**6. Role of complement and proinflammatory cytokines in endophthalmitis**

IL-1 initially mediates the acute-phase response, inducing other inflammatory mediators such as prostaglandins, phospholipase A2, collagenases and other proinflammatory cytokines (IL-6 and tumour necrosis factor alpha [TNF-α]). IL-1 induces the breakdown of the blood–retina barrier and leukocyte recruitment into the intra-ocular tissue [10]. IL-6 induces production of acute phase proteins such as C-reactive protein and fibrinogen by the liver and promotes Band T-cell differentiation [46]. In the eye, IL-6 plays a local role in negative feedback on IL-1 and TNF-α production. TNF-α also provokes an intra-ocular inflammatory reaction and acts synergistically with IL-1. IL-8 promotes the recruitment of neutrophils, and because dense neutrophil infiltration is a characteristic feature of endophthalmitis, its involvement in intraocular infection is probable but has not yet been determined [10].

## **7. Brief overview of pathogenesis**

During bacterial growth, toxin production by virulent organisms results in loss of retinal function. Cell envelopes, fragments of peptidoglycan, and teichoic acid or lipopolysaccharides are released in intra-ocular spaces during intra-ocular growth or antibiotic killing. These components may come in contact with resident immune cells and stimulate them to produce pro-inflammatory cytokines or other immune mediators which initiate a cascade of inflam‐ matory events, including increased permeability of the blood–ocular fluid barrier, with influx of additional soluble mediators and recruitment of phagocytic inflammatory cells to the site of infection. Inflammatory cells may in turn produce more inflammatory cytokines, in addition to toxic enzymes and reactive oxygen species. During the later stages of protracted endoph‐ thalmitis, lymphocytes migrate into inflamed intra-ocular tissues, and an immunoglobulin response results as shown in Figure 5. The ultimate result is the disruption of retinal architec‐ ture and death of non-regenerating retinal photoreceptor cells and a significant intra-ocular inflammatory response which can exacerbate the harmful effects of bacterial growth and toxin production by causing bystander damage [10].

**Figure 5.** Brief overview of pathogenesis of bacterial endophthalmitis

#### **7.1. Diagnosis of bacterial endophthalmitis**

#### *7.1.1. Laboratory diagnosis*

[30]. *E. faecalis* strains frequently harbour conjugative plasmids that encode a cytolysin which effectively lyses both eukaryotic and prokaryotic cells [44]. Cytolysin causes destructive changes in retinal architecture and vitreal structures. Adhesin, aggregation substance, produced by enterococci is a virulence-enhancing factor and helps them to attach to membra‐ nous vitreous structures. *S. aureus* secretes cell wall-associated products and adhesions (e.g. clumping factor, fibronectin-binding protein and protein A) and extracellular virulence factors (e.g. toxins such as alpha-toxin, beta-toxin, gamma-toxin and leukocidin, proteases and lipases) which are responsible for high virulence of this organism in endophthalmitis. These virulence factors are controlled by quorum-sensing systems namely, *agr* (accessory gene regulator) and *sar* (staphylococcal accessory regulator) [45]. Hence, therapeutics designed to inactivate global regulation of *S. aureus* during the early stages of infection may be more

**6. Role of complement and proinflammatory cytokines in endophthalmitis**

IL-1 initially mediates the acute-phase response, inducing other inflammatory mediators such as prostaglandins, phospholipase A2, collagenases and other proinflammatory cytokines (IL-6 and tumour necrosis factor alpha [TNF-α]). IL-1 induces the breakdown of the blood–retina barrier and leukocyte recruitment into the intra-ocular tissue [10]. IL-6 induces production of acute phase proteins such as C-reactive protein and fibrinogen by the liver and promotes Band T-cell differentiation [46]. In the eye, IL-6 plays a local role in negative feedback on IL-1 and TNF-α production. TNF-α also provokes an intra-ocular inflammatory reaction and acts synergistically with IL-1. IL-8 promotes the recruitment of neutrophils, and because dense neutrophil infiltration is a characteristic feature of endophthalmitis, its involvement in intra-

