Filamentous forms of Gram-negative bacteria on microscopy: Report of a case and review of literature.

Gram-negative bacteria (GNB) are known to undergo morphological changes like elongation, abrupt bulge formation at the site of potential cell division and bulge stagnation following exposure to β-lactam antibiotics. Positive blood cultures of a 78-year-old man on Gram staining showed a long filamentous GNB with spherical enlargement along some bacterial filaments. Cultures on solid media grew pan sensitive K. pneumoniae of normal morphology. A total of 16 cases of GNB showing filamentous or spheroplast forms have been reported in literature. Clinicians and microbiologists should be aware of potential Gram stain biases when analysing clinical samples taken under prolonged antibacterial treatment.