Chapter 57
Anaerobic Bacteria
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The eye and ocular adnexa are prone to colonization by anaerobic bacteria that are also part of the flora of periorbital skin, paranasal sinuses, and the oral cavity. Once thought harmless commensals, anaerobes are now implicated in many human eye infections. Advances in microbiologic techniques enable the recovery and identification of obligately anaerobic bacteria from clinical specimens.

The prevalence and incidence of anaerobic infections remain unknown because of variations in culture technique and in criteria used to assess infection. Strict methodologic procedures are required to prevent exposure of anaerobes to atmospheric oxygen. Fortunately, most pathogenic anaerobes are relatively aerotolerant and may survive in culture despite periods of oxygen exposure.

Extensive tissue necrosis, gas in tissue, and foul-smelling discharge are clues to anaerobic infections of soft tissues, but clinical signs of anaerobic ocular infection usually are nonspecific. Some cases of anaerobic ocular infection probably are missed because appropriate laboratory studies are not performed. Many ocular infections involve aerobic, facultative, and anaerobic organisms,1,2 especially when bacteria originate from the patient's indigenous microflora. The polymicrobial composition of anaerobic infections complicates the interpretation of laboratory findings, especially in the presence of other known highly pathogenic microorganisms.

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Anaerobes reported to infect the human eye are listed in Table 1. The taxonomy and classification of anaerobic bacteria continue to undergo revision. The genus Bacteroides now includes the bile-resistant species that were included previously in the B. fragilis group. Saccharolytic species that were formerly in the genus Bacteroides are now included in the genus Prevotella, whereas asaccharolytic or weakly saccharolytic species are now classified as Porphyromonas. Both genera include pigmented and nonpigmented species.3


TABLE ONE. Anaerobic Bacteria of Ocular Importance

Gram-positive rods, spore-formingClostridium bifermentans
 Clostridium perfringens
 Clostridium septicum
Gram-positive rods, nonspore-formingActinomyces spp
 Bifidobacterium spp
 Eubacterium spp
 Propionibacterium propionicum
 Propionibacterium acnes
Gram-positive cocciPeptococcus niger
 Peptostreptococcus spp
Gram-negative rodsBacteroides spp
 Butyrivibrio spp
 Fusobacterium nucleatum
 Porphyromonas spp
 Prevotella melaninogenica
 Prevotella intermedia
Gram-negative cocciVeillonella spp


The genus Peptococcus now contains only one species, P. niger, which is rarely recovered from clinical specimens. Several species from the genus Peptococcus were transferred to the genus Peptostreptococcus when DNA analysis revealed similar G+ C content. Peptostreptococci appear to be closely related to Clostridium. Arachnia propionica, an anaerobic gram-positive nonspore-forming rod, has been reclassified as Propionibacterium propionicum.4

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Clinical studies have shown that anaerobic bacteria are isolated readily from the external ocular structures of healthy individuals. Up to 80% of normal adults harbor anaerobes in their conjunctival sac or on their lid margins.5–8 The relative distribution and composition of the anaerobic bacterial flora of the external eye vary with the geographic location of the study, the population examined, and the age of the individuals.8

Propionibacterium acnes is one of the predominant organisms residing on the ocular surface and has been isolated from approximately 50% of normal eyes.5–7 P. acnes may enter the eye during cataract and other intraocular surgical procedures and has been found to persist inside the eye for up to 3 years.9,10 Other propionibacteria (P. avidum and P. granulosum), P. niger, peptostreptococci, and occasionally Bacteroides species also are indigenous to the external ocular surfaces.5–7,11 The frequency of Bacteroides may be lower than that suggested by the ophthalmic literature since many studies were performed before the reclassification of several Bacteroides species as Prevotella or Porphyromonas.

