In vitro protection of group A beta-hemolytic streptococci from penicillin and cephalothin by Bacteroides fragilis.

beta-Lactamase produced by Bacteroides fragilis could protect from antibiotics group A beta-hemolytic streptococci (GABHS), penicillin-susceptible pathogens frequently isolated from acute tonsillitis in children. To test this hypothesis we determined the minimal bactericidal concentration (MBC) to penicillin and cephalothin of GABHS alone and in mixed culture with eleven beta-lactamase-producing strains of B. fragilis. B. fragilis strains with MBC values less than or equal to 32 micrograms/ml to penicillin and less than or equal to 64 micrograms/ml to cephalothin did not provide any protection for GABHS. However, strains with MBC greater than or equal to 128 micrograms/ml to penicillin and greater than or equal to 256 micrograms/ml to cephalothin provided protection by increasing the MBC of GABHS at least 8,534 times for penicillin and 2,048 times for cephalothin. This phenomenon was most marked in 5% CO2. B. fragilis provided no protection against clindamycin. These results may explain the inability of penicillin therapy to completely eradicate GABHS from cases of recurrently infected tonsils.