During bacterial growth, toxin production by virulent organisms results in loss of retinal function. Cell envelopes, fragments of peptidoglycan, and teichoic acid or lipopolysaccharides are released in intra-ocular spaces during intra-ocular growth or antibiotic killing. These components may come in contact with resident immune cells and stimulate them to produce pro-inflammatory cytokines or other immune mediators which initiate a cascade of inflam‐ matory events, including increased permeability of the blood–ocular fluid barrier, with influx of additional soluble mediators and recruitment of phagocytic inflammatory cells to the site of infection. Inflammatory cells may in turn produce more inflammatory cytokines, in addition to toxic enzymes and reactive oxygen species. During the later stages of protracted endoph‐ thalmitis, lymphocytes migrate into inflamed intra-ocular tissues, and an immunoglobulin response results as shown in Figure 5. The ultimate result is the disruption of retinal architec‐ ture and death of non-regenerating retinal photoreceptor cells and a significant intra-ocular inflammatory response which can exacerbate the harmful effects of bacterial growth and toxin

effective in arresting tissue damage than targeting individual toxins.

ocular infection is probable but has not yet been determined [10].

**7. Brief overview of pathogenesis**

30 Advances in Common Eye Infections

production by causing bystander damage [10].

The clinical diagnosis of endophthalmitis is confirmed by obtaining intra-ocular specimens like aqueous and vitreous specimen [47]. The possibility of isolating a microorganism from the vitreous specimen is 56–70%, whereas it is 36–40% from the anterior chamber (AC) humour [48]. Culture and sensitivity studies on aqueous and vitreous samples are necessary to determine the type of organism and antibiotic sensitivity.[9, 10] If endogenous bacterial endophthalmitis is suspected, a systemic workup for the source of infection is required, with cultures of blood, sputum and urine. Anterior chamber tap can be done by introducing 30 gauge needle on a tuberculin syringe to anterior chamber through limbus to obtain a 0.1 ml sample under topical anesthesia. The vitreous specimen can be obtained either by vitreous tap, vitreous biopsy or by using an automated vitrectomy instrument. In vitreous tap, a 21-gauge needle on a tuberculin syringe is used to obtain 0.1–0.2 ml of vitreous sample under sub-Tenon block. Vitreous biopsy can be taken using a 23-gauge vitrectomy cutter. Direct inoculation of the intra-ocular fluid specimen onto specific culture media is especially important when limited specimens are obtained. Specimens obtained with automated vitrectomy instruments can be processed by two methods. Vitrectomy specimen is either passed through 0.45 mm filter paper that concentrates the microorganisms and particulate matter and filter paper is sectioned and distributed on the appropriate media or vitrectomy specimen is directly inoculated into standard blood culture bottle [49]. Specimens can be inoculated on 5% sheep blood agar for recovery of the most common bacterial and fungal isolates. Sabouraud dextrose agar is also inoculated for recovery of fungal isolates. Chocolate agar: can be used for the recovery of fastidious organisms (i.e. *Neisseria gonorrhoeae* and *Hemophilusinfluenzae*). Thioglycollate broth and anaerobic blood agar are useful for recovery of small numbers of aerobic or anaerobic (including *Propionibacterium acnes*) organisms from ocular fluids and tissues. Blood culture bottles contain specially prepared medium for the recovery of both aerobic and anaerobic bacteria and fungi and it can be directly inoculated by intra-ocular fluids. Immunologic and molecular genetic technologies enable rapid and specific identification of infectious agents. In culture negative cases, the additional use of polymerase chain reaction was reported to aid in the identification of the organism [49]. These real-time techniques have been used in both clinical and experimental settings, and their future use in this area appears promising [50, 51].

In the Endophthalmitis Vitrectomy Study (EVS), Gram stain result did not reveal any sub‐ groups in which vitrectomy had a beneficial value and therefore is not useful in making initial therapeutic decisions [26]. Also in EVS, there was no difference in the culture positivity rate and operative complications between samples obtained by tap and those obtained by vitrec‐ tomy [52].

#### *7.1.2. Imaging studies*

In B-scan ultrasound of the posterior pole, choroidal thickening and ultrasound echoes in the vitreous support the diagnosis of endophthalmitis. Retained lens material and associated retinal detachment are also visible. The ultrasound also provides a baseline prior to intra-ocular intervention and allows assessment of the posterior vitreous face and areas of possible traction [53]. In traumatic cases, a CT scan can be performed, which may show thickening of the sclera and uveal tissues associated with various degrees of increased density in the vitreous and periocular soft tissue structures. In endogenous cases, imaging modalities like two-dimensional echocardiography and chest x-ray can be done to rule out potential sources of infection.