Immune status, antiobiotic exposure, and environment contribute to anaerobic colonization of the conjunctivae. In a study comparing the conjunctival flora of human immunodeficiency virus (HIV)-infected patients and healthy individuals, a slightly higher percentage of cultures from patients with acquired immunodeficiency syndrome (AIDS) yielded anaerobes.12 Different organisms were recovered from persons with AIDS, including Actinomyces and several species of Clostridium. However, all patients with AIDS were hospitalized for secondary infections at the time of the study, and antimicrobial treatment probably was an important factor in the altered anaerobic flora.

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Any ocular structure is vulnerable to anaerobic bacterial infection. The exogenous route of infection is most common, whether from contaminants associated with accidental injury or from organisms of the normal ocular flora. Blood-borne endogenous ocular infection is uncommon but has occurred. Spread from adjacent structures to the eye and ocular adnexa is rare but may be involved in some cases of orbital disease associated with sinusitis.

Systemic anaerobic infections and infections of the eye and ocular adnexa share common features with regard to predisposition to infection. Vascular injury, compression, and tissue edema from trauma, elective surgery, or infection with aerobic or facultative bacteria are the primary factors predisposing to anaerobic infection (Table 2). These processes favor the growth of anaerobic bacteria by decreasing vascular permeability, lowering the local redox potential, and providing a portal of entry for these relatively noninvasive bacteria.13,14


TABLE TWO. Predisposing Factors to Anaerobic Ocular Infection

  Tissue necrosis from trauma, elective surgery, or infection
  Anoxia of tissues from vascular injury or tissue edema
  Altered ocular structures from previous ocular disease
  Retained foreign bodies
  Underlying illness
  Previous antibiotic or corticosteroid use
  Soft contact lens wear


Altered structure of ocular tissues by previous infection or underlying illness (e.g., malignancy, diabetes mellitus, or Sjögren's syndrome) can decrease the host's resistance to infection. The use of corticosteroids or antibiotics can predispose to anaerobic infection by impairing the normal inflammatory and immune responses.

The role of biofilm on contact lenses, intraocular lenses, and other biomaterials also may play a role in anaerobic ocular infection. In one series of contact lens-related keratitis, one third of confirmed anaerobic ocular infections was associated with soft contact lens wear.15

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Anaerobic bacteria produce several virulence factors that contribute to the pathogenesis of infection. Structures mediating attachment and adhesion, the production of hydrolytic enzymes and proteases, and the ability to evade the immune system are important virulence factors.16 In addition to various predisposing host factors, other important aspects of pathogenesis are the polymicrobial nature of most anaerobic infections and the resulting synergistic interactions among multiple bacterial species.


Anaerobic cocci occasionally are isolated from ocular infections.17 Peptostreptococci are causes of keratitis18 and endophthalmitis. Although these bacteria, as a group, are metabolically inactive in vitro,19 enzyme production and capsule formation are variable. The production of gelatinase, collagenase, and hyaluronidase by some species of Peptostreptococcus has been reported.20

Clostridium perfringens can cause conjunctivitis,21 eyelid and orbital infections,22 necrotizing keratitis,23 and panophthalmitis in humans.24–27 Intraocular infection is fulminant, with retinal necrosis and destruction. Other clostridial species (e.g., C. bifermentans and C. septicum) also have been rarely encountered.28,29 Clostridial ocular infections have occurred in dogs, horses, and other animals.30 The only extracellular enzyme directly associated with invasive anaerobic infection is the α-toxin (phospholipase C) of C. perfringens.31,32 Other toxins include collagenase33 and hyaluronidase.

P. acnes is the anaerobe isolated most frequently from ocular infections.6,34,35 Although this anaerobe releases a chemoattractant for leukocytes, P. acnes resists killing by neutrophils, inhibits the generation of specific suppressor T cells, and can persist within macrophages. P. acnes also can act as a nonspecific stimulator of the immune system.

