Conclusion to be derived from in vitro data with respect to in vivo activity

Cefazolin and gentamicin were administered to mice with polymicrobial infection by Escherichia coli and Pseudomonas aeruginosa. Changes in the quantities of these bacteria in blood were investigated. Bacterial replacement was evident. Cefazolin was administered to mice with polymicrobial infection by E. coli and Bacteroides fragilis. Cefazolin was inactivated by B. fragilis-produced beta-lactamases. It was confirmed that the quantities of both B. fragilis and E. coli in blood increased. Even broad-spectrum antibiotics have different antibacterial activity against various bacteria. The present study suggested that an appropriate administration schedule should be determined to first inhibit the growth of a highly resistant bacterium in polymicrobial infection. It was also suggested that an antibiotic that can inhibit the growth of a more resistant bacterium should first be administered in polymicrobial infection. In vivo antibacterial activity of broad-spectrum antibiotics should be separately evaluated in mono-microbial infection and polymicrobial infection.