Antimicrobial activity of nanostructured Amazonian oils against Paenibacillus species and their toxicity on larvae and adult worker bees

Antimicrobial activity of Amazonian oils Andiroba and Copaiba against Paenibacillus larvae has been recently determined, indicating their potential use for the control of American Foulbrood Disease (AFB), but the use of essential oils in the environment still represents a challenge. The oils present several volatile elements in its composition, such high volatility being the cause of a sharp decline in antimicrobial activity. In this context the nanostructuration of these amazon oils may decrease the volatile characteristic of such products. The following research aimed to evaluate the activity of nanoemulsions prepared with Andiroba and Copaiba oils against Paenibacillus species. The toxicity of nanoemulsions has also been investigated with larvae and adult worker bees. Nanoemulsions (NE1, 10% Andiroba oil; NE2, 10% Copaiba oil; and NE3, 10% medium-chain triglyceride as negative control) were prepared in a high pressure homogenizer. The particle sizes were determined as 192, 211, and 178 nm for NE1, NE2, and NE3, respectively. The z potential values were minus56.4, minus47.1, and minus27.2, respectively. NE1 and NE2 showed minimal inhibitory concentration (MIC) values lower than 0.39% for most Paenibacillus species tested. None of the strains were inhibited by negative control NE3. The timeresponse effect of the nanoemulsions has been tested on P. larvae ATCC9545, resulting in a decrease in the number of viable cells to less than 1 log CFU/ml for NE1. The nanoemulsion NE1 showed a significant toxic effect for the larvae (26% mortality) when compared with NE2 (13%) and NE3 (7%). The toxic effect of nanoemulsions has also been evaluated for 72 h in adult worker bees and low mortality rate was only observed for the NE1 treatment (8.3%). This study shows for the first time that nanoemulsions of Copaiba oil can be a potential candidate for the treatment or prevention of AFB.