Control of bacterial pathogens associated with mastitis in dairy cows with natural antimicrobial peptides produced by lactic acid bacteria
Thesis (MSc (Microbiology))--Stellenbosch University, 2008.
Mastitis is considered to be the most costly disease affecting the dairy industry. Management strategies involve the extensive use of antibiotics to treat and prevent this disease. Prophylactic dosages of antibiotics used in mastitis control programmes could select for strains with resistance to antibiotics. In addition, a strong drive towards reducing antibiotic residues in animal food products has lead to research in finding alternative antimicrobial agents. Streptococcus macedonicus ST91KM, isolated from bulgarian goat yoghurt, produces the bacteriocin macedocin ST91KM with a narrow spectrum of activity against Grampositive bacteria. These include mastitis pathogens Streptococcus agalactiae, Streptococcus dysgalactiae, Streptococcus uberis, Staphylococcus aureus and Staphylococcus epidermidis as well as Lactobacillus sakei and Micrococcus varians. Macedocin ST91KM is, according to tricine-SDS PAGE, between 2.0 and 2.5 kDa in size. The activity of macedocin ST91KM remained unchanged after 2 h of incubation at pH 2.0 to 10.0 and 100 min at 100 °C. The peptide was inactivated after 20 min at 121 °C and when treated with pronase, pepsin and trypsin. Treatment with α-amylase had no effect on activity, suggesting that the mode of action does not depend on glycosylation. Precipitation with 60 % saturated ammonium sulphate, followed by Sep-Pak C18 separation recovered 43 % of macedocin ST91KM. Amplification of the genome of strain ST91KM with primers designed from the sequence of the macedocin prescursor gene (mcdA) produced two fragments (approximately 375 and 220 bp) instead of one fragment of 150 bp recorded for macedocin produced by S. macedonicus ACA-DC 198. Strain ACA-DC 198 was not available. However, the DNA fragment amplified from strain LMG 18488 (ACA-DC 206), genetically closely related to strain ACADC 198, revealed 99 % homology to the mcdA of S. macedonicus ACA-DC 198 (accession number DQ835394). Macedocin ST91KM may thus be a related bacteriocin described for S. macedonicus. The peptide adsorbed equally well (66 %) to L. sakei LMG13558 and insensitive cells, e.g. Enterococcus faecalis BFE 1071 and FAIR E92, and Streptococcus caprinus ATCC 700066. Optimal adsorption of macedocin ST91KM was recorded at 37 °C and 45 °C and at pH of 8 - 10. Addition of solvents decreased adsorption by 50%, suggesting that the receptors to which the bacteriocin binds have lipid moieties. The addition of MgCl2, KI and Na2CO3 completely prevented adsorption of macedocin ST91KM to the target cells, possibly due to competitive ion adsorption on the bacterial cell surface. The peptide has a bacteriocidal mode of action, resulting in lysis and the release of DNA and β-galactosidase. Atomic force microscopy of sensitive cells treated with macedocin ST91KM have shown deformation of the cell structure and developing of irregular surface areas. Antimicrobial susceptibility patterns were evaluated against eighteen mastitis pathogens. All isolates tested were resistant to methicillin and oxacillin, but had minimum inhibitory concentrations (MICs) falling in the intermediate and susceptible range against erythromycin. S. agalactiae and S. epidermidis had the highest sensitivity to macedocin ST91KM. A teat seal preparation containing macedocin ST91KM effectively released bacteriocin inhibiting the growth of the bacterial pathogen. Macedocin ST91KM could form the basis for an alternative dry cow therapy to prevent mastitis infections in dairy cows, as it is effective against pathogens that display resistance to conventional antibiotic therapy.