Endophthalmitis Caused by Rhizobium radiobacter

Rhizobium species, formerly known as Agrobacterium, are aerobic, oxidase-positive, catalase- positive, and gram-negative bacilli.^1-4 They are ubiquitous in soil and are primary plant pathogens with a worldwide distribution. Rhizobium radiobacter occasionally acts as an opportunistic pathogen with low virulence.

R radiobacter infection is strongly associated with the presence of foreign bodies, such as catheters, indwelling devices, and surgical apparatus. In one study, 86% of the cases were associated with indwelling foreign material.²R radiobacter-associated endophthalmitis has been reported in patients who have undergone cataract surgery.^5-7 The following is the fifth reported case of R radiobacter endophthalmitis.

Case report
A 62-year-old man with a medical history of type 2 diabetes mellitus but no other significant medical problems presented with complaints of decreased visual acuity, pain, and redness of the left eye. He had undergone cataract removal by phacoemulsification with posterior chamber intraocular lens implantation 3 months earlier. Approximately 3 weeks postoperatively, the patient noted onset of visual changes that he described as a film over his left eye with blurred vision. He was treated with prednisolone drops at a community ophthalmology clinic.

Symptoms stabilized until 4 days before presentation. The patient was afebrile. There was no report of trauma, evidence of postsurgical injury, or an obvious source of direct contamination to the eye. However, the patient had been performing activities such as mowing and yard work within days of his intraocular surgery.

At the time of presentation, his visual acuity was 20/25 in the right eye, but he was able to distinguish fingers only from a distance of 2 feet with the left eye. Intraocular pressure was 15 mm Hg in the right eye and 8 mm Hg in the left. Slitlamp examination of the left eye revealed moderate conjunctival injection and a hypopyon in the anterior chamber with posterior synechiae of the iris (Figure). Because of the hypopyon, the fundus could not be adequately visualized on dilated examination. B-scan ultrasonography verified an organized vitreous and no retinal detachment.

Figure -

Hypopyon and inflammatorydebris shown in the anteriorchamber of the left eye.

Vitreous biopsy and aspiration were performed before the administration of intravitreous vancomycin and ceftazidime. Empiric antibiotic therapy with topical ophthalmic fortified vancomycin and cefazolin was initiated. In addition to topical atropine 1% and oral ciprofloxacin, therapy with a lower dosage of ophthalmic prednisolone was maintained because of the presumed infectious nature of this endophthalmitis.

The vitreous specimen was processed by Gram staining and was inoculated onto standard media that included 5% sheep blood agar, phenylethyl alcohol, MacConkey agar, chocolate agar, and thioglycolate broth. No organisms were visualized on Gram stain. Three days after inoculation, the thioglycolate broth culture yielded a gram-negative rod that was identified as R radiobacter. After a 2-week course of systemic ciprofloxacin, vision in the patient's left eye improved to 20/30, with resolution of the hypopyon.

Discussion
Gram stain of R radiobacter reveals short, pleomorphic bacilli. The organism grows well on 5% sheep blood, chocolate, and MacConkey agars, as well as nutrient broths such as thiogycolate and brain-heart infusion. Detectable growth can be seen within 24 hours on routine laboratory media. On MacConkey agar, R radiobacter is a non-lactose fermenter. It oxidizes glucose, hydrolyzes esculin, and has variable nitrate reduction properties.⁸

Clinical isolates have demonstrated variations in antimicrobial susceptibility patterns supporting in vitro testing. Previous analyses reveal that most strains are susceptible to gentamicin but are resistant to tobramycin. ^2-5 Alnor and colleagues³ performed minimal inhibitory concentration testing on 16 strains of Rhizobium species and found that all were susceptible to ciprofloxacin and imipenem and were resistant to ceftazidime, piperacillin, and aztreonam. R radiobacter strains can produce an inducible cephalosporinase and aminoglycoside acetyltransferase. Some soil strains also have demonstrated production of monobactamases. ⁹ Several of the species contain a Ti plasmid with tumorigenic properties that produce crown gall and hairy root disease in plants.⁶

