[Resistance of nongroupable streptococci to beta-lactams. Detection by antibiogram and the activity of combinations of penicillin G and gentamicin].

Expression of disk-diffusion method of the activity of penicillin G against 68 strains of ungroupable streptococci susceptible to this drug (MIC less than or equal to 0.12 mg/l), 30 strains with a reduced susceptibility ("s") (0.25 mg/l less than or equal to MIC less than or equal to 1 mg/l), and 16 resistant (R) (MIC greater than or equal to 2 mg/l) has been studied. Generally, the R strains exhibited reduced inhibition zone diameters around the penicillin G disks but were best screened with an oxacillin disk (5 micrograms) as it has been proposed for the pneumococci. The study of the relations between the MICs of penicillin G and zone diameters obtained with oxacillin disks showed that the strains with oxacillin zones of less than 21 mm were R or "s". The strains with oxacillin zones less than 26 mm should be tested for sensitivity to penicillin by measurement of MIC. The resistance was crossed towards amoxicillin, cefotaxime, and at a lower degree imipenem. However, the routine test of amoxicillin and cefotaxime did not allow the detection of the resistance to these drugs. Amoxicillin and cefotaxime might not be tested by disk-diffusion method because of the cross resistance to penicillin G. Combinations of penicillin G with gentamicin resulted in bactericidal effects in 6 h or less for 6 of 7 strains tested.