Filamentous gram-negative bacteria masquerading as actinomycetes in infectious endophthalmitis: a review of three cases.

BACKGROUND: To report microbiological diagnostic dilemma posed by observation of unusual morphology of bacteria in the vitreous sample of a series of three cases of bacterial endophthalmitis.

RESULTS: A non-comparative, descriptive case series is described. All three cases presented to the retina-vitreous clinic with a clinical diagnosis of acute endophthalmitis between January and April 2018. Two patients had a past history of cataract surgery, and one had antecedent trauma within 1-2 days of presentation. As per the institutional protocol, patients underwent pars plana vitrectomy with intraocular antibiotics (vancomycin and ceftazidime) and microbiological investigation of the vitreous sample. Microscopic visualization of the stained vitreous fluid revealed the presence of filamentous organisms suggestive of Actinomycetales. However, the culture showed growth of gram-negative bacilli (Pseudomonas aeruginosa, Klebsiella oxytoca, Morganella morganii) which were identified by ViTEK 2 compact system and biochemical tests. Though a combination antibiotic treatment of vancomycin and ceftazidime was given in all cases in view of the short history, the antibiotic susceptibility testing showed multi-drug resistance pattern in two out of three cases leading to unfavorable clinical outcome.

CONCLUSIONS: Gram-negative bacilli can develop abnormal morphology due to stress or sub-inhibitory antibiotic exposure, and it is important for ocular microbiologists and pathologists to be aware of this phenomenon to avoid misinterpretation that may lead to inappropriate treatment.