One contribution of 13 to a theme issue 'Evolutionary ecology of arthropod antimicrobial peptides'. The innate immune system provides protection from infection by producing essential effector molecules, such as antimicrobial peptides (AMPs) that possess broad-spectrum activity. This is also the case for bumblebees, Bombus terrestris, when infected by the trypanosome, Crithidia bombi. Furthermore, the expressed mixture of AMPs varies with host genetic background and infecting parasite strain (genotype). Here, we used the fact that clones of C. bombi can be cultivated and kept as strains in medium to test the effect of various combinations of AMPs on the growth rate of the parasite. In particular, we used pairwise combinations and a range of physiological concentrations of three AMPs, namely Abaecin, Defensin and Hymenoptaecin, synthetized from the respective genomic sequences. We found that these AMPs indeed suppress the growth of eight different strains of C. bombi, and that combinations of AMPs were typically more effective than the use of a single AMP alone. Furthermore, the most effective combinations were rarely those consisting of maximum concentrations. In addition, the AMP combination treatments revealed parasite strain specificity, such that strains varied in their sensitivity towards the same mixtures. Hence, variable expression of AMPs could be an alternative strategy to combat highly variable infections.This article is part of the themed issue 'Evolutionary ecology of arthropod antimicrobial peptides'.

It has been well documented that antibiotic resistance (AR) is a clinical concern that affects both human and animal health but AR in the environment and food-chain is not as well understood. AR bacteria can occur naturally in soil, water and organic fertilizers used in agriculture so there is a risk that AR can pass to humans via the food-chain. This study focuses on lettuce cultivation undergoing four treatments (Normal irrigation water+normal soil, normal irrigation water+manure, UV irrigation water+normal soil, UV irrigation water+manure)to determine the mechanisms by which the AR is transferred to the plants over the growth period of the lettuce (7 time-points – week 0 to week 6). Plasmids (n=318) have been isolated from irrigation water (n=36), soil (n=45) and lettuce (n=42) samples using the exogenous isolation method for week 0 and week 6 initially. Antibiotic susceptibility testingto amikacin, cefotaxime, ciprofloxacin, imipenem, kanamycin, tetracycline has been carried out. Multi-drug resistance profiles were established for soil taken at timepoint 0 and lettuce taken at timepoint 6. Extracted plasmid DNA was sent for metagenomic analysis to determine which genes are involved in the transfer of AR at the interfaces. The results of the sequencing showed that there are multiple AR genespresent, including Tet, Sme, Cmy, Oxa and ANT(4’)-Ib, that confer resistance to bacteria. The identification of multi-drug resistance in soil and lettuce samples is concerning and highlights the need to determine the mechanisms leading to antibiotic resistance in food.