Typing L. monocytogenes

Microbiological typing of strains

The term Listeria monocytogenes is used to refer to a range of closely-related bacteria.  Although the bacteria are similar in terms of their DNA and the genes that their DNA contains, an important point is that all L. monocytogenes are not identical and the different types are called strains.  Bacterial typing is the process of distinguishing between different strains of closely-related L. monocytogenes.  Typing is important for processors that have isolated L. monocytogenes from final product or their plant environment, and that are wondering if they have L. monocytogenes plant resident strains.  Typing allows two or more L. monocytogenes isolates to be compared to determine if they are the same strain or different ones.

It should be made clear that typing of L. monocytogenes is not a routine laboratory test and it is likely to be expensive.  It is likely that typing will be beyond the capabilities of most commercial contract laboratories and thus will need to be undertaken by a specialist lab e.g. a university research lab, government or NHS reference laboratory.

There are a number of different ways to undertake bacterial typing, which include:

  • The use of antibodies for serovar determination.  Although fairly inexpensive, the method is not very discriminatory i.e. it may not pick up differences between similar strains.  However, some contract labs may offer this as a service.  The method is good enough for indication, but not a definitive assessment of the presence of persistent strains.
  • Genetic-based methods such as DNA fingerprinting are much more discriminatory than biochemical-based methods such as antibody serovar determination.
  • Genetic methods include polymerase chain reaction (also called PCR, which is cheap) or pulsed field gel electrophoresis (PFGE, which is the gold standard).
  • PFGE is the preferred typing method by many businesses and research laboratories because if it is done properly, it can allow comparisons to typed strains contained within the Pulsenet database.  Pulsenet is a global database of food and clinical isolates and the history of a PFGE typed strain in terms of causing human illness and the contamination of foods can be viewed.

PFGE

All bacterial cells contain a large molecule called DNA.  Genes on the DNA are blueprints for all the activities the cell undertakes (e.g. breaking down sugar to make energy).  Normally DNA exists as a large single strand in cells.PFGE takes the entire DNA inside a cell and cuts it using molecular scissors, which only cut at specific places on the DNA.  resulting in multiple smaller strands.  These small strands can be separated on the basis of their size (because DNA is negatively charged and moves towards a positively charged electrode).  An example of a typical PFGE result is shown as Figure 1.

An important consideration when typing is to remember that bacteria can evolve to become better suited to their environments over time.  Although all organisms (including humans) do that, a bacterial generation (i.e. the time taken produce daughter cells) can be less than an hour.  Thus, there can be small changes to the DNA fingerprints over time as bacteria evolve to become better suited to the niches they inhabit in processing plants.  A specialist typing lab will be able to use mathematical methods to determine whether a single strain isolated from two places has evolved from a common ancestor to generate different fingerprints over time.  For that reason, it’s a good practice to use two different pairs of molecular scissors, which cut the DNA in different places, as shown as Figure 1.

Typing can be complex, and the above explanation is only a short summary of all the issues and how the process can work.  It is recommended that processors with suspected resident L. monocytogenes strains contact a specialist typing laboratory for additional technical advice and assistance.  A list of suitable contacts is provided below.  Please note, this list is provided only for convenience; none of the contacts below is endorsed in any way by the Food Standards Agency.  Other typing laboratories that would like to have their details included in the list below should contact Marianne James (Marianne.James@fss.scot).

Contact Institute and location
Dr Bob Madden

Agrifood Biosciences Institute, Northern Ireland

bob.madden@afbini.gov.uk
Dr Ken Forbes University of Aberdeen, Scotland k.forbes@abdn.ac.uk
Dr Kathie Grant Public Health England, Colindale, England kathie.grant@phe.gov.uk
Dr Tristan Cogan University of Bristol, England tristan.cogan@bristol.ac.uk