Synergy Studies with <i>Pseudomonas aeruginosa</i> Resistant to Gentamicin and/or Carbenicillin

The predictability of synergy with strains of Pseudomonas aeruginosa highly resistant to gentamicin in combination with carbenicillin has been controversial. 30 clinical isolates of P. aeruginosa resistant to gentamicin and/or carbenicillin were tested by checker-board technique. 14 were found to be highly resistant to gentamicin (minimal inhibitory concentration MIC greater than or equal to 128 microgram/ml) and/or carbenicillin (MIC greater than or equal to 512 microgram/ml). Of these 14, 4 isolates showed synergy. 10 of 16 isolates with moderate resistance demonstrated synergy. It is concluded that the level of resistance to gentamicin with P. aeruginosa cannot be used in predicting whether synergy will occur.