CHAPITRE 1 - Université de Bourgogne

acteria and the ability of producing recombinant phage proteins for specific capture of
bacteria must be considered as a tool with unrivalled properties for bacterial detection
methods.
This novel phage immuno-concentration method was compared to IMS. Nevertheless, with
IMS method, serogroups are immuno-concentrated separately. The IMS reagents are
serogroup specific while with the VIDAS ESPT immuno-concentrates refine serogroups
simultaneously. One must be aware that this test is an automated system.
This study was designed to compare the VIDAS ESPT and IMS method for their ability to
enrich the levels of STEC in samples after enrichment. Sensitivity and specificity studies were
realised on pure cultures and on artificially inoculated meat samples.
For the specificity study, the non-specific strains were tested at high concentrations (10 8
CFU/mL). We could conjectured that the two strains (Citrobacter and “V21”E. coli) were
found positive with the VIDAS ESPT because of such high concentration. In this context,
specificity of the VIDAS ESPT for E. coli O26, O103, O111 and O145 could be enhanced by
the introduction of more selective media. More discriminated media are needed for the
detection of E. coli O26, O103, O111 and O145. Hiramatsu et al. have shown that many
strains of E. coli O26 do not ferment rhamnose. This finding suggested that MacConkey agar
containing rhamnose may be used to enhance the detection of this serogroup from faecal and
food samples (Hiramatsu et al., 2002). Possé et al. (2008) described a new differential and
confirmation media for O26, O103, O111, O145 and O157 isolation from food or faeces.
Our study showed that both the VIDAS ESPT and IMS methods produced comparable results.
However with VIDAS ESPT, less background microflora was presented on the selective
plates. Sensitivity thresholds were around 10 3 CFU/mL in pure culture mixed or not, and
approximatively10 4 CFU/mL in artificially inoculated meat samples.
VIDAS ESPT showed some variability in the sensitivity threshold between serogroups both in
pure culture and in beef matrices tested. Multiple factors might explain the difference in cell-
capture efficiency. For example, interference from components of the ground beef (fat
molecules, background microflora…) might interact with the phage protein or the surface of
the bacteria, which may block the binding. The variability in sensitivity between serogroups
could be explained by different affinity constants between the phage protein and the specific
bacterial binding site.
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