Experimental mastitis with Escherichia coli: kinetics of bacteriostatic and bactericidal activities.

Three dairy cows were infected on a single mammary gland with 10(4) colony forming units of serum resistant Escherichia coli. The inflammatory reaction was monitored by sampling the inoculated glands at 2 h intervals during the first 16 h post-inoculation, then at each milking for five days. Spontaneous sterilization occurred after an initial phase of rapid bacterial growth. Bovine serum albumin and transferrin (Tr) milk levels rose and fell simultaneously, whereas the increase in lactoferrin (LF) levels, which occurred only when acute mastitis developed (two of the three infections), was delayed. Investigation of antibacterial activity of cell-free milk showed that milk, initially a rich growth medium, turned out to be a bacteriostatic and finally bactericidal medium. Heat-labile bactericidal activity against an E. coli serum sensitive test strain appeared at the onset of inflammation. Moderate bacteriostasis appeared at the initial rise in Tr values and increased markedly when LF levels rose. This activity was abolished by ferric ammonium citrate. Added LF (1 mg/ml) in normal milk was unable to slow down the bacterial growth, while in early inflammatory secretion, appreciable enhancement of bacteriostasis occurred. This suggests that citrate, a milk component known to inhibit LF activity, became unable to suppress the iron-binding proteins effect, presumably through decreased synthesis and influx of blood derived bicarbonate. Further in the course of inflammation, a bactericidal activity against the two serum-resistant strains appeared. This activity proved resistant to catalase and was inhibited only by a massive amount (1 mg/ml) of ferric ammonium citrate. The mechanism is unknown.