Filamentous Bacteria Resembling Fungal Hyphae: A Diagnostic Pitfall

Abstract

Abstract Introduction/Objective When exposed to antibiotics or a host immune response, Gram-negative bacilli grow with inhibited septation producing long continuous cells; a phenomenon known as filamentation. Methods/Case Report We present a case of a 68-year-old man with quadriplegia, which was caused by the right vertebral artery dissection and subarachnoid hemorrhage requiring an emergent posterior inferior cerebellar artery direct bypass surgery. Lung lower lobe infiltrates were noted 3 days after the surgery. A bronchial wash culture was negative for growth, and a sputum specimen grew Serratia marcescens. The patient was treated with piperacillin/tazobactam, vancomycin, cefepime, cefazolin. After the surgery, he suddenly developed severe cerebral vasospasm of the posterior circulation with cerebellar strokes requiring emergent posterior fossa decompression. He remained on ventricular drainage by external ventricular drainage. After two days, the patient's cerebrospinal fluid (CSF) was sent for evaluation, on which a high segmented neutrophil count of 35% and Gram-negative filamentous organisms with occasional swellings were noted. Multi-drug resistant Serratia marcescens was isolated from the CSF specimen four days later. Neurosurgical procedures were reported to be highly associated with S. marcescens meningitis. The filamentous organisms noted on Gram-stained direct smear most likely represented Gram-negative S. marcescens with filamentation due to administration of sub-inhibitory concentrations of antibiotics. Results (if a Case Study enter NA) NA. Conclusion Gram-negative bacilli can develop abnormal morphology including elongation, swelling, and filamentation due to stress or sub-inhibitory antibiotic exposure, and it is important for microbiologists and pathologists to be aware of this phenomenon to avoid misinterpretation that may lead to inappropriate treatment.