A novel, selective synthetic acetamide containing culture medium for isolating Pseudomonas aeruginosa from milk

A selective synthetic medium has been developed both in liquid (Z-broth) and solid (Z-agar) forms for selective isolation of Pseudomonas aeruginosa from foods. The simple, easy to prepare peptone-free synthetic medium contained acetamide that is metabolized to ammonia and acetic acid providing nitrogen and carbon supply. The medium contained no inhibitors. Selectivity of the liquid medium was tested by inoculation of pure cultures of different bacteria belonging to the groups Bacillus, Pseudomonas, Enterobacteriaceae and Staphylococcus. It was found that the selectivity of the medium was complete for the examined range of bacteria. However, a similar result was obtained when nitrofurantoin broth was used. Applicability of the synthetic agar medium was also tested by a nation-wide inter-laboratory test using two milk samples containing 103/ml (sample I) and 105/ml (sample II) Pseudomonas aeruginosa. According to this test, no microbiologically relevant differences were found between the results obtained by Z-agar and cetrimide-agar a frequently used selective agar in case of sample II. However, a relevant and statistically significant difference was found in the results of sample I in favour of the Z-agar, that could indicate the presence of a low number of bacteria. Concerning repeatability and reproducibility, Z-agar proved to be superior to cetrimide agar.