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Chapter 3
previously [163]. These environmental persisters may be related to the
formation of a mature biofilm [56]. After a mature biofilm has developed it
may be difficult to eradicate the pathogen from the environment due to
the number of metabolic processes as outlined in earlier chapters.
3.1.1. Repeated S. enterica outbreaks
As discussed in detail in chapter 1 the serovars studied in this research (S.
Agona, S. Enteritidis and S. Typhimurium) have been involved in multiple
food-borne outbreaks. Repeated isolation of PFGE indistinguishable
isolates from a source suggests that the isolates represent a strain that is
resident in that niche. Isolation of PFGE indistinguishable isolates from
different sources is generally interpreted as indicative of a recent common
origin and my help to link cases of infection with a specific niche [194].
Nesse and colleagues analysed Salmonella isolated from the environment
and the raw ingredients used in four fish feed factories over an extended
period of time [163]. All isolates collected over three years were assessed
by PFGE and plasmid analysis. In addition, strains taken retrospectively
from the factories spanning a ten year period were also examined. It was
evident from the results that there were numerous distinguishable PFGE
profiles found within the ingredients [163]. Specific clonal lineages of
Salmonella (based on indistinguishable PFGE profiles) were recognized to
persist within the environment over the 3 year period, with the same
serovar (no PFGE data) reported in the environment for up to ten years
[163].This indicates a particular strain may enter and persist in a premises
and that contamination of products may come from this source rather than
from raw materials [163].
Following the PFGE analysis of S. enterica collected in the fish feed
factories the same research group sought to link persistence in the
environment with dense biofilm formation [56]. Seven strains were
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