Effect of zinc-reversible growth-inhibitory activity in human empyema fluid on antibiotic microbicidal activity

Abscess fluid supernatants have zinc-reversible microbial growth-inhibitory activity that is mediated by calprotectin, a zinc-binding protein. Because it inhibits microbial growth, this activity might interfere with killing by antibiotics that require their target organisms to be proliferating. In the present study, we cultured bacteria in human empyema fluid and used zinc to overcome the growth-inhibitory effect of calprotectin. We then compared the effect of zinc on killing by the beta-lactams ampicillin and cefazolin with that of the fluoroquinolone trovafloxacin, since the latter may be better able to kill nonproliferating organisms. In empyema fluid diluted 1:5 in normal saline, addition of zinc (30 microM) increased growth of two strains of Staphyloccocus aureus and two strains of Escherichia coli but did not affect the MICs or MBCs of the three antibiotics in Mueller-Hinton broth. For one strain of S. aureus, no effect of zinc was found on killing by either ampicillin or cefazolin. However, with the other strain of S. aureus and both strains of E. coli, significant enhancement of killing by both drugs was observed with zinc addition. On the other hand, no effect on the killing of any of the organisms was observed for trovafloxacin when zinc was added. These results suggest that the zinc-reversible growth-inhibitory activity of abscess fluid may interfere with the microbicidal activity of antibiotics requiring proliferating target organisms, although antibiotics better able to kill nonproliferating organisms may be less affected by this phenomenon.