P. acnes is implicated in several forms of ocular infection, ranging from chronic inflammation to acute suppuration and necrosis. P. acnes may contribute to the pathogenesis of rosacea and other forms of Meibomian gland dysfunction and blepharitis. P. acnes also has been isolated from rare cases of acute suppurative keratitis.36 Corneal infection due to this anaerobe has occurred in association with contact lens wear, corneal trauma, recurrent corneal erosion, herpetic keratitis, and other predisposing conditions.

P. acnes endophthalmitis is characterized by its delayed onset and often indolent course.37–56 Most infections follow cataract surgery, and its features mimic phacoantigenic or phacoanaphylactic uveitis. Endogenous P. acnes endophthalmitis also has occurred.57 The mechanism of pathogenesis is hypothesized to involve an interaction of residual lens cortex with P. acnes, resulting in hypersensitivity to lens protein41 and postoperative inflammation. Recent evidence indicates that Actinomyces also can produce a delayed-onset, chronic, postoperative endophthalmitis that simulates P. acnes infection.58

Infections of the lacrimal drainage system can be caused by a variety of bacteria. Among the anaerobes, Propionibacterium species probably are the most prevalent isolates.59–61 Although most canalicular infections have been attributed to Actinomyces israelii,62–66 it is possible that a number of these isolates should be reclassified as P. propionicum.66 Fusobacteria67,68 and other gram-negative anaerobes60,61 also are encountered, although uncommonly, in cases of dacryocystitis.


Prevotella melaninogenica and P. intermedia, formerly included in the genus Bacteroides, account for approximately 12% of the ocular infections caused by anaerobes.69 Isolates of B. fragilis and P. melaninogenica recovered from human infections have polysaccharide capsules (Kasper antigen), which, like the capsules of pneumococci, markedly suppress macrophage phagocytic activity.70–74 Metabolic byproducts of B. fragilis growth, such as succinic acid,75 ammonia,76 and other short-chain fatty acids (e.g., lactate, formate, fumarate), can inhibit phagocytic killing, reduce neutrophil migration and chemotactic responsiveness, and lyse mucosal cells.77

Although the lipopolysaccharide of Bacteroides, Porphyromonas, and Prevotella species is less toxic than that of Enterobacteriaceae,16 it still activates Hageman factor and initiates the intrinsic pathway of coagulation in vitro. B.78 fragilis and several species of Porphyromonas and Prevotella produce hyaluronidase, chondroitin sulfatase, heparinase, and several enzymes that hydrolyze carbohydrates. These enzymes cause tissue damage and also provide nutrients for the bacteria. Prevotella melaninogenica and P. intermedia also produce immunoglobulin A (IgA) protease.16

Fusobacteria elaborate a number of extracellular enzymes including a leukocidin, a collagenase, and a hemolysin. They possess the classic gram-negative endotoxic lipopolysaccharide that is biologically and chemically active. Butyric acid, a major byproduct of fusobacterial fermentation, adversely affects the functioning of neutrophils in vitro.79 The tissue damage in infections with F. nucleatum is believed to be associated with the production of amines by this species.16

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Unlike other animal models of microbial ocular infections, models of anaerobic infections have limited applicability because they fail to simulate human disease adequately. Most of the models are of endophthalmitis,80–83 although models of keratitis84 and conjunctivitis21 also have been developed. These models require abnormally large inocula to establish infection, result in uncharacteristically fulminant infection, and usually have a self-limiting course. In general, there has been insufficient effort to correlate the clinical signs of infection with either the presence of the inoculated organisms or the density of the microbial load. Because of the obvious difficulties in studying the complex interactions among multiple organisms, polymicrobial anaerobic infections have not been examined.
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Clues for the clinical diagnosis of anaerobic ocular infections are nonspecific, except possibly for eyelid crepitus and visualization of intraocular gas bubbles in clostridial infections. Proper specimen collection and laboratory investigations are necessary to isolate and to identify anaerobes.

