Inhibiting Factors in the Determination of Penicillin in Human Sera

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Abstract
Sera from adults and children never treated with penicillin or sulfonamides were tested for their inhibitory activity on the strept. strain C203 by the Gyorgy method. The same sera were used to detmn. the sensitivity of a strain of Bacillus subtilis using the tube dilution method. Tube results were read at 16, 24 and 40 hrs. at 37[degree]C. Subcultures were made on blood agar plates from tubes with no visible growth at 24- and 40-hr. periods. Human sera contained substances inhibitory to Streptococcus C203 and B. subtilis. Inhibition of normal adult sera was greater against B. subtilis than against the streptococci. In the latter the effect was only bacteriostatic and was nullified by subculture after 24 hrs''. incubation. The effect was similar in children. Sera from sick adults was bacteriostatic and bactericidal against both organisms. Heating normal adult sera at 56[degree]C for 5 min. decreased bacteriostasis against the streptococci and B. subtilis but the bactericidal effect against the latter was not eliminated. Neither the streptococcus nor B. subtilis methods were reliable to determine low penicillin concs. in sick adults. A strain of Staphylococcus aureus required 0.08 units of penicillin per tube for inhibition as compared to 0.02 units for the Streptococcus C203.