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Gram staining is one of the most crucial staining techniques in microbiology.

The use of Gram stain facilitates rapid use of appropriate antimicrobial agents. In bacterial pneumonia, the most useful sample which reflects the status of inflammation in the lung is supposed to be sputum from the infectious areas in the lung. The changing patterns of Gram-stained sputa can be used as the therapeutic marker of effectiveness of antimicrobial agents. When the first administered antimicrobial agent is effective against the target pathogen; S. pneumoniae, M. catarrhalis or Haemophilus influenzae, a decrease in number of the pathogen in the sputa was clear and almost no or little pathogen were seen in the sputum obtained several-hours after the first administration of antimicrobial agent or before the second administration, which is mostly administered 8-12 hours-after the first one, showing that Gram-stained sputum is a definite effective marker of the effectiveness of the agent. In pneumococcal pneumonia, a loss of gram-positive-staining of pneumococci was another early marker of the effectiveness of the agent. We can expect the effectiveness at least 1 h after completion of the first administration of the agent, when a loss of gram-positive-staining of pneumococci with a decrease in the number of cocci is found in the sputa. The reason for the loss of staining is supposed to be by reduction in peptidoglycan synthesis induced by antimicrobial agents distributed in the sputa. To find effective marker showing the effectiveness of administered antimicrobial agent in bacterial pneumonia or bronchitis, we compared the white blood cell count (WBC), serum C-reactive protein (CRP) level, and the decrease in the bacterial density in gram-stained sputa in which the administered antimicrobial agents were effective. The data showed that at least 2 to 4 days were needed to evaluate the effectiveness when the WBC or CRP level was used as a therapeutic marker, but the median duration needed to determine the effectiveness of the agent was 6.5 hours (range, 1 to 12 hours) in Gram-stained sputa, which showed that Gram-stained sputum after the first administration of antimicrobial agents can be used as the quickest therapeutic marker in treating bacterial respiratory infections. We showed that Gram staining of sputum is a useful and effective tool to check the effectiveness of administered antimicrobial agents in bacterial pneumonia and bronchitis.