Streptococcus pneumoniae (pneumococcus) resides asymptomatically in the nasopharynx but can progress from benign colonizer to lethal pulmonary or systemic pathogen. Both viral infection and aging are risk factors for serious pneumococcal infections. Previous work established a murine model that featured the movement of pneumococcus from the nasopharynx to the lung upon nasopharyngeal inoculation with influenza A virus (IAV) but did not fully recapitulate the severe disease associated with human co-infection. We built upon this model by first establishing pneumococcal nasopharyngeal colonization, then inoculating both the nasopharynx and lungs with IAV. In young (2 months) mice, co-infection triggered bacterial dispersal from the nasopharynx into the lungs, pulmonary inflammation, disease and mortality in a fraction of mice. In old mice (20-22 months), co-infection resulted in earlier and more severe disease. Aging was not associated with greater bacterial burdens but rather with more rapid pulmonary inflammation and damage. Both aging and IAV infection led to inefficient bacterial killing by neutrophils ex vivo. Conversely, aging and pneumococcal colonization also blunted IFN-α production and increased pulmonary IAV burden. Thus, in this multistep model, IAV promotes pneumococcal pathogenicity by modifying bacterial behavior in the nasopharynx, diminishing neutrophil function, and enhancing bacterial growth in the lung, while pneumococci increase IAV burden likely by compromising a key antiviral response. Thus, this model provides a means to elucidate factors, such as age and co-infection, that promote the evolution of S. pneumoniae from asymptomatic colonizer to invasive pathogen, as well as to investigate consequences of this transition on antiviral defense.
Resistance of pathogenic bacteria to standard antibiotics is an issue of great concern, and new treatments for bacterial infections are needed. Antimicrobial peptides (AMPs) are small, cationic, and amphipathic molecules expressed by metazoans that kill pathogens. They are a key part of the innate immune system in both vertebrates and invertebrates. Due to their low toxicity and broad antimicrobial activities, there has been increasing attention to their therapeutic usage. Our previous research demonstrated that four peptides—DAN1, DAN2, HOLO1 and LOUDEF1—derived from recently sequenced arthropod genomes exhibited potent antimicrobial effects in-vitro . In this study, we show that DAN2 protected 100% of mice when it was administered at a concentration of 20 mg/kg thirty minutes after the inoculation of a lethal dose of E . coli intraperitoneally. Lower concentrations of DAN2—10mg/kg and 5mg/kg protected more than 2/3s of the mice. All three dose levels reduced bacterial loads in blood and peritoneal fluid by 10-fold or more when counted six hours after bacterial challenge. We determined that DAN2 acts by compromising the integrity of the E . coli membrane. This study supports the potential of DAN2 peptide as a therapeutic agent for treating antibiotic resistant Gram-negative bacterial infections.
Using tBLASTn and BLASTp searches, we queried recently sequenced arthropod genomes and expressed sequence tags (ESTs) using a database of known arthropod cecropins, defensins, and attacins. We identified and synthesized 6 potential AMPs and screened them for antimicrobial activity. Using radial diffusion assays and microtiter antimicrobial assays, we assessed the in vitro antimicrobial effects of these peptides against several human pathogens including Gram-positive and Gram-negative bacteria and fungi. We also conducted hemolysis assays to examine the cytotoxicity of these peptides to mammalian cells. Four of the six peptides identified showed antimicrobial effects in these assays. We also created truncated versions of these four peptides to assay their antimicrobial activity. Two cecropins derived from the monarch butterfly genome (Danaus plexippus), DAN1 and DAN2, showed minimum inhibitory concentrations (MICs) in the range of 2–16 μg/ml when screened against Gram-negative bacteria. HOLO1 and LOUDEF1, two defensin-like peptides derived from red flour beetle (Tribolium castaneum) and human body louse (Pediculus humanus humanus), respectively, exhibited MICs in the range of 13–25 μg/mL against Gram-positive bacteria. Furthermore, HOLO1 showed an MIC less than 5μg/mL against the fungal species Candida albicans. These peptides exhibited no hemolytic activity at concentrations up to 200 μg/ml. The truncated peptides derived from DAN2 and HOLO1 showed very little antimicrobial activity. Our experiments show that the peptides DAN1, DAN2, HOLO1, and LOUDEF1 showed potent antimicrobial activity in vitro against common human pathogens, did not lyse mammalian red blood cells, and indicates their potential as templates for novel therapeutic agents against microbial infection.
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