The unique outer membrane of gram-negative bacteria, containing phospholipids, LPS, and proteins, has been implicated as a major player in pathogenesis. The LPS portion of the envelope is both a major cell surface antigen and a major molecule triggering host response to bacterial infection. It is made up of three regions: the O-specific region (region I), responsible for the O or somatic antigenicity of the species and subspecies; the core polysaccharide (region II); and lipid A (region III), which is closest to the cell wall. The term “endotoxin” was used before discovering that it is the lipid A portion of the LPS. Different sugar sequences, linkage groups, and substituents cause different antigenic (serologic) specificities. The presence of O-antigen in Serratia marcescens enhances its adherence to inert and biologic surfaces.⁴⁴ Region II is less variable than region I. Region III is highly conserved among eukaryotes, with little microheterogeneity among genera and species. Several members of the family Enterobacteriaceae express additional antigenic polysaccharides in the form of capsular or K antigens.

Lipid A is crucial in that it is responsible for the pathophysiologic effects associated with gram-negative bacterial infections and bacteremia, including pyrogenesis and hematologic, immune, and endocrinologic effects. Systemic exposures to endotoxin may result in hypotension, disseminated intravascular coagulation, and death. Most of these effects are mediated by cytokines and some by clotting and complement activation.

Encapsulated strains of bacteria have capsules that consist of high-molecular-weight polysaccharides that form gels and allow bacteria to adhere to the host target cell surface. The capsule is shieldlike because it is hydrophilic and poorly immunogenic. First, the polysaccharide-rich composition of the capsule strongly inhibits phagocytosis by the hydrophobic surface of the host cell. Second, many capsules are poor immunogens⁴⁵ and complement activators.⁴⁶ If phagocytosis occurs, it requires specific opsonizing antibodies against the capsule. The thickness of the capsule in Francisella has been correlated with virulence, and in Brucella, encapsulated smooth colonies are ingested less readily than rough colonies.⁴⁷ The virulence of Klebsiella⁴³ and Yersinia⁴⁸ species has also been correlated with capsular presence and protection. Immunologic diversity of the capsule within the same species explains the basis for serotyping. Although most ocular isolates of Haemophilus influenzae are nonencapsulated, the encapsulated form of type b is more virulent than the encapsulated type d.⁴⁹

When seen with transmission electron microscopy (TEM), the outer membrane of a gram-negative rod can take on a peritrichous appearance because of the flagella and more numerous pili that surround it. Flagella are larger than pili (they measure 16 to 18 nanometers in diameter) and allow for motility of the organism toward and within the host tissue. Ninety-five percent of clinical isolates of Pseudomonas are flagellated,⁵⁰ and the burned-mouse model of P. aeruginosa has shown that flagella-deficient mutants are significantly less virulent.⁵¹ Flagellar proteins have been studied as vaccines. Rudner and associates⁵² showed that systemic or topical immunization of mice with strain-specific flagellar proteins or antiflagellar antibody homologous to the specific strain of bacteria protected them from pseudomonal keratitis.

TEM also reveals fimbriae, also known as pili, that measure 4 to 10 nanometers in diameter. These microfibrils can vary in number (2 to 12 in Pseudomonas) and can be classified according to their function as adhesins, lectins, evasins, aggressins, and sex pili. Fimbriated cells adhere to surfaces with specificity and thus allow colonization of a specific host tissue cell. Attachment to host cells is a first step in Pseudomonas keratitis. Stern and colleagues⁵³ demonstrated that murine corneal trauma predisposes to ulceration, not by increasing the exposed area of de-epithelialized stroma for entry of the organisms, but by providing an injured epithelial edge to which P. aeruginosa can adhere. Purified Pseudomonas pili were found to compete with whole bacteria in saturating the binding receptors of host tissue.⁵⁴ Knutton and coworkers,⁵⁵ in their experiments with enterotoxigenic E. coli, showed that before invasion, these microorganisms adhere to the mucosal surface of intestinal cells with pili that allow them to hold on to specific mucosal receptors.

Exotoxin A,⁵⁶ alkaline protease,⁵⁷ exoenzyme S,⁵⁸ phospholipase C,⁵⁹ hemolysin,⁶⁰ and elastase⁶¹ have all been implicated as players in the pathogenesis of Pseudomonas infections. Whereas endotoxins are part of the cellular wall, exotoxins (also known as cytolysins) are extracellular enzymes and are easily separated from the envelope. Most of the cytolysins that have been isolated and purified are made up of two components. One part binds to the target cell and allows the other part, the enzyme, to pass through the cell membrane. After preparing a specific combination of amino acids and a metal-chelating moiety as an inhibitor to Pseudomonas elastase (a zinc metalloendopeptidase), Kessler and associates⁶¹ demonstrated that intrastromal injection of the inhibitor first, followed by elastase, prevented corneal melting. Further, subconjunctival injections of the elastase and inhibitor only delayed corneal melting, suggesting that the reversal of corneal melt by the elastase is better carried intrastromally. Purified P. aeruginosa hemolysin injected into the corneas of rabbits was found to induce an extensive leukocytic invasion of the corneal stroma.⁶⁰ In Moraxella angular conjunctivitis, the pathogenesis for lid maceration is caused by proteases from inflammatory cells rather than proteases elaborated by Moraxella.⁶² Both encapsulated and untypeable isolates of H. influenzae produce an IgA protease that degrades the protective secretory IgA elaborated by host mucosal surfaces.^63,64

Adherence of bacteria to the corneal epithelium is a prerequisite for keratitis. Both P. aeruginosa and Staphylococcus aureus were found to bind to rabbit corneal epithelial cells in vitro. P. aeruginosa bound, in multiple layers, to the periphery of cells grown on glass slides, whereas S. aureus bound more randomly to the cell surface. E. coli did not bind significantly to those cells. The peripheral location of Pseudomonas binding is probably caused by its affinity to macromolecules of the cell surface involved in cell-cell interaction.⁶⁵ Panjwani and coworkers⁶⁶ also demonstrated that P. aeruginosa binds to rabbit corneal neutral glycosphingolipids. They later demonstrated that P. aeruginosa also binds to specific phospholipids (phosphatidylinositol and phosphatidylserine) extracted from rabbit corneal epithelium.⁶⁷ These two molecules in ocular mucus or at the corneal surface may function as bacterial receptors and allow specific host-bacterium interaction and initial colonization.