Essential oils inhibit the bovine respiratory pathogens Mannheimia haemolytica, Pasteurella multocida and Histophilus somni and have limited effects on commensal bacteria and turbinate cells in vitro.

The objective of this study was to determine antimicrobial activities of essential oils (EOs) against bovine respiratory disease (BRD) pathogens and nasopharyngeal commensal bacteria, as well as cytotoxicity in bovine turbinate (BT) cells in vitro. The chemical composition of 16 EOs was determined using gas chromatography-mass spectrometry. All EOs were first evaluated for growth inhibition of a single BRD pathogen Mannheimia haemolytica serotype 1 strain (L024A). The most inhibitory EOs (n = 6) were then tested for antimicrobial activity against multidrug-resistant strains of M. haemolytica (serotypes 1, 2 and 6); the BRD pathogens Pasteurella multocida and Histophilus somni, as well as commensal bacteria that were isolated from the nasopharynx of feedlot cattle. The cytotoxicity of 10 EOs was also evaluated using a BT cell line. The EOs ajowan, thyme and fennel most effectively inhibited all BRD pathogens tested including multidrug-resistant strains with minimum inhibitory concentrations (MIC) of ≤0·025% (volume/volume, v/v). For these EOs, the MIC was 2-32 fold greater against commensal bacteria, compared to BRD-associated pathogens. No cytotoxic effects of EOs against BT cells were observed within the tested range of concentrations (0·0125-0·4%, v/v). The EOs ajowan, thyme and fennel inhibited M. haemolytica, P. multocida and H. somni at a concentration of 0·025% and had minimal antimicrobial activity against nasopharyngeal commensal bacteria and cytotoxicity against BT cells. This study demonstrated that EOs may have potential for intra-nasal administration to mitigate bovine respiratory pathogens in feedlot cattle.

Amat S ，Baines D ，Timsit E ，Hallewell J ，Alexander TW ... -  《-》

A vapour phase assay for evaluating the antimicrobial activities of essential oils against bovine respiratory bacterial pathogens.

The objectives of this study were to develop a new assay for the evaluation of the antimicrobial activities of essential oils (EOs) in vapour phase and to demonstrate the antimicrobial activities of commercial EOs against BRPs. To achieve the first objective, a microtube cap containing 100 μl of EO was embedded in an agar plate. An agar plug (diameter 13 mm) inoculated with a bacterial suspension containing108  CFU per ml was then placed over the cap and incubated at 37°C for 24 h. Subsequently, bacteria were recovered from the agar plug by immersion in 5 ml of broth for 10 min, followed by vortexing for 30 s, and the broths were then plated for enumeration. To demonstrate the usefulness of the assay, nine commercial EOs derived from the following specific plants: ajowan, carrot seed, cinnamon leaf, citronella, fennel, ginger grass, lavender, rosemary and thyme were first evaluated for their vapour phase antimicrobial activities against Mannheimia haemolytica serotype 1. Selected EOs were further tested against Pasteurella multocida and Histophilus somni. The EOs of ajowan, thyme and cinnamon leaf completely or partially inhibited BRPs growth. This new assay provided reproducible results on the vapour phase antimicrobial activities of EOs against BRPs. These results support further study of EOs as a potential mitigation strategy against BRPs. In this study, we present a new vapour phase assay for evaluating the antimicrobial activities of essential oils (EO) against bovine respiratory pathogens (BRPs). Using this assay, we identified EOs, such as ajowan, thyme and cinnamon leaf, that can effectively inhibit growth of the BRPs Mannheimia haemolytica serotype 1, Pasteurella multocida and Histophilus somni. This is the first study to demonstrate the vapour phase antimicrobial activity of EOs against BRPs.

Amat S ，Baines D ，Alexander TW 《-》

Prevalence and antimicrobial susceptibility of Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni isolated from the lower respiratory tract of healthy feedlot cattle and those diagnosed with bovine respiratory disease.

