Abstract
Bovine mastitis is a worldwide disease of dairy cattle associated with significant economic losses for the dairy industry. One of the most common pathogens responsible for mastitis is Staphylococcus (S.) aureus. Due to the development and spreading of antibiotic resistance, the search for novel antimicrobial substances against S. aureus is of great importance. The aim of this study was to evaluate two dihydroxybenzaldehydes for the prevention of bovine mastitis. Therefore we determined the minimal inhibitory concentration (MICs) of gentisaldehyde (2,5-dihydroxybenzaldehyde) and 2,3-dihydroxybenzaldehyde of a diverse set of 172 bovine mastitis S. aureus isolates using an automated robot-based microdilution method. To characterize the bovine isolates we determined the genotype by spa-typing, the antimicrobial resistance to eight antibiotic classes using the disk diffusion method and the MICs of three commonly used antiseptics (benzalkonium chloride, chlorhexidine, and iodine). Further we investigated the cytotoxicity of gentisaldehyde and 2,3-dihydroxybenzaldehyde in bovine mammary epithelial MAC-T cells using the XTT assay. The S. aureus strains showed a high genetic diversity with 52 different spa-types, including five novel types. Antibiotic susceptibility testing revealed that 24% of isolates were resistant to one antimicrobial agent and 3% of isolates were multi-resistant. The occurrence of antibiotic resistance strongly correlated with the spa-type. Both dihydroxybenzaldehydes showed antimicrobial activities with a MIC50 of 500 mg/L. The MIC of gentisaldehyde significantly correlated with that of 2,3-dihydroxybenzaldehyde, whereas no correlation was observed with the MIC of the three antiseptics. Cytotoxicity testing using bovine mammary epithelial MAC-T cells revealed that gentisaldehyde and 2,3-dihydroxybenzaldehyde show low toxicity at MIC50 and MIC90 concentrations. In conclusion, gentisaldehyde and 2,3-dihydroxybenzaldehyde exhibited antimicrobial activities against a diverse range of bovine mastitis S. aureus strains at low-cytotoxic concentrations. Therefore, both compounds are potential candidates as antiseptics to prevent bovine mastitis and to reduce the use of antibiotics in dairy cows.
Highlights
Mastitis is an inflammatory disease of the mammary gland that is usually caused by bacterial infection [1]
In this study we investigated the antimicrobial activities of these two phenolic benzaldehydes using 172 bovine mastitis S. aureus isolates
In addition to minimum inhibitory concentrations (MICs) determinations, we investigated the cytotoxicity of these phenolic benzaldehydes using bovine mammary epithelial MAC-T cells to evaluate their potential for use in the prevention of bovine mastitis
Summary
Mastitis is an inflammatory disease of the mammary gland that is usually caused by bacterial infection [1]. Bovine mastitis is today a worldwide problem in dairy cows leading to significant economic losses due to decreased milk yield. In the case of severe clinical mastitis, animals experience pain and discomfort leading to depressed appearance, weight loss, and abnormal postures [3]. The most common mastitis pathogens include Gram-negative bacteria such as Escherichia coli, Pseudomonas spp. and Klebsiella spp. and the Gram-positive species Staphylococcus (S.) aureus, Streptococcus agalactiae, Streptococcus dysgalactiae, and Streptococcus uberis [1]. S. aureus is known to be the primary agent responsible for bovine mastitis, including clinical as well as subclinical mastitis. Its special abilities to form biofilms or hide within host phagocytes and epithelial cells of the mammary gland might account for its ability to evade antibiotics, leading to prolonged therapy or even persistence [4]
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