Accurate prediction of the fate of microbial foodborne pathogens in foods is of great
concern for anyone involved in the food chain. Factors that may influence microbial
responses in foods and food environments, such as food structure and composition,
microbial interactions and mode of growth were identified and assessed in the present
study.
The fate of Listeria monocytogenes, Salmonella Typhimurium and Escherichia coli
O157:H7 was monitored both in and on teewurst, a raw spreadable sausage, at different
storage temperatures. Regardless of the storage temperature and inoculation type,
pathogen numbers decreased during storage. The increase of endogenous lactic acid
bacteria and the concomitant reduction of pH mostly accounted for this reduction. The
inactivation of all three pathogens inoculated into batter or onto slices varied
considerably among trials possibly due to variations in the initial batch-to-batch levels
of lactic acid microflora and the associated microbial interactions.
The effect of structure, composition and microbial interactions on the growth kinetics of
L. monocytogenes was evaluated in different growth substrates, including broth, agar,
sterile meat blocks, naturally contaminated meat blocks and minced meat. The growth
responses of L. monocytogenes were significantly different in the different growth
media and food products tested. These differences were more pronounced at low
temperatures. The validation of a model based on data from broth against the observed
growth of the pathogen in the rest of the tested media showed that broth models may
result in significant prediction errors.
The potential for mono- or multi-strain cultures of Escherichia coli O157:H7 to attach
and form biofilm in combinations of food-contact surfaces, growth substrates and
storage temperatures was examined. The susceptibility of biofilms to sanitizers was also
evaluated. Attachment and biofilm formation was strain dependent. The presence of
food residues (liquid or solid) facilitated the attachment/transfer of E. coli O157:H7 on
food-contact surfaces. At moderately cold temperatures culture broth was more conducive to subsequent growth. At chill temperatures the presence of natural
microflora in liquid residues enhanced further attachment of the pathogen. Biofilms
were less susceptible to sanitation treatments as compared to planktonic cells. Biofilm
cells surviving sanitation were able to survive and present slight increases at
refrigeration and abuse temperatures, respectively, in cross-contaminated ground meat.
Acylated homoserine lactones (AHLs) and autoinducer-2 (AI-2) signalling molecules in
the cell-free supernatants of Pseudomonas aeruginosa, Yersinia enterocolitica-like,
Serratia proteamaculans and a mixture of two Yersinia enterocolitica strains were
found to affect the growth kinetics of two Salmonella Enteritidis and S. Typhimurium
strains, respectively. P. aeruginosa synthesized quorum sensing signals that accelerated
the metabolic activity of Salmonella strains. All other quorated bacteria tested had a
negative effect on both initiation of growth and metabolic activity. The effect seems to
be strain and QS signal dependent
