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Increasing antibiotic resistance complicates therapy and increases the morbidity and mortality of bacterial infections. For this reason, there is a great need for new antimicrobial substances that are not affected by the resistance mechanisms of the germs. Therefore, antimicrobial peptides, which are part of the innate immunity of both prokaryotes and eukaryotes, have also become the focus of new investigations. In addition to the pathomechanisms of the bacteria, the body's own immune response also causes damage to the tissue through a prolonged inflammatory response.
The present study investigated 23 insect-derived antimicrobial peptides synthesised as analogues for their efficacy against pathogenic Streptococcus pneumoniae (antibiotic-sensitive and -resistant), Moraxella catarrhalis and the commensals Escherichia coli and Enterococcus faecalis in liquid culture and in the macrophage infection model as well as their influence on the release of the cytokines MCP1, MIP-1-α, IL-1ß, IL-6, IL-8, IL-10, IL-12p70, IL-23, LAP and TNF-α. Antibiotic-sensitive and -resistant Streptococcus pneumoniae were only inhibited by Defensin 1, while eleven of the insect peptides showed a dose-dependent growthinhibiting effect on Moraxella catarrhalis. In particular, Defensin 1 of Tribolium castaneum and Sarcotoxin 1C of Lucilia sericata significantly inhibited bacterial growth. Testing of the two peptides on the commensals Escherichia coli and Enterococcus faecalis revealed sensitivity of Escherichia coli to Sarcotoxin 1 C, whereas Enterococcus faecalis growth remained unaffected.
Defensin 1 had a small growth-reducing effect on Enterococcus faecalis but no effect on Escherichia coli. To investigate the cytokine response in an in vitro-model, macrophages differentiated from human blood were infected with different doses of Streptococcus or
Moraxella. As a simulation of clinical therapy, different concentrations of Defensin 1 were added to the infected macrophages after different incubation periods. The investigation of the expression of IL1-ß- and IL-8-mRNA by quantitative reverse transcription polymerase chain reaction revealed a significantly reduced bacterially induced expression after treatment with Defensin 1. These results were confirmed by a multiplex enzyme-linked immunosorbent assay. Defensin 1 resulted in significantly reduced release of the proinflammatory cytokines IL-1β, IL6, IL-8 and TNF-α after infection with Streptococcus pneumoniae, and of IL- 1ß, IL-10, IL-12p70 and IL-23 after infection with Moraxella catarrhalis. Investigations of Defensin 1 showed low cell toxicity to human macrophages and low haemolytic activity in porcine erythrocytes.
In summary, the results shown here underline the potential of the substance group of antimicrobial peptides in general as an alternative therapy option of bacterial infections. The knowledge gained in the in vitro studies can potentially be used to develop new specific drugs
in the fight against infectious diseases and the increasing antibiotic resistance worldwide.