**4. Etiology**

Multiple microorganisms may cause APIE [3, 4]. Bacterial pathogens are the most common [4]. Gram-positive cocci are responsible for 65–80% of APIE cases, mainly *Staphylococcus spp.* [12, 13].

Staphylococci belong to the *Micrococcaceae* family and have a diameter of between 0.2 and 12 microns [3, 14, 15]. The most common staphylococci species that cause endophthalmitis are coagulase-negative Staphylococci and *Staphylococcus aureus*.

Among all Staphylococci, *Staphylococcus epidermidis*, a coagulase-negative staphylococci, has emerged as the main cause of APIE [4, 13]. These bacteria have the property of producing an exopolysaccharide, which can be a factor that hampers phagocytosis and induces antibiotic resistance, including methicillin and betalactam antibiotics. However, these microorganisms are almost always susceptible to vancomycin [3].

*Staphylococcus aureus* is a non-spore-forming facultative aerobic microorganism that colonizes human skin. It produces different enzymes such as catalase, coagulase, beta-lactamase, many of which, are related to its pathogenicity [14, 15]. *Staphylococcus aureus* is the second most common bacteria isolated in cases of APIE [3, 13].

Other causes of APIE include Streptococci, Gram-positive bacilli, Gramnegative cocci, and Gram-negative bacilli [4].

Streptococci are facultative Gram-positive, aerobic microorganisms or obligate anaerobes and produce various toxins that increase their virulence. They are also sensitive to vancomycin [3].

Gram-positive bacilli causative agents of endophthalmitis include bacteria from the Bacillus genus. The most common intraocular Gram-positive bacilli pathogen is *Bacillus cereus* [3, 4]. Bacillus is a spore-forming rod that is Gram-positive or Gramvariable. It produces extracellular products, including toxins that induce severe inflammation when injected into the eye. Bacillus infection risk factors include foreign bodies, immunosuppression from malignant tumors, corticosteroid use, penetrating and perforating trauma, as well as acquired immunodeficiency syndrome. The infection is quite virulent and may significantly damage the eye in less than 24 hours. Systemically, it may induce fever and leukocytosis [3]. Vancomycin is the first-line drug used against *Bacillus spp.*

The genus *Pseudomonas* are Gram-negative, strictly aerobic organisms found in soil and water. They are part of the normal human flora but are predominantly isolated in cases of nosocomial infection [3]. *Pseudomonas aeruginosa* is the most common Gram-negative bacteria causing APIE, but other species have also been isolated [3, 16]. The pathogenesis of the infectious disease caused by *Pseudomonas* includes the production of extracellular enzymes and other toxic proteins and hemolysin, endotoxin, and exotoxin A, which explain the fulminant and severe nature of its clinical presentation [3]. *Pseudomonas spp.* are usually sensitive to aminoglycosides and ceftazidime [3].

Other bacteria members of the *Enterobacteriaceae* family may cause APIE. *Enterobacteriaceae* is a group of Gram-negative, facultative aerobes. They are distributed in the soil and plants, and colonize the human and animals' gastrointestinal tract [3].


**Table 1.**

*Most frequently isolated microorganisms in different types of eye surgery [10, 12].*

Fungal endophthalmitis is an infrequent cause of APIE; however, it should be considered as a possible pathogen [4]. *Candida, Aspergillus spp, Histoplasma*, and *Blastomyces dermatitidis* are some of the fungal microorganisms that may cause APIE.

*Candida albicans* is frequently found as part of the normal flora on the mucosal surfaces, and is the most common cause of fungal APIE, followed by *Aspergillus spp.* Patients that become immunocompromised by debilitating conditions such as AIDS, and other malignancies may also carry a higher risk for developing *Candida* APIE **Table 1** [4]. Summarizes the most frequently isolated microorganisms in different types of eye surgery.

### **5. Diagnosis**

#### **5.1 Signs and symptoms**

The diagnosis of APIE is eminently clinical at onset. Intraocular surgery, mainly cataract surgery, is usually a painless procedure in the vast majority of cases [4, 12]. The sudden appearance of red eye, pain, and blurred vision as symptoms in the early postoperative period of patients that have undergone any intraocular surgery should always alert the surgeon to the possibility of APIE, although it is considered a rare but feared and devastating postoperative complication [3].

Common signs that may occur at onset are palpebral erythema and edema, ciliary injection, and conjunctival chemosis, corneal edema, hypopyon, anterior chamber cells, and vitreous haze due to vitritis (**Figures 1** and **2**). It is essential to mention that most endophthalmitis cases present between the third and tenth postoperative day, and 88% of the cases occur within six weeks after surgery (**Table 2**) [3, 4].

Fundus evaluation should be performed to determine the vitreous clarity, and to establish the status of the retina and the optic nerve. If fundus visualization is not feasible, linear B-ultrasound examination is mandatory to evaluate the posterior segment and rule out vitreous hemorrhage, retained lens material, retinal detachment, choroidal thickening, or the presence of membranes [4, 17].

#### **5.2 Microbiology diagnosis**

Samples obtained from anterior-chamber aspiration and vitreous needle biopsy or pars plana vitrectomy (PPV) should be process for smear and cultured *Acute Postoperative Infectious Endophthalmitis: Advances in Diagnosis and Treatment DOI: http://dx.doi.org/10.5772/intechopen.97545*

#### **Figure 1.**

*Clinical image of a case of an acute postoperative infectious endophthalmitis caused by* Pseudomonas aeruginosa, *showing prominent ciliary injection and conjunctival chemosis, a 3 mm hypopyon, and marked anterior chamber inflammatory infiltrate that obstructs visualizing the anterior segment details.*

#### **Figure 2.**

*Clinical image of a case of an infectious endophthalmitis 3 days after phacoemulsification surgery caused by*  Staphylococcus epidermidis*. The presence of hypopyon, corneal folds and edema, ciliary injection and cloudy media are observed.*

separately. The sample obtained from the vitreous should be undiluted and taken directly from the vitrectomy line. This has the potential advantage of having an adequate amount of bacterial load to grow in the culture plates, thus increasing the sensitivity of the culture to identify the possible APIE causative microorganism.

Alternatively, cassette washings from PPV should be concentrated by a centrifuge before culture and staining [3, 4]. Samples are placed on glass slides and stained using Gram and Giemsa stains. Obtained samples are plated on blood, thioglycolate, chocolate, and Saboraud agars and cultured under both, anaerobic and aerobic conditions. Whenever possible, it is advisable to place the obtained samples directly on agar plates in the operating room for better yield. They should be at room temperature by the time they are used, avoiding using them at refrigeration temperature because microbial growth might be reduced. Care should be taken to avoid contamination while placing the samples on the plates or transport media.


#### **Table 2.**

*Percentage of presentation of common symptoms and signs in acute postoperative infectious endophthalmitis [4].*

The endophthalmitis vitrectomy study (EVS) study reported positive cultures from 69.3% of biopsied cases using traditional agar plates and broth culture methods [13].

These conventional microbiology methods are commonly used for laboratory identification and antibiotic sensitivity tests of pathogens in APIE cases. Disadvantages of culture, include a low sensitivity and specificity for bacterial detection in the aqueous and vitreous humor, and are time-consuming [3, 4]. Nonetheless, whenever a minimal suspicion of infectious endophthalmitis exists, smear and culture are mandatory. Vitreous sample for culture gets a better yield than aqueous humour.

Disadvantages of lack of a microbiological confirmation in cases of APIE include non-response to IAI, increased morbidity from prolonged infection, repeated biopsies and IAI, and the potential to require performing more surgeries [4].
