By Andy Muirhead – Company Microbiologist, ALS Food & Pharmaceutical (UK)
One of the fundamental principles in all microbiological techniques is selectivity. It is acknowledged that in any food matrix, the target bacterium (Salmonella, Listeria etc) may not be the predominate organism, so we have to encourage the growth of our target organism so it achieves levels at which it becomes detectable, whilst at the same time suppress the growth of the other competing bacteria which, if left unchecked may hinder our chances of successfully detecting the pathogen. We normally try to utilise a unique growth characteristic of the target organism and incorporate this into the selective media. For example, Salmonella can tolerate very high levels of bile salts, so incorporation of sodium desoxycholate into the selective agars (for example XLD and DCA) helps to promote the growth of Salmonella over the other organisms which may be present in the sample. If we know that our target organism has developed resistance to a particular antibiotic, we can incorporate that into the selective broth or agar in the hope that it will still be effective against, and suppress the growth of other organisms. We can also incubate our plates and broths at temperatures which we know that the target organism can tolerate, but which are too high for the majority of competing organisms to be able to grow, for example incubation of Rappaport-Vassiliadis broth at 41.5°C. To date all established selective broths are unique for the individual target organism, but a new research paper published this month in the Journal of Food Safety claims to have developed a “universal” selective broth which is suitable for Salmonella, Listeria, Staphylococcus aureus and E coli 0157. The selective agents include acriflavine, lithium chloride, sodium chlorate and the antibiotic nalidixic acid. The advantages of having a single enrichment platform for multiple organisms is obvious, but it has to be acknowledged that in food microbiology we are often trying to recover cells which are “stressed” due to the conditions which the bacteria have been subjected to in the manufacturing process, and we have to make sure that the selective agents are not having a detrimental effect on the target, as well as the non-target bacteria. Although the paper stated that the broth gave a 5 log increase of all 4 target bacteria, and successfully inhibited the growth of the background flora, it did not state how the broth compared to the established individual selective broths for the individual organisms.