Control of Listeria monocytogenes in an avocado processing facility
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Date
2015-11
Authors
Strydom, Amy
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
Listeria monocytogenes contamination of food is a growing concern for the food industry
since it is the causative agent of human listeriosis. Despite increased awareness and
strict microbiological standards for this pathogen, countries such as France, Austria and
Germany have reported increases in listeriosis outbreaks. The research in this thesis
shows how Listeria contamination in a South African avocado processing was almost
eradicated. The first aim of this project was to isolate and genetically type the L.
monocytogenes strains isolated from the facility. Pulsed field gel electrophoresis (PFGE)
was used to group a subset of strains (n=80) according to the digestion of their genomes
with the restriction enzyme AluI. These results were compared to polymerase chain
reaction (PCR)-restriction fragment length polymorphism (RFLP) of the inlA gene
(n=140). The results of the two methods compared well with each other but both indicated
a lower genetic diversity among the isolates than expected. These strains were isolated
over a period of four years and only two major groups were identified with the PFGE and
the PCR-RFLP resulted in only three banding patterns. Also, the origin of the bacterial
contamination could not be identified, but results indicated that cross contamination
played a role in the persistence of these bacteria in this food processing facility.
Secondly, the commercial product ListexTM P100 was assessed for possible use
as a biocontrol agent in the facility. The host range of the phage product was determined
based on 239 L. monocytogenes strains isolated from this facility, but only 26.7% of these
strains were susceptible to the bacteriophage. The strains were also analysed for
serotype and no correlation was observed between typing methods or isolation source
and date of isolation of the strains, as well as the susceptibility to the phage product. This
could indicate that cross contamination played a role in the transfer of bacterial cells since
these strains were distributed randomly in the facility. Inoculation of the phage product
with L. monocytogenes T162 in brain heart infusion (BHI) broth resulted in significant
reductions in the bacterial concentration. However, activity of the phage in avocado pulp
and guacamole was very low and no significant reduction in the L. monocytogenes
concentration was measured.
Lastly, the population of the Listeria strains in the facility were continuously
monitored over five years. The final products and processing environment, including
floors, equipment, work areas and personnel were tested on-site for Listeria with the ISO
11290-1 method. Based on the prevalence of Listeria, the facility introduced new
strategies in processing to counter cross contamination. Results from the 2014
guacamole production season showed almost complete eradication of Listeria spp. in final
products (0.17%, n=1170) and the processing facility (0.79%, n=1520). These results
indicate that successful management of Listeria spp. in an avocado processing facility
can be accomplished with in-house monitoring of the bacterial population and subsequent
adjustments to the processing system.
The results from this project indicated that the cause of contamination by L.
monocytogenes in the facility was due to cross contamination, although a strict quality
control system was followed. Despite low genetic variability between the L.
monocytogenes strains, the commercial phage product was only effective against 26.7%
of strains tested. This is surprising since literature reported very high percentages of
susceptible strains to this specific product. Although bacteriophage biocontrol with
ListexTM P100 was not effective in this facility, it cannot be concluded that this will be the
case for other facilities. Also, bacteriophage product with a broader host range such as
a cocktail of different phages, may work well in the processing environment to minimise
transfer of bacterial cells to the final product. Control of L. monocytogenes will, however,
only be effective if the processing conditions counter cross contamination.
Description
Keywords
Listeriosis, Communicable diseases, Listeria monocytogenes, Food -- Microbiology, Thesis (Ph.D. (Microbial, Biochemical and Food Biotechnology))--University Free State, 2015