Human infections with Rhizobium are rare. R radiobacter appears to be the only species that causes human disease. An opportunistic pathogen with low virulence, the bacterium has been associated with rare clinical infection in immunocompromised persons, particularly in the setting of hematological malignancies, solidorgan neoplasms, and chronic kidney disease.^2,4

In 1980, Plotkin¹⁰ reported the first case of human disease attributed to R radiobacter in a patient with prosthetic valve endocarditis. More typical syndromes include bacteremia and peritonitis.^2,4 Most cases are as associated with the use of central venous or intraperitoneal catheters. Several cases of urinary tract infections have been reported in patients with nephrostomy tubes.²

Endophthalmitis following cataract surgery also has been reported. Miller and colleagues⁶ reported a case of postsurgical endophthalmitis due to R radiobacter infection that rapidly resolved with empiric intraocular corticosteroid, vancomycin, and gentamicin therapy without further surgical intervention.⁶ In a case presented by Pierre-Filho and colleagues,⁵ the intraocular implant was left in place and the patient was treated with intraocular ceftazidime, vancomycin, and dexamethasone and with systemic cefazolin, gentamicin, and prednisone. This intervention cleared the infection, but low visual acuity remained. In a case reported by Namdari and colleagues⁷ chronic R radiobacter endophthalmitis was cured with a 2-week course of ciprofloxacin and concomitant removal of the intraocular implant. To our knowledge, ours is the fifth case of R radiobacter endophthalmitis reported in the literature.

Ciprofloxacin has been suggested as the drug of choice for systemic infection. Indwelling foreign bodies, including catheters and lens implants, may serve as a source of chronic infection. Therefore, removal must be considered in the setting of persistent infections to achieve cure.

References:

• Kersters K, de Ley J. Genus III. Agrobacterium Conn 1942, 359. In: Krieg NR, Holt JG, eds. Bergey’s Manual of Systematic Bacteriology. Vol 1. Baltimore: Williams & Wilkins; 1984:244-254.

• Edmond MB, Riddler SA, Baxter CM, et al. Agrobacterium radiobacter: a recently recognized opportunistic pathogen. Clin Infect Dis. 1993;16:388-391.

• Alnor D, Frimodt-Moller N, Espersen F, Fredericksen W. Infections with the unusual human pathogens Agrobacterium species and Ochrobacrum anthropi. Clin Infect Dis. 1994;18:914-920.

• Lai CC, Teng LJ, Hsueh PR, et al. Clinical and microbiological characteristics of Rhizobium radiobacter infections. Clin Infect Dis. 2004;38:149- 153.

• Pierre-Filho Pde T, Ribeiro AP, Passos ED, et al. Endophthalmitis caused by Agrobacterium radiobacter. Scand J Infect Dis. 2003;35:410-411.

• Miller MJ, Novy C, Hiott M. Case of bacterial endophthalmitis caused by an Agrobacterium radiobacter-like organism. J Clin Microbiol. 1996; 34:3212-3213.

• Namdari H, Hamzavi S, Peairs RR. Rhizobium (Agrobacterium) radiobacter identified as a cause of chronic endophthalmitis subsequent to cataract extraction. J Clin Microbiol. 2003;41:3998-4000.

• Forbes BA, Sahm DF, Weissfeld AS, Bailey WR. Bailey & Scott’s Diagnostic Microbiology. 12th ed.St Louis: Elsevier Mosby; 2007.

• Sykes RB, Cimarusti CM, Bonner DP, et al. Monocyclic β-lactam antibiotics produced by bacteria. Nature. 1981;291:489-491.

• Plotkin GR. Agrobacterium radiobacter prosthetic valve endocarditis. Ann Intern Med. 1980;93:839-840.