The LOUIS Test
The LOUIS algorithm, a screening test for Salmonella and Shigella
A rapid protocol using only three Diatabs for the screening of lactose non-fermenting colonies for the presence of Salmonella and Shigella cultured on conventional microbiological media.
The method is a rapid, cost-effective screening test that reduces unnecessary identifications by using the minimum number of biochemical characters for the selection of isolates from conventional agars for further testing or discarding of isolates not relevant for further testing.
It reduces the burden on the laboratory of full biochemical identifications and serological tests.
|The LOUIS Test|
|L||=||Lysine Decarboxylase (LDC)|
"The LOUIS test is a screening protocol for Salmonella and Shigella using rapid enzyme tests. It had a sensitivity of 100% and a specificity of 94%, and achieved presumptive reporting of Salmonella three hours after colony isolation with savings of time and money compared to commercial identification systems" (2).
|LDC/IND (Double Test Tablet)||ONPG||URE|
1. Suspect oxidase negative colonies are inoculated in 1ml of sterile saline to give a dense suspension (McFarland 4).
2. Dispense 200ul into 3 test tubes and inocultae an agar plate to confirm purity.
3. Add one Diatab to each tube
|Oil (3 drops of mineral oil)||Yes||No||Yes|
|Incubation||37oC for 3 hours||37oC for 3 hours||37oC for 3 hours|
Negative: Yellow, Green, Grey
After scoring the LDC reaction:
Add 3 drops of Kovac's reagent to the tube for the indole reaction.
Positive: Red (surface layer)
|Further testing or discarding of isolates depends on the profile according to the LOUIS test algorithm|
* Lactose non-fermenting colonies from MacConkey or Deoxycholate-Lactose-Sucrose agar (DCLS). If black or translucent colonies from Xylose-Lysine-Deoxycholate (XLD) agar are picked up, use the Urease/TDA double tablet since retarded urease reactions may be seen. The TDA reaction is unaffected by the XLD agar and more specific for Proteus spp.
The LOUIS algorithm
|LDC||ONPG||URE||IND||Possible ID||Step 1||Step 2|
Proteus spp. or
|+||0||0||0||Salmonella spp.||Confirm by serology||
(LDC neg. Salmonella spp.)
Shigella sonnei or
|Confirm by serology||
|503-11||ONPG (Beta Galactosidase)||ONPG|
|Alternative to Urease|
1. Wilson G. The LOUIS test: a rapid biochemical based protocol for the screening of lactose non-fermenting colonies for the presence of Salmonella and Shigella. Poster. ECCMID 2003.
2. Wilson G. Rapid and Economical Method for the Biochemical Screening of Stool Isolates for Salmonella and Shigella Species. J. Clin. Microbiol. 42, 4821-3, 2004.