Influence of antibiotics and intramammary antibiotic products on phagocytosis of Staphylococcus aureus by bovine leukocytes

SUMMARY Antibiotics and vehicles used in the treatment and prevention of mastitis were tested in vitro to determine effects on phagocytosis of 32P-labeled Staphylococcus aureus ([32P]sa) by bovine polymorphonuclear leukocytes (pmn) isolated from milk. Ampicillin, lincomycin, erythromycin, tetracycline, neomycin, penicillin G, streptomycin, cephalothin, nitrofurantoin, gentamicin, dicloxacillin, rifampin, trimethoprim, amikacin, novobiocin-penicillin, polymyxin B, and tiamulin were tested at in vitro concentrations of 1,000, 500, and 10 μg/ml, and sulfamethoxazole and chloramphenicol at 4,000, 2,000, and 10 μg/ml of incubation mixture. These dilutions were equivalent to antibiotic values in milk at 0, 6, and 12 hours after injection into the mammary gland. Vehicles included peanut oil, mineral oil, 3% cabosil in peanut oil, 2% aluminum monostearate in peanut oil, 25% polyethylene glycol (peg) in an aqueous base, 25% peg in mineral oil, and 25% peg in peanut oil. These were tested at dilutions of 1:100, 1:1,000, 1:5,000, and 1:10,000. The percentage of phagocytosis was determined and was compared with the percentage of phagocytosis in phosphate-buffered saline solution (pbss). Antibiotics and vehicles were diluted in 0.0132M pbss (pH 7.4); combined with skimmed milk, pmn (12.5 × 106), and [32P]sa (50 × 106); and incubated for 1 hour (37 C, no rocking) followed by rocking for 1 hour. A minimum of 2 runs for the antibiotics and 3 runs for the vehicles were performed with all concentrations or dilutions assayed in a single day's run. Phagocytosis was significantly (P < 0.05) reduced at the high and intermediate concentrations for tiamulin, nitrofurantoin, rifampin, chloramphenicol, and amikacin, and at the high concentrations for gentamicin, tetracycline, and novobiocin-penicillin. The only vehicle that inhibited phagocytosis significantly was peanut oil without additives at the 1:100 dilution.