When a round underperforms (<80% overall or <90% in specific cases), we verify sample integrity end-to-end: • Pre-dispatch QC (passed) – the batch met homogeneity and stability acceptance criteria prior to release with all 10 samples testing positive. • Retained-unit confirmation – batch records were rechecked and retained units re-tested; Salmonella was recovered from 100% of the 10 retained samples, confirming target content and viability. • Logistics assessment – we analyse time-to-receipt, transit distance and climate band for potential transport effects. No relationship was found. For lyophilised PT materials, organisms are effectively in suspended animation, so these variables are not typically associated with performance differences.
With stability and logistics excluded we continue to a full technical investigation, following a defined sequence.
1. Strain characterisation – each PT strain undergoes full culture analysis to understand behaviour across conditions: colony morphology on a range of agars, plus biochemical and serological profiling.
2. Method review – participant submissions are screened to identify any systematic differences between methods. In practice, pronounced method bias is rarely observed, as laboratories use validated Salmonella detection methods and performance is broadly comparable across methods.
CULTURE PROFILE – GROUP 6
LGC CODE
ORGANISM NAME
617 v2
Salmonella Bredeney
ORIGIN
SOURCE
Unknown
Wild strain
GROUP MORPHOLOGY
Enteric pathogens. Catalase positive and Oxidase negative. Salmonella - On Brilliant Green typical Salmonella growth Is pink colonies with pink agar. On MRSV Salmonella typically generates a central colony with a large pale halo of motility. On XLD Salmonella generates red colonies with black centres. Shigella sonnei - On SS agar produces no growth. On MC No.3 typical colonies are straw coloured. XLD typical growth is red colonies. COLONY MORPHOLOGY – GENERAL (from initial stocks) Incubation temp. and duration 24 hours @ 37 o C aerobic Blood Entire, slightly raised pale beige colonies ~2mm, α -haemolysis MacConkey Entire, flat pale pink colonies, irregular edges ~2mm, pale pink agar Nutrient Entire, flat shiny cream colonies ~2mm COLONY MORPHOLOGY - SALMONELLA Brilliant green Entire, flat pale green colonies, irregular edges ~2-3mm, green agar XLD Entire, slightly raised opaque yellow colonies ~1-2mm, yellow agar CASE 1-2mm entire shiny circular blue/green colonies BROTH GROWTH BU HQ +++ RV +++ BIOCHEMICAL TESTS Oxidase Negative Catalase Positive API 20 E profile 6704552 API 20 E identification Salmonella species 89.4% SEROLOGICAL TESTS
Method
% Satisfactory
77%
MKTT/RV + XLD + BGA 37 (various)
Broth 37 agar 37 (various)
86%
VIDAS
94%
Rapid test (various)
100%
Chromogenic agar
88%
PCR
100%
ELISA
100%
WKL scheme Serogroup
Positive
O:4 (B)
Salmonella latex
Other
90%
O antigens
1,4,12,27
H phase 1 antigens
I, v
H phase 2 antigens 1, 7
Prolab O antisera
Factor 4 Factor 12
Prolab phase 1 H antiosera
lv
Prolab phase 2 H antiosera
Poly 1 complex +z6 7
3. Historical comparison – we examine prior PT rounds that used the same strain to detect recurring patterns. Where similar outcomes recur, this supports a strain- or matrix-linked effect; where prior performance was strong, the result is more likely round-specific. Subsequent rounds have gradually improved over time, suggesting participants have learned to recognise the characteristics of the strain.
MICROSCOPY
Motility
Motility observed
Gram stain
Short Gram negative rods in pairs and short chains GROWTH PROFILES
55°C (NA 48hrs) 44°C (NA 48hrs) 42°C (NA 48hrs) 37°C (NA 48hrs) Aerobic (NA 72 hrs)
-
30°C (NA 72hrs) 25°C (NA 3-5days) 22°C (NA 6 days) 6°C (NA 10 days) Anaerobic (NA 72 hrs)
+++ +++ +++
+++ +++ +++ +++
-
+++
GENERAL INFORMATION
Enumeration media Resuscitation media
NA SP/ PCA PP 24 hours @ 37 o C
Bead details Lot number
Sub x + 1 on 23/4/99
NA SP 24 hours @ 37 o C
Lactose positive and H 2 S variable Salmonella enterica subspecies enterica
Notes/Atypical reactions
All
Schemes suitable for
PROFILE APPROVAL
AC
Date
7/8/25
Approved by
7/8/25
Progressive improvement in Salmonella Bredeney detection across PT rounds
Author: Andrew Cheetham Authorized by: Andrew Cheetham Year
Issue No.: 4 Issue date:6/8/25 Result
Round
Sample Contents
No. of results
Inoculum Level
% Correct
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S. Bredeney
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