Current information on prevalence and antimicrobial resistance (AMR) of bacterial respiratory pathogens is crucial to guide antimicrobial choice for control and treatment of bovine respiratory disease (BRD). The objectives were to describe the prevalence of three BRD-associated bacteria (Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni) in the lower airways of feedlot cattle, and to analyze AMR in these bacteria. Cattle with (n=210) and without (n=107) BRD were sampled by trans-tracheal aspiration at four feedlots (Nov. 15-Jan. 16). These cattle had received 2.5mg/kg of tulathromycin on arrival at the feedlot for BRD control and two in-feed pulses of chlortetracycline (5g/animal/day for 5days) within the first 21days on feed to prevent histophilosis. Bacteria were detected by culture and AMR was tested by microdilution. Pasteurella multocida was the most frequent bacterium isolated in cattle with BRD (54.8%), followed by M. haemolytica (30.5%) and H. somni (22.9%). Compared to those with BRD, healthy cattle were less likely to be positive for P. multocida (OR=0.27), M. haemolytica (OR=0.32), or H. somni (OR=0.25). There were high levels of resistance (>70%) against tulathromycin and oxytetracycline in M. haemolytica and P. multocida isolates and high levels of resistance against oxytetracycline (67%) and penicillin (52%) in H. somni isolates. None or few isolates were resistant to florfenicol, enrofloxacin and ceftiofur. The high prevalence of resistance against tulathromycin and oxytetracycline suggests that these antimicrobials should not be repeatedly used for both control and treatment of BRD and/or histophilosis.

Timsit E ，Hallewell J ，Booker C ，Tison N ，Amat S ，Alexander TW ... -  《-》

Development of Bacterial Therapeutics against the Bovine Respiratory Pathogen Mannheimia haemolytica.

Bovine respiratory disease (BRD) is a major cause of morbidity and mortality in beef cattle. Recent evidence suggests that commensal bacteria of the bovine nasopharynx have an important role in maintaining respiratory health by providing colonization resistance against pathogens. The objective of this study was to screen and select bacterial therapeutic candidates from the nasopharynxes of feedlot cattle to mitigate the BRD pathogen Mannheimia haemolytica In a stepwise approach, bacteria (n = 300) isolated from the nasopharynxes of 100 healthy feedlot cattle were identified and initially screened (n = 178 isolates from 12 different genera) for growth inhibition of M. haemolytica Subsequently, selected isolates were evaluated for the ability to adhere to bovine turbinate (BT) cells (n = 47), compete against M. haemolytica for BT cell adherence (n = 15), and modulate gene expression in BT cells (n = 10). Lactobacillus strains had the strongest inhibition of M. haemolytica, with 88% of the isolates (n =33) having inhibition zones ranging from 17 to 23 mm. Adherence to BT cells ranged from 3.4 to 8.0 log10 CFU per 105 BT cells. All the isolates tested in competition assays reduced M. haemolytica adherence to BT cells (32% to 78%). Among 84 bovine genes evaluated, selected isolates upregulated expression of interleukin 8 (IL-8) and IL-6 (P < 0.05). After ranking isolates for greatest inhibition, adhesion, competition, and immunomodulation properties, 6 Lactobacillus strains from 4 different species were selected as the best candidates for further development as intranasal bacterial therapeutics to mitigate M. haemolytica infection in feedlot cattle.IMPORTANCE Bovine respiratory disease (BRD) is a significant animal health issue impacting the beef industry. Current BRD prevention strategies rely mainly on metaphylactic use of antimicrobials when cattle enter feedlots. However, a recent increase in BRD-associated bacterial pathogens that are resistant to metaphylactic antimicrobials highlights a pressing need for the development of novel mitigation strategies. Based upon previous research showing the importance of respiratory commensal bacteria in protecting against bronchopneumonia, this study aimed to develop bacterial therapeutics that could be used to mitigate the BRD pathogen Mannheimia haemolytica Bacteria isolated from the respiratory tracts of healthy cattle were characterized for their inhibitory, adhesive, and immunomodulatory properties. In total, 6 strains were identified as having the best properties for use as intranasal therapeutics to inhibit M. haemolytica If successful in vivo, these strains offer an alternative to metaphylactic antimicrobial use in feedlot cattle for mitigating BRD.

Amat S ，Timsit E ，Baines D ，Yanke J ，Alexander TW ... -  《-》

A ten-year (2000-2009) study of antimicrobial susceptibility of bacteria that cause bovine respiratory disease complex--Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni--in the United States and Canada.

Portis E ，Lindeman C ，Johansen L ，Stoltman G ... -  《-》