Insect antimicrobial peptides show potentiating functional interactions against Gram-negative bacteria

Abstract

Antimicrobial peptides (AMPs) and proteins are important components of innate immunity against pathogens in insects. The production of AMPs is costly owing to resource-based trade-offs, and strategies maximizing the efficacy of AMPs at low concentrations are therefore likely to be advantageous. Here, we show the potentiating functional interaction of co-occurring insect AMPs (the bumblebee linear peptides hymenoptaecin and abaecin) resulting in more potent antimicrobial effects at low concentrations. Abaecin displayed no detectable activity against Escherichia coli when tested alone at concentrations of up to 200 μM, whereas hymenoptaecin affected bacterial cell growth and viability but only at concentrations greater than 2 μM. In combination, as little as 1.25 μM abaecin enhanced the bactericidal effects of hymenoptaecin. To understand these potentiating functional interactions, we investigated their mechanisms of action using atomic force microscopy and fluorescence resonance energy transfer-based quenching assays. Abaecin was found to reduce the minimal inhibitory concentration of hymenoptaecin and to interact with the bacterial chaperone DnaK (an evolutionarily conserved central organizer of the bacterial chaperone network) when the membrane was compromised by hymenoptaecin. These naturally occurring potentiating interactions suggest that combinations of AMPs could be used therapeutically against Gram-negative bacterial pathogens that have acquired resistance to common antibiotics.

Behavioral and Immunological Features Promoting the Invasive Performance of the Harlequin Ladybird Harmonia axyridis

“…The importance of diverse AMP repertoires and high induction levels became clearer when evidence emerged that insect AMPs show potentiating functional interactions against microbial pathogens (Rahnamaeian et al, 2015). For example, ctype-lysozymes from H. axyridis boost the antibacterial activity of co-expressed coleoptericins (Beckert et al, 2015), which are a family of AMPs restricted to the Coleoptera (Mylonakis et al, 2016).…”

Antimicrobial combinations: Bliss independence and Loewe additivity derived from mechanistic multi-hit models

“…As an example, if an inhibitor targeting the cell membrane facilitates the travel of a second inhibitor to its molecular target within the cell, the rate at which the second inhibitor hits, a, depends on the number of hits the cell has received from the first AMP-a clear instance of synergy. This type of interaction has been observed in the case of combinations of AMPs [10,32] and antibiotics [19]. The reference models describe additive interactions, and our multi-hit models are only used to describe additivity of mixtures based on the two approaches introduced by Loewe and Bliss.…”

“…Except for a few clear-cut cases in which one of the inhibitors is effective only in combination with the other [10,32], or in the case of suppression [19,33], the classification of a combination as synergistic or antagonistic depends on the reference model. For high concentrations of inhibitors, Bliss independence is the more conservative choice of a reference model if one seeks to determine synergistic effect.…”

“…Moreover, multiple AMPs are expressed in concert [4][5][6][7][8]. The co-expression is hypothesized to be due to synergy between AMPs [9,10].…”

Mode of Action of Antimicrobial Peptides Identified from Insects

“…Honey bee venom is a mixture of over 20 compounds, including active peptides such as melittin, apamine [3], apidaecin [8], and abaecin [8,43]. Among them, melittin is the principal active component (40 to 50%) of bee (Apis mellifera) venom and is a powerful lytic peptide [50].…”