An assessment of the inhibitory activity of rat serum on staphylococci

A study has been made on the effect of rat serum on staphylococci. By following the respiration and growth of 5 strains ofStaphylococcus aureus and an equal number ofS. epidermidis in fresh, normal rat serum, we found thatS. aureus grew in, and oxidized rat serum better thanS. epidermidis. Difference in growth was not correlated with nutritional requirements. The antibacterial agent of fresh, normal, undiluted rat serum was stable to heating at 56 C for 1 hr, but its activity was completely destroyed after heating at 60 C for 2 hr. A 50 per cent dilution of rat serum with nutrient broth significantly reduced the antibacterial activity and treatment of rat serum with 0.4m solutions of sodium citrate reduced it drastically. Once the serum had been treated with sodium citrate, or oxalate, addition of equimolar solutions of calcium chloride or magnesium chloride failed to restore the antibacterial activity. Addition of ferric ions in high concentrations allowed the coagulase-negative strains of staphylococci to grow well in this serum. The antibacterial agent of rat serum was absorbed by heat-killed cells ofStaphylococcus aureus andS. epidermidis but not byStreptococcus pyogenes andEscherichia coli. Treatment of rat serum with bentonite at a concentration of 100 mg per ml decreased its antibacterial activity.