In vitro activity of cefazolin against recent clinical bacterial isolates.

The in vitro antibacterial activity of cefazolin was determined, by an agar dilution technique, against 546 recent isolates of bacteria recovered from patients at Boston City Hospital. The activity of cefazolin was comparable to that of cephalothin against most gram positive cocci, but the former was more active against most strains of E. coli and Klebsiella and against some isolates of most strains of E. coli and Klebsiella and against some isolates of Enterobacter. Neither of these two cephalosporins had significant activity against Serratia or Pseudomonas. The median MICs of undiluted cultures of some gram negative bacilli tested in agar were 32-85 times higher than those of the same cultures diluted 1:1,000 before inoculation. The results of these tests of susceptibility were influenced by the composition of the medium, sometimes by a factor of 16; none of the four media gave consistently higher or lower values with strains of all species tested.