Effects of neutrophils on cefazolin activity and penicillin-binding proteins in Staphylococcus aureus abscesses.

Bacteria survive within abscesses despite antimicrobial therapy, usually necessitating drainage. Our previous work showed that bacterial killing is diminished within the neutrophils of animals with abscesses. To further assess the role of neutrophils in Staphylococcus aureus survival and the poor activities of -lactams in abscesses, tissue cage abscess-bearing rats were given polymorphonuclear leukocyte (PMN)-depleting antibody prior to and several times following inoculation of the tissue cages with S. aureus. Cefazolin (300 mg/kg of body weight/day) was administered to all animals in appropriately divided doses. After 7 days of antimicrobial therapy, the 17 animals that received anti-PMN serum had significantly fewer abscess neutrophils than the 18 controls and fewer abscess bacteria (5.55 versus 3.79 log10 CFU/ml [P 0.04]) than the 18 controls. The data were consistent with the premise that cefazolin is more effective in abscesses depleted of neutrophils. To investigate further, S. aureus was incubated with rat peritoneal neutrophils; and bacterial cell membrane proteins were isolated, labeled with biotinylated ampicillin, separated by electrophoresis, blotted onto nitrocellulose, and stained for biotin reactivity. PBP 2 expression was consistently and significantly decreased after a brief, nonkilling PMN exposure. These experiments showed that PMN depletion enhanced the activity of cefazolin in the abscess milieu. Furthermore, altered bacterial cell wall cefazolin targets may be the mechanism by which the PMN diminishes antimicrobial activity, suggesting the importance of the staphylococcus-PMN interaction in the outcome of established infections. The rapid delivery of neutrophils over the first 4 t o6ht o a site of infection is known to play an important role in controlling an acute infection (6). Once the infection is established, however, the activities of antimicrobials in a suppurative environment are limited. Most abscesses require surgical drainage for cure. However, some abscesses respond to antimicrobial therapy without drainage (2), and the mechanisms by which antimicrobial activity is diminished in a suppurative environment are only partially understood. Although the diminished growth rate of bacteria in abscesses is likely important in affecting the reduced effectiveness of antimicrobials in a suppurative environment, we had previously been unable to correlate antimicrobial activity in an abscess with the antimicrobial activity against stationary-phase organisms (3). We found that -lactam antimicrobials had only marginal efficacies, despite adequate antimicrobial concentrations, and that the activities of antimicrobials in polymorphonuclear leukocytes (PMNs) correlated with in vivo efficacy. The PMN may act as a sanctuary since we have previously found that killing by PMNs, but not phagocytosis of staphylococci, (i) is inhibited systemically by the presence of an abscess and (ii) may be further inhibited in the abscess milieu (9). In this study, we sought to directly determine the role of the PMNs in diminishing the activities of -lactams in an abscess. We postulated that neutrophils, although important in the immediate control of pyogenic infections, may inhibit the activities of -lactams in an established abscess and that a reduction in the rate of PMN influx into an abscess could enhance the killing effects of -lactams. Previous studies have provided some evidence to support this hypothesis, but the question either was not studied directly or conflicting results were pre