AXIO Proficiency Testing | Coliform Test Results in LGC Proficiency Testing Schemes
Observations from a Proficiency Testing (PT) Scheme
The LGC AXIO Proficiency Testing Microbiology schemes have been organised over the past 30 years in order to enable laboratories performing the microbiological analysis of food and dairy products to monitor their performance and compare it with that of their peers. LGC AXIO Proficiency Testing also aims to provide information to participants on technical issues and methodologies relating to testing of food and dairy products. AXIO QMS Samples 16 (PT-MC-16D for the dairy matrix and PT-MC-16F for the food matrix) include tests for the enumeration of total aerobic mesophilic count, Escherichia coli , Enterobacteriaceae and coliforms. Results over the past few years show that the proportion of participants obtaining satisfactory z-scores for these tests is generally around 95%. However, in Round 292 distributed in August 2020, the number of participants obtaining satisfactory z-scores for coliforms was much lower than usual (with respectively 78.7% and 74.6% satisfactory results for MC-16D and MC-16F). These MC-16D and MC-16F samples contained Escherichia coli , Enterobacter aerogenes and Citrobacter koseri. On investigation it was found that the Citrobacter koseri strain is an example of a delayed fermenter. In VRBL pour plates, the colonies appear typical of a coliform. However, confirmation using BGBB broth according to ISO 4832:2006 gives variable results. After 48 hours at 37°C no gas was observed and only 1mm of gas at 72 hours . H owever , when 100µl of BGBB was subbed into fresh BGBB and incubated , 2mm of gas was produced after only 24 hours. More gas is produced after subbing due to Citrobacter koseri adapting over time to ferment the lactose. (see Figures 1 & 2).
Figure 1: From left to right, cultures of strains of C.diversus , C. freundii , C.koseri and C.freundii in BGBB, showing lack of gas production by C. diversus and C.koseri after 48 hours incubation at 37°C _______________________________________________
Figure 2: From left to right, cultures of the same strains of C.koseri and C.diversus from Fig 1, subbed into BGBB, and now showing gas production after a further 24 hours incubation at 37°C
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