Contributor: Gordon K. Klintworth
Bacteria are a common cause of ocular infection, which may be restricted to specific parts of the eye: cornea [keratitis - bacterial], conjunctiva [conjunctivitis - bacterial], intraocular contents [endophthalmitis - bacterial] and eyelid [blepharitis - bacterial]. Bacterial infection is caused gram-positive bacteria [bacterial infection - gram-positive], gram-negative bacteria [bacterial infection - gram-negative], spirochetes [spirochetal infection], Mycobacterium [mycobacterium infection] and members of the Actinomycetales [actinomycetes infection]. Some bacteria produce a heterogeneous group of extremely potent exotoxins (Bacillus anthracis[anthrax],Clostridium botulinum[botulism], Corynebacterium diphtheriae[diphtheria],Pseudomonas),endotoxins (Escherichia coli, Klebsiella, Enterobacter, Pseudomonas, and Serratia) and/or enzymes (Pseudomonas) that account for many of the deleterious effects of the infections caused by them. Important pathogenic bacteria include Actinomyces israelii[actinomycosis], Bacillus anthracis[anthrax], Brucella abortus, Brucella melitensis, Brucella suis, Clostridium tetani [tetanus], Clostridium perfringens [gas gangrene], Corynebacterium diphtheriae [diphtheria], Enterobacter aerogenes, Enterobacter cloacae, Escherichia coli, Francisella tularensis, Haemophilus aegyptius, Haemophilus ducreyi, Haemophilus influenzae, Klebsiella pneumoniae,  Listeria monocytogenes [listeriosis], Mycobacterium avium, Mycobacterium tuberculosis hominis [tuberculosis], Neisseria gonorrhoeae [gonorrhea], Neisseria meningitidis [meningitis - meningococcal], Nocardia asteroides [nocardiosis]),  Pasturella multocida, Proteus, Pseudomonas aeruginosa, Salmonella, Shigella, Serratia marcesens, Staphylococcus aureus [staphylococcal infection], Streptococcus pneumoniae, Streptococcus pyogenes [streptococcal infection] and Treponema pallidum [syphilis].
The human flora rarely contains highly virulent bacteria. With minimally virulent bacteria the inflammatory response may be absent or insignificant (as in infectious pseudocrystalline keratopathy is frequently caused by the growth of minimally virulent bacteria, such as Streptococcus viridans, and usually occurs in eyes having had previous accidental or surgical trauma.
Several factors contribute to the host defense against bacterial infections. These include the mechanical barrier created by ocular structures, defensins (α-defensins and β-defensins), antibacterial compounds in the tears, as well as the inflammatory reaction(including  phagocytosis by neutrophils and the mononuclear phagocytic system) and immune responses. The effectiveness of the immune response depends mainly on the host's ability to damage the bacterial cell wall. Neutrophil phagocytosis of bacteria involves opsonization and is important in the defense against numerous bacteria incuding Streptococcus pneumoniae, Pseudomonas, Klebsiella, and certain strains of Staphylococcus aureus.
The pathogenicity and virulence of different bacteria varies considerably and depends to some extent upon the natural and acquired immune defense of the host. 
Diseases of the host that predispose to bacterial infection include those with impaired defense mechanisms. Such disorders may have defective neutrophils (Chédiak-Higashi syndrome, chronic granulomatous disease of childhood), an immunodeficiency state (such as Good syndrome), chronic granulomatous disease of childhood, Chédiak-Higashi syndrome, C3 deficiency, impaired opsonization [opsonization - impairment]  and corticosteroid therapy. Congenital disorders that predispose to bacterial infections include DiGeorge syndrome. Acquired defects in cell mediated immunity occur with a variety of diseases including AIDS, multiple myeloma, lymphomas, sarcoidosis, uremia, malnutrition, and diabetes mellitus. Certain systemic infections may also depress cell mediated immunity, particularly measles and tuberculosis. Abnormalities in cell mediated immune function are also found in elderly persons [aging], heroin addicts, and in patients on chemotherapy. Diseases that predispose to bacterial infection, especially those with defective cell-mediated immunity, may lead to an opportunistic infection, due to bacteria of low pathogenicity (such as Streptococcus viridans). As a rule the offending microorganisms differ in the different types of underlying diseases. Other factors that predispose to ocular infections include trauma and contact lens wear [contact lens - complications]. 
The factors that determine the pathogenicity of bacteria include their ability to adhere to different structures and their invasive properties. The adherence of some bacteria (such as Pseudomonas) to certain structures, such as the corneal stroma, is enhanced by components of the bacterial cell wall (such as lectins or pili  of Pseudomonas).  In persons with functionally abnormal leukocytes marginal keratitis [keratitis - marginal] and blepharoconjunctivitis are common, perhaps because of an impaired  control over normal eyelid flora. Inactive chorioretinal scars, which are thought to represent localized sequalae of previous blood-borne infection, are also often detected.
The risk of developing an opportunistic bacterial infection of the eye varies with the type and severity of the underlying disorder. Some bacterial infections, such as syphilis, Mycobacterium avium and tuberculosis may complicate AIDS. Mycobacterium avium is the most common bacterial infection in patients with AIDS. A focal endophthalmitis that responds to antibiotics has been observed in patients with AIDS. It is presumably due to blood-borne bacteria, but attempts to identify an offending microorganism has often been unsuccessful. 
Other factors that predipose to ocular infection are contact lenses [contact lens - complications]. The eye is constantly exposed to numerous bacteria, but relatively few microorganisms result in a corneal infection. Most bacteria cannot penetrate the corneal surface directly and only enter the tissue following accidental or surgical trauma. Some highly virulent bacteria (Neisseria gonorrhoeae [gonorrhea], Neisseria meningiditis [meningococcal infection], and Corynebacterium diphtheriae [diphtheria]) can invade an intact corneal epithelium. After disruption of the corneal epithelium, bacteria vary in their ability to adhere to the the underlying corneal stroma. Factors influencing this adherence include structural components of the cell wall of the pathogen (such as pili or fibriae of Pseudomonas). Gram-negative bacteria also cause ulcerative keratitis, but primary ocular infection rarely results in sepsis or endotoxic shock.