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Review of Antibacterial Drugs Used For Treatment of Ocular Infections

Diane P. Yolton , PhD, OD, FAAO

 

Contents

 

Introduction

 

The eyes and adnexa are constantly exposed to a variety of bacteria. In most cases, the bacteria do not produce infection because the skin, mucous membranes, and the immune system provide effective barriers against infection. However, some bacteria can penetrate these defenses and cause an infection. When this happens, an antibacterial drug (e.g., an antibiotic) can help the immune system kill the bacteria.

Choosing an antibiotic requires knowledge about the bugs as well as the drugs. This course describes the epidemiology of ocular infections as well as drug actions, spectra of activity, resistance, clinical uses, and potential side-effects for each of the major antibacterial drugs categories.

At the end of the course, a summary table (Table 1) is provided that lists antibacterial drugs of choice and dosages for initial treatment of the most common ocular infections.

 

Bacteria That Causes Ocular Infections

 

Bacteria are single-celled microorganisms that, in most cases, produce their own energy and cellular components. Of the many species of bacteria, only a few are pathogenic in humans. The largest division of bacteria can be classified on the basis of shape and gram stain reaction.

Gram Stain Bacterial Classification

Gram-positive spherical bacteria (cocci) arranged in clusters belong to the classification staphylococci. Staphylococcus aureus (Figure 1) and Staphylococcus epidermidis (Figure 2) together cause about half the ocular infections that occur in humans and many non-ocular infections.

Figure. 1. Gram-stain of Staphylococcus epidermidis.

 

Figure 2. Gram stain of Staphylococcus aureus within a tissue sample.

 

The streptococci are the other group of gram-positive cocci that cause ocular infections; they are arranged in chains (Figure 3). This group includes Streptococcus pneumoniae that is a common cause of pneumonia, Streptococcus pyogenes that causes strep throat, and viridans streptococci that can cause endocarditis.

 

Figure 3. Gram stain of Streptococcus pneumoniae seen as chains of two (diplococci)(arrow) within a tissue sample.

 

Neisseria gonorrhoeae are gram-negative cocci (Figure 4) that cause gonorrhea and Neisseria meningitidis are gram-negative cocci that can cause meningitis.

 

Figure 4. Gram stain of Neisseria gonorrhoeae seen as gram-negative diplococci (arrow) inside a polymorphonuclear neutrophil (two arrows).

 

The gram-negative rods include Haemophilus influenzae. In a gram stain, these rods have many shapes ranging in appearance from very short to very long rods (Figure 5). H. influenzae typically infects children under five years of age causing otitis media, respiratory infections, and meningitis.

 

Figure 5. Gram stain of Haemophilus influenzae. Arrows show very short, gram-negative rods and also long rods in the same specimen.

 

Another group of gram-negative rods includes Escherichia coli, Serratia marcescens, Proteus, and Pseudomonas aeruginosa. These rods are typically short, even-length in the gram stain (Figure 6). They are typically found in the intestinal tract and commonly cause urinary tract infections.

 

Figure 6. Gram stain of Pseudomonas aeruginosa.

 

Bacterial Classification Based on Structure

In addition to the bacteria that are characterized using the gram stain, several groups of pathogenic bacteria have a unique structural structures or metabolism. Chlamydia lack the ability to produce enough energy to grow independently so they mimic viruses by growing and multiplying inside living cells (Figure 7). Chlamydia trachomatis is transmitted by finger-to-eye or fomite-to-eye in trachoma, or by self-contamination in inclusion conjunctivitis.

 

Figure 7. Chlamydia growing inside a conjunctival epithelial cell (intracellular inclusion body).

 

The spirochetes, including Treponema pallidum that causes syphilis, have a special structure consisting of flexible spirals (Figure 8). Possible syphilitic eye disease findings include interstitial keratitis, uveitis, pigmentary retinopathy, vitritis, retinal vascular sheathing, and papillitis.

 

Figure 8. Treponema pallidum.