Antibacterial activity of a Tribolium castaneum defensin in an in vitro infection model of Streptococcus pneumoniae

Abstract

Streptococcus pneumoniae (S. pneumoniae) is the most common bacterial cause of communityacquired pneumonia. Increasing rates of antibiotic-resistant S. pneumoniae strains impair therapy and necessitate alternative treatment options. In this study, we analysed insect-derived antimicrobial peptides (AMPs) for antibacterial effects on S. pneumoniae in a human in vitro infection model. AMP effects on bacterial growth were examined by colony forming unit (CFU)-assays, and growth curve measurements. Furthermore, cytotoxicity to primary human macrophages was detected by measuring lactate-dehydrogenase release to the supernatant. One AMP (Defensin 1) was tested in a model of primary human monocyte-derived macrophages infected with S. pneumoniae strain D39 and a multi-resistant clinical isolate. Inflammatory reactions were characterised by qPCR and multiplex-ELISA. In total, the antibacterial effects of 23 AMPs were characterized. Only Tribolium castaneum Defensin 1 showed significant antibacterial effects against S. pneumoniae strain D39 and a multiresistant clinical isolate. During in vitro infection of primary human macrophages with S. pneumoniae D39, Defensin 1 displayed strong antibacterial effects, and consequently reduced bacteria-induced cytokine expression and release. In summary, Tribolium castaneum Defensin 1 showed profound antibacterial effectivity against Streptococcus pneumoniae D39 and a multi-resistant clinical isolate without unwanted cytotoxic or inflammatory side effects on human blood-derived macrophages.

Tribolium castaneum defensin 1 kills Moraxella catarrhalis in an in vitro infection model but does not harm commensal bacteria

“…Defensin 1 also inhibited the bacteria-induced secretion of key cytokines IL-1β, IL-10, IL-12p70 and IL-23 at both postinfection time points and both MOI values, as determined by multiplex ELISA (Figure 3). The sterile activation of BDMs by LPS was not significantly affected by the presence of defensin 1, as previously also reported for TNF-α [7].…”

“…All donors gave informed written consent (Ethics approval number: 161/17). Human BDMs were cultured as previously described [7]. Briefly, primary human monocytes were isolated from donor buffy coats by selection for CD14 + cells using CD14 Microbeads (Miltenyi Biotec, Bergisch Gladbach, Germany).…”

“…We therefore tested the ability of defensin 1 to induce inflammation in an in vitro infection assay with BDMs. Previous work has shown that defensin 1 has minimal hemolytic activity up to a concentration of 100 µM and is nontoxic toward macrophages [7]. We exposed BDMs to defensin 1 at a concentration of 12.5 µM 1 or 5 h after infection with M. catarrhalis at MOI = 0.5 or MOI = 1 to mimic a clinical setting.…”

“…Their spectrum of activity includes viruses, bacteria, fungi and parasites. Insects provide a source of particularly diverse AMPs, which can be classified on the basis of their chemical attributes [7]. AMPs demonstrate a broad range of cellular mechanisms, from the promotion of angiogenesis to host cell chemotaxis, but electrostatic interaction with bacterial membranes and subsequent pore formation is the most important mechanism of direct bactericidal action [8].…”

“…AMPs demonstrate a broad range of cellular mechanisms, from the promotion of angiogenesis to host cell chemotaxis, but electrostatic interaction with bacterial membranes and subsequent pore formation is the most important mechanism of direct bactericidal action [8]. In this regard, we previously described the effect of defensin 1 from the red flour beetle (Tribolium castaneum) against Streptococcus pneumoniae [7].…”