Fourier-transform infrared (FT-IR) spectroscopy is a promising tool for monitoring the population dynamics of microorganisms in food stuff

The routine identification and enumeration of large numbers of bacteria in complex microbial consortia is expensive and time consuming. In the present study, FT-IR spectroscopy has been used for the first time to rapidly monitor the population dynamics of bacteria in a food system. The bacterial cheese surface flora from two different batches was examined throughout the production process, starting with the inoculum. The ripening process was monitored over 12 or 29 days. One hundred bacterial isolates were investigated at each sampling point resulting in a total of 1307 isolates. Classical microbiological methods and Fourier-transform infrared (FT-IR) spectroscopy, partly supplemented by partial 16S rDNA sequencing, were used for identification. In the smear brine of cheese A, Staphylococcus equorum was found to dominate, while corynebacteria dominated the smear liquid of cheese B. The flora consisted mainly of the coryneform bacteria Corynebacterium casei and C. variabile, which made up 80-90% of the aerobic surface flora after 5-6 days of ripening and stayed in that order of magnitude until the end of ripening. The other flora components consisted of staphylococci, gram-negative bacteria, catalase-negative bacteria, and aerobic spore-formers. Only 1% of the coryneform bacteria could not be identified by FT-IR spectroscopy. The results demonstrate that FT-IR spectroscopy is a suitable method for the rapid monitoring of the population dynamics of coryneform bacteria on the surface of smeared cheeses. Generally, application of this method will enable a food producer to obtain a detailed overview on the microbiological status of a product down to the species level, which allows for correction measures to be taken early in the process.