Detecting Listeria with MALDI-TOF mass spectrometry

Researchers at Swinburne University of Technology, in partnership with bioMérieux Australia, found very low levels of the pathogen could be identified from different food samples.

An experiment spiked ultra-high-temperature (UHT) milk as a model food and then detected the bacteria from three different foods: chicken pate, cantaloupe and camembert cheese.

The aim was to see if MALDI–TOF MS could detect L. monocytogenes directly from a food enrichment broth (following sample preparation) without culturing the bacteria on solid media.

Milk samples tested​

Milk samples spiked with single species and multiple species cultures were incubated in a selective enrichment broth for 24 hours, followed by six hours secondary enrichment.

As few as one colony-forming unit (cfu) of L. monocytogenes per mL of initial selective broth culture could be detected within 30 hours.

“On applying the same approach to solid foods previously implicated in listeriosis, namely chicken pâté, cantaloupe and camembert cheese, detection was achieved within the same time interval at inoculation levels of 10 cfu/mL,”​ said the researchers.

Time dependant on dose​

Professor Enzo Palombo, ​lead researcher, said detection time depends on the initial dose of bacteria.

“The 30 hour method we described ensures detection of low levels of bacteria. In our initial experiments, higher doses resulted in earlier detection​,” he told FoodQualityNews.com.  

“The method works once a certain threshold of bacteria is reached. So, the greater the initial number, the sooner this occurs. However, in the 'real world', the level of contamination would be unknown. Plus, standard laboratory enrichment procedures require at least 24 hours.”​

Spiked foods were enriched in non-selective brain heart infusion (BHI) or the selective Oxoid Novel Enrichment (ONE) broth.  

The partnership with bioMérieux Australia began in 2011 and it is their first work together.

Detection of foodborne pathogens can be challenging due to the likely presence of multiple bacteria in a single sample, said the researchers.

Technology challenge​

Palombo, who is also chair of the department of chemistry and biotechnology faculty of science, engineering and technology Swinburne University of Technology, said the major challenge is initial cost of instrumentation.

“However, following the US FDA approval of MALDI-TOF MS as a diagnostic tool, it is predicted that this technology will be more widely adopted, thus reducing costs. The initial investment is off-set by the low running costs and most laboratories claim a return on investment after about 12 months.​

“Another challenge is that the spectral databases that enable identification are biased towards clinical microorganisms rather than food-related microbes. Again, as the technology is adopted more widely, food-related data will become incorporated into the databases.”​

Other methods may be quicker but additional costs may be prohibitive, said Palombo.

“DNA and immuno-based methods are likely to be more rapid but require additional expertise that may not be available in commercial testing laboratories​,” he said.

“Our method is designed to operate within the normal workflow and technical capabilities of food testing laboratories. In addition, MALDI-TOF MS is a 'universal' test method. DNA and immuno-based methods require specific reagents to detect other bacteria of concern, further adding to costs.” ​

Source: Journal of Proteomics Volume 97, 31 January 2014, Pages 100–106

Online ahead of print, DOI: 10.1016/j.jprot.2013.09.014​

“Detection of ​Listeria monocytogenes​ from selective enrichment broth using MALDI–TOF Mass Spectrometry”​

Authors: Snehal Jadhav,​Danielle Sevior, Mrinal Bhave, Enzo A. Palombo