Interaction during infection by neutrophil bactericide LL‐37 and pharmaceutical antibiotics

Abstract

The neutrophil (PMN) peptide LL‐37 possesses innate bactericidal activity although diminished bacterial killing by some pharmaceutical antibiotics is reported in the presence of high numbers of PMN. Our earlier studies showed reduced S. aureus penicillin binding protein (PBP) binding when S. aureus was exposed to non‐killing incubation with PMN. This study investigated whether inhibition of pharmaceutical antibiotic bactericidal activity by LL‐37 involves that PBP‐PMN interaction. Rats received intra‐abdominal perforated spheres that were infected with S. aureus after 5‐wk encapsulation. Treatment utilized commercial antibiotics with either PBP‐2 specificity (cefotaxime) or lacking PBP‐2 specificity (cephalexin). Both had similar killing profiles vs. the S. aureus. Control capsules received no commercial antibiotic but had high PMN counts. Abscess fluid was sampled on days 3, 5, and 7 when bacteria and PMN were counted and LL‐37 quantified by Western blot.Over the 7 day treatment time, cephalexin produced a 2.1 ± 1.7 log10 drop in bacterial cfu/mL from day 0, (p=0.029, t‐test). Cefotaxime exhibited only a 0.81 ± 2.5 log10 drop in bacterial cfu/ml from day 0 (p= NS). Day 7 abscess fluid cefotaxime concentration was 1.56 ± 0.99 μg/mL and cephalexin concentration was 1.45 ± 1.1 µg/ml. Both values exceeded the MIC of the infective microbe. Abscess fluid LL‐37 levels correlated negatively with S. aureus counts, showing intra‐capsule bactericide in the presence as well as absence of both antibiotics. These data suggest that the PMN "antibiotic" LL‐37, effective in bactericide in this rat model, appears to function by mechanisms unrelated to the PBP‐2 of the S. aureus cell wall.