Resistance patterns of nonfermentative gram-negative rods and aeromonads to beta-lactam antibiotics: a diagnostic aid.

Consistent resistance to certain antimicrobial agents has in the past been of help in the diagnosis of nonfermentative gram-negative rods. In order to establish patterns of consistent resistance to 15 mostly newer betalactam antibiotics, 561 strains of such organisms plus Aeromonas and Plesiomonas were tested with the Kirby-Bauer method and also against the selective agent C-390 in three concentrations in Mueller-Hinton agar. No species-specific patterns emerged but the resistance data can be used to exclude certain species. C-390 was almost specifically selective for Pseudomonas aeruginosa.