The number of large studies describing the prevalence of Bacillus endophthalmitis is sparse. A decade ago, the incidence of Bacillus endophthalmitis was reported to be 4% in postoperative infection and 14.4% following trauma [19, 20]. The largest series till date, reported in 2001, included 31 cases of Bacillus endophthalmitis [21]. In this series, 90.3% cases were secondary to trauma. The commonest species across studies causing endophthalmitis has been Bacillus cereus. Other known species to cause endophthalmitis are Bacillus subtilis, Bacillus licheniformis, Bacillus laterosporus, and Bacillus macerans.

Bacillus species are known to produce several toxins during intraocular growth. These include phospholipases, enterotoxins, hemolysins, and proteases [22, 23]. These toxins set up an intense intraocular inflammation that causes extensive tissue necrosis. In addition to the toxins, Bacillus shows a unique behavior among all Gram-positive organisms. Once inoculated into the eye, the Bacillus rapidly migrates to all parts of the eye from the anterior chamber to the retina within 6 h [24]. This leads to a prolific spread of infection and clinical worsening in the first 6–12 h if not treated timely, often resulting in the loss of the eye.

As Bacillus is a Gram-positive organism, the first proposed combination for intraocular usage was clindamycin with gentamicin [25]. Vancomycin is as potent as clindamycin against Bacillus. Currently the commonly used drug for empiric management is intravitreal vancomycin. Bacillus is known to produce a β-lactamase enzyme, which makes it inherently resistant to penicillin group of antibiotics. In previously reported literature, the susceptibility of the Bacillus isolates to amikacin, gentamicin, and ciprofloxacin was over 90% [21]. We analyzed data of 86 eyes with Bacillus endophthalmitis [26]. In our study there was over 95% sensitivity to vancomycin, ciprofloxacin, and amikacin. Thus vancomycin continues to be the preferred intravitreal antibiotic for Bacillus endophthalmitis. Oral ciprofloxacin is the preferred antibiotic for systemic usage; the adult dose is 750 mg two times a day. This calculation is based on the fact that vitreous concentration of one 750 mg dose of ciprofloxacin exceeds the minimum inhibitory concentration for Bacillus [21]. The antibiotic sensitivity of two large series from the LV Prasad Eye Institute is shown in the Table 20.1.

Though the role of early vitrectomy has not been studied in humans, studies in a rabbit model have suggested a definite role of early vitrectomy [27]. In this model, it was shown that vitrectomy was effective in preserving significant retinal function if the surgery was initiated in the first 4 h following endophthalmitis. Though the results may not be extrapolated accurately to humans, it underlines the importance of prompt surgical management in cases of Bacillus endophthalmitis.

Accurate and immediate identification of the infecting organism causing endophthalmitis is important for appropriate management. Standard microbiologic diagnostic methods include smear preparation for Gram stain and growth on selected culture media. Most Bacillus spp. grow readily on nutrient agar or peptone media. Growth is sometimes improved by glucose, but not by blood or serum. The optimum temperature for growth varies from 25 to 37 °C. The commonly used media are chocolate agar, 5% sheep blood agar, brain heart infusion broth, and thioglycollate broth. In the vegetative form, the bacilli are killed in 1 h by moist heat at a temperature of 55 °C. The spores of B. subtilis may withstand boiling for hours.

Identification is confirmed by Gram stain as described above (large Gram-positive to Gram variable rods, arranged in pairs or chains with rounded or square ends). In the B. cereus group, colonies grown on culture media appear flat and irregular and are 2–5 mm in diameter with a gray/white color and ground-glass appearance on blood agar with β hemolysis. Colonies show a tenacity that allows them to be pulled up and stay upright on teasing with a loop. In the B. subtilis group, colonies are large (2–7 mm) with a frosted-glass appearance but may become opaque on blood agar with β hemolysis. Variable colonial morphology is seen as some species may produce mucoid or smooth or raised wrinkly colonies.

Species differentiation of the genus is complex and, in some instances in a routine laboratory, a combination of Gram stain and colonial appearance, motility, and biochemical identification using commercial automated microbial identification system, like API kits or ViTEK (bioMérieux), machine using BCL cards [28]. Identifications are made after 14 h incubation, and the database allows for identification of up to 46 species. Recently, matrix-assisted laser desorption ionization–time-of-flight mass spectrometry (MALDI-TOF MS) is being applied for identification of species, and this is based on the protein composition of a bacterial cell. This technique is also has also been found to be a good alternative to 16S rRNA sequencing and even a more powerful tool in the accurate classification of Bacillus species, especially for differentiating B. subtilis and B. cereus from Bacillus amyloliquefaciens and Bacillus thuringiensis, respectively [29]. However, further studies are still required to test this technology with a large collection of Bacillus of diverse origins. A variety of other rapid identification methods have been developed for isolates from clinical samples which include molecular techniques such as pulsed field gel electrophoresis (PFGE), multilocus sequence typing (MLST), and 16S rRNA gene sequencing [30]. While all of these approaches enable subtyping of strains, they remain accessible to reference laboratories only and are difficult to implement for routine bacterial identification in a clinical laboratory.