[Usefulness and limit of Gram staining smear examination]

Gram staining is one of the most simple and inexpensive methods for the rapid diagnosis of bacterial and fungal infections. It yields results much faster than culture, and provides important data for the patient's treatment and prognosis. However, a difference exists in the quality and quantity of information yielded by Gram staining smears based on the experience and knowledge of those conducting the tests. Therefore, a risk of misdiagnosis based on the information obtained from Gram staining smears is also present. The Gram staining conditions and morphology of bacteria sometimes change due to antimicrobial therapy. Species of Gram-negative rods sometimes become filamentous and pleomorphic. Gram-positive bacteria may become gram variable (change in staining condition) after antimicrobial therapy. Even bacteria that are easy to mis-identify exist, because the morphology of bacteria may be similar. Enterococcus faecalis is a Gram-positive diplococcus, forming Gram-positive clustered cocci in specimens from blood culture bottles, resembling Streptococcus pneumoniae. Acinetobacter baumannii is a Gram-negative diplococcus in sputum, resembling Moraxella (Branhamella) catarrhalis. Pasteurella multocida is a small-sized, Gram-negative short rod in the sputum, resembling Haemophilus influenzae. Prevotella intermedia is a small-sized, Gram-negative short rod in sputum, resembling Haemophilus influenzae. Capnocytophaga sp. is a Gram-negative fusiform (thin needle shape) rod present in clinical specimens, resembling Fusobacterium nucleatum.