[Inhibition of Proteus sp. by Nalidixic acid]

The investigation comprised 192-positive and Gram-negative strains of bacteria, including 111 Proteus sp. The degree of resistance of these strains to nalidixic-acid was investigated on blood agar with 25 mug, 40 mug, and 50 mug nalidixin-acid per ml medium. None of the Proteus strains were able to grow on medium with 50 mug nalidixic-acid per ml, whereas Streptococcus sp., Corynebacterium pyogenes, Peptococcus indolicus, Pseudomonas aeruginosa, and beta-toxic staphylococci all were found resistant to this concentration (Tables I and II), and could be reisolated in pure culture after having grown together with a Proteus strain on such medium (Table III). Blood agar with 50 mug nalidixic-acid per ml is therefore suitable for isolation of the above mentioned Gram-positive bacteria and Pseudomonas aeruginosa from Proteus contaminated material.