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article imageNew rapid methods for Salmonella detection

By Tim Sandle     May 23, 2016 in Health
Salmonella is a major agent of food-borne illness globally. Researchers are developing a range of new, rapid microbiological methods to speed up detection and to improve the accuracy of the results.
Salmonella is a Gram-negative rod shaped bacterium (the Gram-negative refers to the outcome of a cell staining test, relating to the structure of the organism’s cell wall.) in the U.S. alone, the organism causes around one million cases of food poisoning each year.
The Salmonella genus is composed of two species, enterica and bongori, with six subspecies of S. enterica. This is more complex, since there are several different serotypes based on the presence of specific surface molecules. Some estimates put the number of serotypes as high as 2,500.
Until the 1990s cases of Salmonella infection were primarily related to contaminated poultry, meat and dairy food stuffs. However, in recent years, as Karen Graham’s regular reports for Digital Journal testify, illnesses have become linked to a wider range of foods such as ready‐to‐eat produce like tomatoes, melons, sprouts, leafy greens and berries.
A new review paper considers the latest tests designed to speed up the detection of the bacteria from food samples, and also to improve the accuracy of reporting. Such rapid microbiological methods include qPCR, whole genome sequencing and metagenomics. The review is published in the journal Microbial Biotechnology (“Recent and emerging innovations in Salmonella detection: a food and environmental perspective.”)
Although rapid testing has advanced over the past two decades, in order to best trace origins of contamination most identification and subtyping strategies require a pure isolate, and it takes time to culture such organisms.
This problem can be overcome through advances in DNA sequencing technologies. Here PCR and real‐time, quantitative PCR (qPCR) are the methods of choice. The polymerase chain reaction (PCR) is a technique used in molecular biology to amplify a single copy or a few copies of a piece of DNA across several orders of magnitude. A real-time polymerase chain reaction monitors the amplification of a targeted DNA molecule during the PCR, that is in real-time, and not at its end, as in conventional PCR.
A limitation with PCR is with separating out different bacteria. Here immunomagnetic separation (IMS) allows for the specific separation of Salmonella from other organisms within the food sample. This involves mixing using a magnetic bead.
A further technology being developed is next‐generation sequencing (NGS) and whole genome sequencing (WGS). These techniques refer to highly automated genome sequencers, designed to sequence the entire genome(s) of bacterial pathogens within a few hours.
Another advancement is with the identification of bacteria by mass spectrometry (MS). Here a method called matrix‐associated laser desorption ionization–time of flight mass spectrometry (or MALDI-TOF).
More about food pathogens, Salmonella, rapid method, Microbiology
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