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Evaluation of antibiotic effectiveness against Staphylococcus aureus surviving within the bovine mammary gland
M S Sanchez1, C W Ford, R J Yancey
1Upjohn Company, Kalamazoo, Michigan 49001.
Abstract:
Bovine mastitis due to Staphylococcus aureus may become chronic and refractory to antibiotic therapy because of the organism's ability to survive within the mammary gland macrophages and/or polymorphonuclear neutrophils (PMNs). Therefore, phagocytosis and killing of S. aureus by bovine udder macrophages, udder and blood neutrophils and blood monocytes were studied. Gland and blood PMNs were about equally effective at phagocytosing (2.5 log reduction in supernatant) and killing the bacteria (92% reduction of viable bacteria by two hours). Gland macrophages phagocytosed at a lower rate (1.5 log reduction) and were less effective at killing the bacteria (73% reduction by two hours). Blood monocytes phagocytosed and killed S. aureus at the lowest rate. An udder macrophage monolayer system was developed and used to evaluate the ability of antibiotics to kill surviving intracellular S. aureus. This assay was similar to our previously described system with blood PMNs. Several classes of antibiotics were investigated. These included naphthalenic ansamycin, lincosaminide, tetracycline, coumarin, peptide, paulomycin, quinolone, macrolide, cephalosporin, and penicillin-class antibiotics. The activity of these compounds was compared to positive (rifampicin), negative (cloxacillin), and nonantibiotic treated controls. Only naphthalenic ansamycin class antibiotics, paulomycin, paldimycin and ciprofloxacin caused significant reduction in viable intracellular bacteria in the macrophage system. These results were similar to those obtained in the blood PMN monolayer system. Because a low intraphagolysosomal pH could affect an antibiotic's ability to kill intracellular bacteria by affecting the drug itself or inhibiting bacterial growth, the effect of low pH on the minimum inhibitory concentration and the minimum lethal concentration of clindamycin and rifampicin against three strains of S. aureus was also tested. While the activity of clindamycin at pH 5.0 compared to pH 7.0 was not affected greatly, the activity of rifampicin was greatly enhanced at acidic pH. These results suggest that at least some of the excellent activity of rifampicin for intracellular S. aureus is due to potentiation of its activity in the intracellular acidic compartment of the phagolysosome.
Insights
Bovine mastitis treatment fails as Staphylococcus aureus survives inside immune cells. New tests show certain antibiotics, like naphthalenic ansamycins and ciprofloxacin, effectively kill intracellular bacteria, offering hope for chronic infections.
Area of Science:
- Veterinary Immunology
- Bacterial Pathogenesis
- Antimicrobial Resistance
Background:
- Bovine mastitis caused by Staphylococcus aureus can become chronic and resistant to antibiotics.
- This resistance is linked to Staphylococcus aureus's ability to survive within host immune cells like macrophages and neutrophils.
- Understanding the interaction between Staphylococcus aureus and bovine immune cells is crucial for developing effective treatments.
Purpose of the Study:
- To investigate the phagocytosis and killing capabilities of bovine udder macrophages, neutrophils, and blood monocytes against Staphylococcus aureus.
- To evaluate the efficacy of various antibiotic classes against intracellular Staphylococcus aureus using a macrophage monolayer system.
- To determine the impact of acidic pH on the activity of specific antibiotics against Staphylococcus aureus.
Main Methods:
- Compared phagocytosis and killing rates of Staphylococcus aureus by bovine gland and blood neutrophils, gland macrophages, and blood monocytes.
- Developed an udder macrophage monolayer system to test antibiotic effectiveness against intracellular Staphylococcus aureus.
- Assessed the minimum inhibitory and lethal concentrations of clindamycin and rifampicin against Staphylococcus aureus at different pH levels (5.0 and 7.0).
Main Results:
- Bovine neutrophils were more effective than macrophages at phagocytosing and killing Staphylococcus aureus.
- Naphthalenic ansamycin class antibiotics, paulomycin, paldimycin, and ciprofloxacin significantly reduced intracellular Staphylococcus aureus in the macrophage system.
- Rifampicin's activity against Staphylococcus aureus was greatly enhanced at acidic pH (5.0) compared to neutral pH (7.0), suggesting potentiation within the phagolysosome.
Conclusions:
- Certain antibiotic classes, including naphthalenic ansamycins and ciprofloxacin, show promise for treating intracellular Staphylococcus aureus infections in bovine mastitis.
- The acidic environment of the phagolysosome may enhance the efficacy of some antibiotics, like rifampicin, against intracellular bacteria.
- Further research into antibiotics effective against intracellular pathogens is warranted for managing chronic and refractory bovine mastitis.