Filamentous bacteria masquerading as fungi: a diagnostic pitfall in direct smear interpretation with report of two cases.

Bacteria, particularly Gram-negative bacilli, can develop abnormal morphology after the administration of subinhibitory concentrations of antibacterial agents. Filamentation is a common response in which bacteria replicate but incompletely divide, leading to long slender chains that resemble fungal hyphae. Pathologists are frequently consulted to examine direct smears of body fluids, which often contain microorganisms. Antibiotic-related filamentous morphology may resemble fungal hyphae and this potential misinterpretation can lead to inappropriate treatment for presumed fungal infections. Two cases are described in which direct smears of body fluids were examined by on-call pathology residents who misinterpreted filamentous bacteria as fungal organisms, with one case leading to the initiation of antifungal medication. Although well-established within the field of microbiology, many residents and practising pathologists are less familiar with antibiotic-related bacterial morphology, as it may not be routinely encountered. It is important for pathologists to be aware of this phenomenon in order to avoid misinterpretation.