and Propionibacterium species, which form the resident flora. Given a chance, any environmental organism can become a transient flora in the eye. The intraocular tissues and spaces, however, are sterile. While the conjunctiva is protected by blood supply, the cornea is avascular; therefore, the types of organisms invading these tissues may vary. The intraocular tissues are relatively immune-privileged and can be infected by any organism that manages to enter the inside of the eye. Trauma is an important predisposing factor for infection of the cornea and intraocular tissues. While exogenous infections are most common, eye infections may develop by the spread of infection from neighboring organs or hematogenous.

Ocular infections may be caused by bacteria, fungi, parasites, or viruses, and each of these may produce a spectrum of diseases. It is usually challenging to determine the causative agent based on clinical features because they may or may not be distinctive. Many a time, it may not even be possible to discriminate between infective or non-infective conditions. In addition, the prevalence and distribution of the type of infectious agents associated with eye infections widely vary and are dependent on a variety of factors. In recent times, in vivo methods, such as confocal microscopy, have come a long way in putting clinical diagnosis on firmer grounds, especially for Acanthamoeba and fungal keratitis. However, apart from its limited application, the equipment may not be available in all settings. Therefore, confirmation by microscopic examination and culture of the clinical samples remains the gold standard for etiological diagnosis. Currently, molecular diagnosis has added the sensitivity, specificity, and speed that have been the concerns with the conventional techniques of microscopy and culture. This review will describe the role of various methods that are currently available for the diagnosis of ocular infections.