Collecting the specimen from the infected eye requires preparation and knowledge of the likely spectrum of responsible micro-organisms.85 Because anaerobes rarely can be excluded, care should be taken to facilitate their recovery. Exudate and intraocular fluids should be aspirated with a needle and syringe, being careful to expel any air bubble and to seal the needle tip with a rubber stopper. If a swab is used to collect material, moistening the tip with sterile saline or a liquid medium minimizes trapped air pockets in the pledget. Material collected by scraping with a metallic instrument can be transferred to a moistened swab or inoculated directly onto anaerobic media under appropriate anaerobic conditions. If direct inoculation is impossible, the specimen should be transported immediately to the laboratory for processing.

Inoculation of thiol or thioglycolate broth provides the minimally accepted level for anaerobic culture. These liquid media contain free sulfhydryl groups that, in addition to binding free oxygen, can chelate and neutralize a number of antibiotics. Chopped meat broth with or without supplementation also may be used, but it may not be available readily in all clinical laboratories. The preferred method for culturing anaerobes involves the use of prereduced anaerobically sterilized (PRAS) tubed media,86 although the proper use of these media may prove technically difficult. The disadvantage of using thiol or thioglycolate as the only anaerobic medium is the inability to monitor contamination and to quantify growth. Aerobic or facultative bacteria also may outgrow the anaerobic organisms in broth at 35°C. Thus, a PRAS solid medium should ideally be included among the materials for laboratory investigation of many ocular infections.

Nonselective agar media (e.g., Brucella blood agar and Schaedler's blood agar) and selective plating media (e.g., anaerobe paromomycin-vancomycin blood agar and anaerobe phenylethyl alcohol blood agar) are recommended for the primary isolation of anaerobes.19 These media are nutritionally complete when supplemented with vitamin K and hemin. All culture media should be placed in an appropriate, closed anaerobic system that allows observation for at least 7 days. The slow growth of some anaerobes may account for delays in detectable growth on culture plates.87 Clinical suspicion of anaerobic infections warrants a minimal holding period of 2 weeks before plates are discarded as negative.

The identification of anaerobes was historically based on staining characteristics, cell morphology, biochemical reactivity, and gas-liquid chromatographic analysis of free fatty acids and alcohol profiles of metabolic end-products of fermentation. Most microbiology laboratories now rely on commercial rapid enzymatic identification systems (e.g., RapID-ANA II, AN-Ident,88 and BBL Crystal ANR89) or microbiochemical systems (e.g., API-20A87). DNA analysis or genetic sequencing can be used to definitively identify anaerobes; however, molecular methods usually are limited to special circumstances.

Susceptibility testing of anaerobes often is not routinely available for patient treatment. Methods include agar dilution, broth microdilution, and E test.90 Interpretation of the minimum inhibitory concentration (MIC) can be difficult, and MICs have not been widely applied to ocular disease.

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The treatment of anaerobic infections usually is empiric. In vitro susceptibility testing is unconventional, time-consuming, and difficult. Antibiotic sensitivity testing requires several days to perform because of the time needed for the recovery of anaerobes. Multiple analytic procedures may be needed in instances of polymicrobial recovery, and important therapeutic decisions may be required long before the results of these tests are available. As with other serious ocular infections, the clinician may need to design broad-spectrum antimicrobial coverage before the identification and sensitivity testing of individual isolates. Cumulative data on anaerobic susceptibility patterns may be helpful in making these decisions.

Penicillin G is active against many gram-positive and gram-negative anaerobes, excluding B. fragilis. Ampicillin may approximate the activity of penicillin G, but most beta-lactams show variable efficacy. Imipenem and the combination of a beta-lactam with a beta-lactamase inhibitor are potentially useful. Chloramphenicol and clindamycin phosphate are active against a variety of anaerobic bacteria, but frequent resistance among various genera occurs. Metronidazole is another broad-spectrum antibiotic used in nonocular anaerobic infections, but P. acnes generally is metronidazole resistant.91 Fluoroquinolones have poor activity in most anaerobic infections. All anaerobes are resistant to aminoglycosides.92

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