Listeria monocytogenes Infection of the Brain

Summary

During infection, Listeria monocytogenes is capable of crossing the blood-brain barrier to colonize the brain. In this protocol, we demonstrate how to assess bacterial colonization of organs following infection of mice. A procedure to perform whole organ perfusion for specific determination of bacterial numbers in the brain parenchyma is provided.

Abstract

Listeria monocytogenes is an intracellular bacterial pathogen that is frequently associated with food-borne infection. The ability of L. monocytogenes to cross the blood-brain barrier (BBB) is concerning as it can lead to life-threatening meningitis and encephalitis. The BBB protects the brain microenvironment from various toxic metabolites and microbial pathogens found in the blood following infection, and therefore supports brain homeostasis. The mechanisms by which L. monocytogenes present in the bloodstream cross the BBB to cause brain infections are not fully understood and there is also a lack of a robust model system to study brain infections by L. monocytogenes. Here, we present a simple mouse infection model to determine whether bacteria have crossed the BBB and to quantitate the burden of bacteria that have colonized the brain in vivo. In this method, animals were infected intravenously with L. monocytogenes and were humanely euthanized by exposure to CO₂ followed by cervical dislocation. Cardiac perfusion of the animals was performed prior to harvesting infected organs. Blood was collected before perfusion and the number of bacteria per organ or mL of blood was determined by plating dilutions of the blood or organ homogenates on agar plates and counting the number of colonies formed. This method can be used to study novel receptor-ligand interactions that enhance infection of the brain by L. monocytogenes and can be easily adapted for the study of multiple bacterial pathogens.

Introduction

The Gram-positive bacterium Listeria monocytogenes is a facultative intracellular pathogen and one of the most deadly food-borne pathogens worldwide. Ingestion of L. monocytogenes contaminated food can lead to listeriosis in humans, a severe invasive disease targeting mostly pregnant women, newborns, the elderly, and immunocompromised individuals¹. L. monocytogenes is among the leading causes of death by a food-borne pathogen in the U.S. and case fatality rates from listeriosis are as high as 20–30%, the highest for all food-borne pathogens². No vaccine currently exists for L. monocyt....

Protocol

All animals are to be maintained and handled with maximum care to minimize discomfort during the course of the procedure. The procedure is to be conducted in compliance with the Institutional Animal Care and Use Committee and all federal, state and local laws. Also note that the laboratory experiments are to be conducted in accordance with Biosafety Level 2 guidelines.

1. Growth of L. monocytogenes for Mouse Infection Studies

1. Autoclave 500 mL of Brain Heart Infusio.......

Discussion

L. monocytogenes is able to cause life-threatening meningoencephalitis in humans. Prior studies have demonstrated the ability of bacteria to cross the blood-brain-barrier (BBB) and to colonize the brain. Three routes of brain invasion have been proposed during infection: direct penetration of the BBB by bacteria, stealth transport by bacteria contained inside of mononuclear cells³, and axonal migration by L. monocytogenes strains that cause rhombencephalitis¹⁵

Materials

Name	Company	Catalog Number	Comments
Brain Heart Infusion media	Becton Dickinson	237200
Streptomycin sulfate	Amresco	0382-50G
Petri dishes	VWR	25384-342
Glycerol	VWR	97062-832
IKA T18 ULTRA-TURRAX Basic Homogenizer	IKA	3352109	Model: T18BS1
Spectrophotometer	Beckman Coulter	DU 800 series
BALB/c mice	Jackson Laboratory	Model #000651
1 mL syringes	Becton Dickinson	309659
26-gauge needles	Becton Dickinson	305115
21-gauge butterfly needles	Becton Dickinson	367281
Ethylenediaminetetraacetic acid	Sigma-Aldrich	60004
15 mL conical tubes	VWR	21008-918
Round-bottom test tubes	VWR	60819-546
Phosphate-buffered saline	Corning	46-013-CM
Stainless steel spatula	VWR	82027-520
Stainless steel scissors (6.5 in)	VWR	82027-592
Stainless steel scissors (4.5 in)	VWR	82027-578
Stainless steel blunt forceps  (4.5 in)	VWR	82027-440
Stainless steel fine tip forceps  (6 in)	VWR	82027-406