In vitro susceptibility of swine pathogens to feed additives and active ingredients with potential as antibiotic replacements.

AIMS: The rise in antibiotic resistance requires the reduction of antibiotic use in all sectors. In animal production, many commercial alternatives to antibiotics have been developed for incorporation into feeds, but a lack of evidence on their antibacterial activity limits confidence in their application. We aim to compare the antibacterial activity of feed additives and active ingredients to better understand their usefulness. METHODS AND RESULTS: The antibacterial activity of 34 active ingredients and feed additives, including medium- and short-chain organic acids and essential oils, was tested against pure cultures of five bacterial swine pathogens. Antibacterial activity was observed using an agar plug diffusion method and quantified via broth microdilution. A diverse range of antibacterial activities were observed. The highest inhibitory activity against Staphylococcus aureus and Streptococcus suis was exhibited by the C12 monoglyceride (0.49 mg ml-1 ). The monoglyceride of C12 was more effective than C12:0 against Strep. suis, but neither C12:0 nor its monoglyceride showed efficacy against the gram-negative micro-organisms tested. The most active against Escherichia coli were the C6:0 medium-chain organic acids and potassium diformate (1.95 mg ml-1 ). For Salmonella Typhimurium, potassium diformate, sodium diformate, and a blend of C8:0/C10:0 (each 1.96 mg ml-1 ), and for Actinobacillus pleuropneumoniae, eugenol (0.49 mg ml-1 ) showed the most promising activity. CONCLUSIONS: We identified broad-spectrum antibacterial activity, such as the C6:0 MCOA, and those with interesting narrow-spectrum activity, notably the killing of Strep. suis by C12 monoglyceride. We have identified additives that show the most promising bioactivity against specific pathogens. SIGNIFICANCE AND IMPACT OF THE STUDY: We broadly compare a large collection of feed additives and active ingredients for their antibacterial activity against a diverse panel of bacterial swine pathogens. This provides a solid base of evidence which can drive the development of feed supplementation strategies with the aim of reducing dependency on antibiotic use in swine production.

Size fractionation of bioaerosol emissions from green-waste composting.

Particle size is a significant factor in determining the dispersal and inhalation risk from bioaerosols. Green-waste composting is a significant source of bioaerosols (including pathogens), but little is known about the distribution of specific taxa across size fractions. To characterise size fractionated bioaerosol emissions from a compost facility, we used a Spectral Intensity Bioaerosol Sensor (SIBS) to quantify total bioaerosols and qPCR and metabarcoding to quantify microbial bioaerosols. Overall, sub-micron bioaerosols predominated, but molecular analysis showed that most (>75%) of the airborne microorganisms were associated with the larger size fractions (>3.3 µm da). The microbial taxa varied significantly by size, with Bacilli dominating the larger, and Actinobacteria the smaller, size fractions. The human pathogen Aspergillus fumigatus dominated the intermediate size fractions (>50% da 1.1-4.7 µm), indicating that it has the potential to disperse widely and once inhaled may penetrate deep into the respiratory system. The abundance of Actinobacteria (>60% at da < 2.1 µm) and other sub-micron bioaerosols suggest that the main health effects from composting bioaerosols may come from allergenic respiratory sensitisation rather than directly via infection. These results emphasise the need to better understand the size distributions of bioaerosols across all taxa in order to model their dispersal and to inform risk assessments of human health related to composting facilities.