Screening and selection of essential oils for an intranasal spray against bovine respiratory pathogens based on antimicrobial, antiviral, immunomodulatory, and antibiofilm activities

Introduction

The rise in antibiotic resistant pathogens associated with bovine respiratory disease (BRD) poses a serious challenge, particularly to the beef feedlot industry, as they currently depend on antibiotics to prevent BRD to mitigate the financial burden (approx. $1 billion annual loss) inflicted by BRD-associated high mortality and morbidity in feedlot cattle. Thus, there is an impetus need for the development of antimicrobial alternative strategies against BRD. This study aimed to screen and select candidate essential oils (EOs) for the development of an intranasal EO spray that can inhibit BRD pathogens and promote microbiota-mediated respiratory health.

Methods

The effects of selected EOs (ajowan, cinnamon leaf, citronella, grapefruit, fennel, and thyme) on a bovine nasopharyngeal microbiota culture were evaluated using 16S rRNA gene sequencing. The microbiota culture was enriched by incubating nasopharyngeal swabs obtained from finishing beef heifers in brain heart infusion broth with and without EOs (0.025%, v/v). These EOs were then also evaluated for their immunomodulatory effects on bovine turbinate (BT) cells by analyzing the concentrations of 15 cytokines and chemokines in cell culture after 24 h incubation. The crystal violet assay was done to assess the antibiofilm activity of EOs against Escherichia coli UMN026 strain. Finally, 15 EOs were screened for their antiviral activity against the bovine viral diarrhea virus 1 (BVDV-1) using BT cells and a fluorescence-based method.

Results

Ajowan, fennel, and thyme resulted in a moderate reduction of overall nasopharyngeal microbiota growth with significant alterations of both alpha and beta diversity, and the relative abundance of predominant bacterial families (e.g., increasing Enterobacteriaceae and decreasing Moraxellaceae) compared to the control (p < 0.05). Co-incubation of BT cells with selected EOs resulted in minimal alterations in cytokine and chemokine levels (p > 0.05). Ajowan, thyme, fennel, and cinnamon leaf exhibited antibiofilm activity at concentrations of 0.025 and 0.05%. Reduction of BVDV-1 replication in BT cells was observed with thyme (strong), and ajowan and citronella (moderate) at 0.0125% concentration.

Discussion

Accordingly, ajowan, thyme, fennel, cinnamon leaf, and citronella EOs were selected for further development as an intranasal EO spray to prevent and control of BRD pathogens in feedlot cattle.

10 Development of an Essential Oil-Based Intranasal Spray Against Bovine Respiratory Pathogens: in vitro Selection Based on Antimicrobial, Antiviral, Immunomodulatory, and Antibiofilm Activities

The objective of this study was to screen and select candidate essential oils (EOs) for the development of an intranasal EO spray against bacterial and viral pathogens associated with bovine respiratory disease (BRD) as an alternative to antibiotics. The effects of previously selected EOs (N = 6; ajowan, cinnamon leaf, citronella, grapefruit, fennel, and thyme) on bovine nasopharyngeal microbiota culture was evaluated using 16S rRNA gene sequencing. The microbiota culture was prepared by growing nasopharyngeal swabs obtained from finishing beef heifers in brain heart infusion broth with and without the presence of EOs (0.025%, vol/vol). The sequencing was performed on genomic DNA extracted from the microbiota culture harvested after 24 h incubation. These EOs were then evaluated for their immunomodulation effect on bovine turbinate (BT) cells by analyzing the concentrations of 15 cytokines and chemokines in cell culture after 24 h incubation. The EOs were further characterized by assessing their antibiofilm activity against E. coli UMN026 using a crystal violet assay. Finally, 15 EOs were screened for their antiviral activity against the bovine viral diarrhea virus 1 (BVDV1) using BT cells and a fluorescence-based method. Ajowan, fennel, and thyme resulted in a moderate reduction of overall nasopharyngeal microbiota growth, and significant alterations of both α- and β-diversity, and relative abundance of predominant bacterial families (e.g., boosting Enterobacteriaceae while suppressing Moraxellaceae) compared with control (P &lt; 0.05). Only IL-6 and IL-8 cytokine concentrations were affected by EOs (P &lt; 0.05). Ajowan, thyme, fennel, and cinnamon leaf exhibited antibiofilm activity at the concentration of 0.05%. Reduction of BVDV1 viral replication on BT cells was observed with thyme (strong), ajowan, and citronella (moderate; 0.0125%). Accordingly, ajowan, thyme, fennel, cinnamon leaf, and citronella EOs were selected for development as an intranasal EO spray to prevent and control BRD pathogens in feedlot cattle.