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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.

louis_salmonella_screen_reags

Introduction

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)
O	=	ONPG (o-nitrophenyl-beta-D-galactopyranoside)
U	=	Urease (URE)
I	=	Indole (IND)
S	=	Screen Test

"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).

Procedure

	LDC/IND (Double Test Tablet)	ONPG	URE
Media^*/Suspension	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	37^oC for 3 hours	37^oC for 3 hours	37^oC for 3 hours
Results I	Positive: Blue/Violet Negative: Yellow, Green, Grey	Positive: Yellow Negative: Colourless	Positive: Red/Purple Negative: Yellow/Orange
Results II	After scoring the LDC reaction: Add 3 drops of Kovac's reagent to the tube for the indole reaction. Positive: Red (surface layer) Negative: Yellow
	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
+	+	0	+	E. coli	Discard
+	0	0	+	E. coli	Discard
0	0	+	+	Proteus spp. or Morganella morgani	Discard
0	0	+	0	Proteus spp. or Morganella morgani	Discard
+	0	0	0	Salmonella spp.	Confirm by serology	Serology: Positive: Identify Negative: Discard
0	0	0	0	Shigella spp. (LDC neg. Salmonella spp.)	Full biochemical identification	Serology: Positive: Identify Negative: Discard
0	0	0	+	Shigella spp.	Full biochemical identification	Serology: Positive: Identify Negative: Discard
0	+	0	0	Shigella sonnei or Shigella dysenteriae	Confirm by serology	Serology: Positve: Identify Negative: Discard

Typical results:

LDC	ONPG	URE	IND	Identification

+	+	0	+	E. coli

+	0	0	0	Salmonella spp.

0	0	+	+	Proteus spp.

Rosco Products

584-11	Lysine Decarboxylase/Indole	LDC/IND
503-11	ONPG (Beta Galactosidase)	ONPG
575-11	Urease	URE
920-31	Kovacs' Indole	-
Alternative to Urease
579-11	Urease/TDA	URE/TDA

References

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.