Influence of human serum on bactericidal activity of ceftizoxime sodium

The influence of normal human serum on bactericidal activity of ceftizoxime sodium (CZX) was studied in vitro against Escherichia coli and Staphylococcus aureus. Subminimal inhibitory concentrations of CZX inhibited the growth of bacteria. Normal human serum as a source of complement and low doses of CZX, such as 1/20 or 1/10 the minimal inhibitory concentration, had a strong synergic action on the gram-negative bacteria E. coli, but not on the gram-positive bacteria S. aureus. However, the combination of CZX and fresh human serum had a stronger inhibitory effect on the growth of S. aureus than CZX alone. It is concluded that subminimal inhibitory concentrations of CZX inhibited the growth of bacteria in vitro in the presence of human serum, indicating that smaller dosages of CZX may act to eliminate the invading bacteria in vivo together with serum complement.