Selective medium for isolation of saprophytic coryneform bacteria.

Out of 64 test chemicals for which the toxicity was analysed on pure cultures of Gram-positive and Gram-negative bacteria, nalidixic acid, polymyxin B sulfate, colistin, methyl red, cefotaxim, amikacin, 2-chlorphenol, metronidazole and potassium tellurite were shown to perform best for the selective isolation of coryneform microorganisms; sodium azide, potassium thiocyanate and lithium chloride were not suitable. A new selective agar medium (NPC) for coryneform bacteria containing standard agar, plate count agar or columbia agar together with nalidixic acid (20 mg/l), polymyxin B sulfate (40,000 IU/l) and cycloheximide (100 mg/l) was tested with samples of milk, surface water, waste water and soil. The NPC agar was not inhibitory to micrococci, probably due to their close relatedness with coryneforms. Use of sugar-free nutrient agar (standard agar) restricted growth of Streptococcus faecalis. Pseudomonads, flavobacteria and chromobacteria, which often accounted for 1-5% of the NPC agar-resistant Gram-negative rods, could be distinguished from the Gram-positive bacteria due to differences in colony morphology, since on this medium colony shape is not masked by color reactions. The NPC medium was more effective in eliminating Gram-negative bacteria, bacilli, moulds and yeasts than beta-phenethyl alcohol media and potassium tellurite media.