Introduction: Listeria monocytogenes is an important food borne pathogen found across a wide range of foods and food ingredients. It is important to be able to isolate and identify this pathogen during the food manufacturing process from the food components and environmental surfaces where it can persist in biofilms. Food testing laboratories consider Listeria identification as one of their top priorities.
Chromogenic agars offer a very discrete method of highlighting the target bacteria from competitors and can grow single isolated colonies in 24 hours. Microgen Listeria ID is designed to identify individual Listeria species within 18-24 hours so the time to identification from the enrichment broth can be completed with 48 hours using these two products in combination.
Method: Using isolates from our extensive culture collection we tested 56 Listeria species isolates and 31 closely related bacteria to determine how efficient this combination of tests to identify the individual Listeria species would be. All isolates were grown initially in Fraser broth.
Results:
Listeria species
|
Organism |
Total |
EZCHROM Listeria |
Microgen Listeria ID |
|
|
|
|
Blue/Green |
Halo |
|
|
L. monocytogenes |
20 |
YES |
YES |
L. monocytogenes (20/20) |
|
L. innocua |
16 |
YES |
NO |
L. innocua (16/16) |
|
L. ivanovii |
2 |
YES |
YES |
L. ivanovii (2/2) |
|
L. welshimeri |
5 |
YES |
NO |
L. welshimeri (5/5) |
|
L. seeligeri |
9 |
YES |
NO |
L. seeligeri (9/9) |
|
L. grayii |
3 |
YES |
NO |
L. grayii (3/3) |
Closely related bacterial species
|
Bacterial Species |
EZCHROM LISTERIA |
Microgen Listeria ID |
||
|
Blue/Green |
Halo |
Growth |
Result |
|
|
Staphylococcus xylosus |
No |
No |
Very Poor |
Not Listeria |
|
Citrobacter freundii |
No |
|||
|
Salmonella enteritidis |
No |
|||
|
Staphylococcus aureus (2) |
No |
|||
|
Staphylococcus aureus |
White |
No |
Very Poor |
Not Listeria |
|
Staphylococcus aureus |
No |
No |
Very Poor |
Not Listeria |
|
Bacillus subtilis |
No |
|||
|
Enterobacter aerogenes |
No |
|||
|
Proteus mirabilis |
No |
|||
|
Proteus vulgaris |
No |
No |
Very Poor |
Not Listeria |
|
Salmonella typhimurium |
No |
|||
|
Streptococcus spp. (2) |
No |
|||
|
Bacillus cereus (2) |
No |
|||
|
Carnobacterium divergens |
No |
|||
|
Carnobacterium gallinarum |
No |
|||
|
Carnobacterium piscicola |
No |
|||
|
Lactobacillus casei subspecies casei |
No |
|||
|
Lactobacillus delbrueckii subspecies lactis |
No |
|||
|
Lactobacillus plantarum |
No |
|||
|
Bacillus mycoides |
No |
|||
|
Bronchothrix thermosphacta |
No |
No |
Very Poor |
Not Listeria |
|
Kurthia zopfii |
No |
|||
|
Propionibacterium freudenreichii |
No |
|||
|
Erysipelothrix rhusiopathiae |
No |
|||
|
Micrococcus spp. |
No |
No |
Very Poor |
Not Listeria |
|
Rhodococcus equi |
No |
|||
|
Enterococcus durans |
No |
|||
|
Corynebacterium renale |
No |
No |
Very Poor |
Not Listeria |
Summary:
- Percentage of Listeria species Confirmed correctly = 100% (56/56)
- Percentage of False Identification of non-Listeria spp. = 0% (0/31)
Conclusions:
- The combination of EZ-CHROM Listeria & Microgen Listeria ID produces a confirmed identification of individual Listeria species in 48 hours
- All of the results confirm the correct identification of the Listeria species