Selective colorimetric detection of Gram-negative re-contaminants in pasteurised milk products by a novel application of the BacT/ALERT 3D system

The BacT/ALERT 3D equipment was applied as a fast and sensitive method for the detection of Gram-negative re-contaminants in pasteurised dairy products, particularly drinking milk and cream. The iAST medium was modified and then applied to eleven certified reference strains and nine relevant wild-type isolates originating from a dairy plant. Gram-negative strains could be detected at low levels in pure culture and in combination with high numbers of Gram-positive bacteria as background microflora. Additional experiments with qualified reference material (Escherichia coli, Pseudomonas aeruginosa) showed that low levels of contamination of these bacteria were detectable within 1 and 1.5 days, respectively. The milk matrices exerted no relevant effect on the detectability of the Gram-negative re-contaminants tested. Based on the technique developed, a faster hygiene monitoring and enhanced approval of dairy product lots at the production site is enabled.