Gastrointestinal Pathogens Causing Diarrhea, Dysentery, Enteric Fever, Pseudomembranous Colitis and Food Poisoning

Lecture 34 - Paradigms of Gastrointestinal Infections - Invasive Enteric Pathogens -including Shigella spp. and Enteroinvasive E. coli (EIEC), Salmonella spp. and Campylobacter jejuni.

Reading Assignments: (1) Text Chapters 16, 17, 57, 73 (2) “Colonization I: Adherence”, pp. 189-201, In McClane, B.A. and T.A. Mietzner. 1999. Microbial Pathogenesis: A Principles-Oriented Approach, Fence Creek Publishing, Madison, CT. (3) “Infectious Diarrhea”, In Gates, R. H. 1998. Infectious Disease Secrets, Hanley and Belfus, Inc., Philadelphia., (4) Gilligan, P.H., M. L. Smiley, and D. S. Shapiro. 1997. (Cases 22, 23, 26), Cases in Medical Microbiology and Infectious Diseases (2^nd. Ed.), ASM Press, Washington, D.C., (5) Cunha, B.A. 1999. Infectious Disease Pearls (Cases 73 and 79), Hanley and Belfus, Inc., Philadelphia. (6) India Jones and The Temple of Porcelain – Medscape.

INTRODUCTION

1. Shigella spp. and Enteroinvasive E. coli (EIEC) (See Text Case 1 “Infant V”, pp. 185-186; Case

23 in Reading Assignment (4)), and Case 73 in Reading Assignment (5).

A. Introduction

Shigella and EIEC closely related genetically. Both cause bacillary dysentery (Amebic

dysentery is caused by the amoeba, Entamoeba histolytica.) These two bacteria are

prototypes of locally invasive pathogens– they invade into the intestinal

epithelial cells, but usually do not invade beyond the lamina propria into the

mesenteric lymph nodes or reach the bloodstream. With both pathogens,

the invasive phenotype is mediated by both chromosomal and plasmid loci.

B. Epidemiology

1. Shigella spp. – S. dysenteriae, S. flexneri, S. boydii, and S. sonnei –

exclusively pathogens of humans and primates

a. Transmission - human carriage with person-to person spread;

primarily transmitted by fecal-oral route by people with contaminated hands.

b. Infectious dose– very low (10 organisms)

c. Easily spread among young children in day-care centers, nurseries, and

custodial institutions. Also a problem in male homosexuals.

2. EIEC

a. Infective dose higher than Shigella spp. – less person-to person spread.

b. EIEC oubreaks usually foodborne or waterborne

C. Disease – Bacillary Dysentery

1. Dysentery – a clinical triad consisting of cramps, painful straining to pass stools

(tenesmus), and frequent, small-volume bloody, mucoid diarrhea.

2. Clinical course:

a. Incubation period – 1-4 days fever, malaise, anorexia, and sometimes

myalgia.

b. Watery diarrhea ( large numbers of leukocytes can be seen in the stool by

light microscopy.)

c. Diarrhea turns bloody – symptoms of dysentery begin.

d. With healthy adults, symptoms subside in 2-5 days. In children and the

elderly, loss of water and electrolytes may lead to dehydration and even

death.

e. In a small number of cases there can be neurological symptoms and kidney

damage (HUS- due to the production of Shiga toxin by Shigella dysenteriae

strains.)

(EIEC disease presents most commonly as watery diarrhea, and fewer

patients present with dysentery. To date, HUS has not been associated with

EIEC because these strains do not produce Shiga toxin.)

D. Pathogenesis

1. Overview - The fundamental event in the pathogenesis of bacillary dysentery is

invasion of the epithelial cells of the colon. The bacteria reach the lamina propria and

trigger an acute inflammatory response with mucosal ulceration and abscess

formation. In well-nourished individuals, the infection does not extend beyond the

lamina propria. Some Shigella produce Shiga toxin, which is not essential for

invasion, but which can cause more severe disease and lead to HUS.

2. Step by Step Process: (see Text p. 187)

a. Invasion via the M cells across the colonic mucosal membrane.

b. Bacteria released into the lamina propria and some are phagocyosed by

macrophages. The inflammatory response is triggered; cytokines are

released.

c. Epithelial cell penetration (via the basolateral membrane- invasion

proteins (Ipa proteins) carried on a plasmid

d. Escape from the endocytic vesicle by lysis

e. Intracellular multiplication

f. Directional movement through the cytoplasm via polymerization

of host cell actin – propelled by an actin-motor.

g. Extension into adjacent cells and continued cell-to cell invasion.

This sequence of events elicts a strong inflammatory response which causes ulcer

formation. Neutrophils are shed and appear in the feces.

3. Virulence Factors (including genetics) – genes necessary for invasion are carried

on 120MDa (S. sonnei) and 140Mda (other Shigella spp. and EIEC) plasmids (pInv) .

The plasmids genes encode:

(1) a type III secretion apparatus (see Paradigm p. 189) which is

necessary for the secretion of

(2) multiple proteins (the Ipa proteins:Ipa A - IpD) which are the

effectors of cell invasion.

(3) VirG is a surface protein necessary for actin polymerization and

formation of the actin-motor.

4. Toxin Production

S. dysenteriae strains produce Shiga toxin (discussed above)

2. Salmonella spp.

A. Nomenclature

1. Ewing – 3 species within the genus

a. S. enteritidis- 2000 serotypes – most cause enterocolitis

b. S. choleraesuis- one serotype – more often associated with invasive disease

c. S. typhi – one serotype – enteric fever

2. Kaufmann and White – assign different species for each serotype – 1500 species

named for where they were first isolated.

B. Comparison of Important Features of Salmonella and Shigella (See Table 18-8) at end of

lecture notes.

C. Diseases

1. Enterocolitis (all Salmonella except S. typhi) (See Case 4, “Mr. T”, p.665-666)

a. Transmission to man – via large animal reservoir; transmitted via

contaminated food, especially poultry and dairy products.

Waterborne less frequent.

b. Infectious dose – high (lowered by altering gastric acidity)

c. Symptoms - acute self-limiting diarrhea is the most common

manifestation of disease; occasional spread to the bloodstream

(see below).

d. Pathogenesis (see Text pp. 193-195)

1. Overview: Enterocolitis is characterized by invasion of the epithelial and

subepithelial tissue of the small and large intestines. The organisms

penetrate the mucosal barrier into the lamina propria with resulting

inflammation and diarrhea. There is no enterotoxin production. The influx

of neutrophils from the inflammatory response limits the infection to the gut

and the adjacent mesenteric lymph nodes; bacteremia is infrequent.

(Virulence genes are carried on a large plasmid.)

2. Step by Step Process:

a. Entry – large infectious dose (10⁵ or higher)

b. Contact with the cell surface causes a rearrangement of actin and

deformation of a large area of the host cell membrane (ruffling).

c. Organisms are taken into the cell in endocytic vesicles.

d. Organisms remain in the vesicles for many hours and resist lysosomal

contents of the cell.

e. Bacteria travel in the vesicle to the basement membrane and pass into

the lamina propria.

f. The inflammatory response draws neutrophils into the lamina propria

where most bacteria are phagocytosed and killed. A subpopulation of

bacteria that escape may invade to the mesenteric lymph nodes and

cause transient bacteremia. Usually bacteremia is not sustained unless

there are underlying host defects, or unless the infection is caused by

one of a few Salmonella serotypes that are associated with invasion.

2. Bacteremia and Metastatic Infections

Associated with certain serovars of Salmonella (S. cholerasuis and others), with

children and other patients with predisposing conditions especially sickle cell

anemia, or cancer. Many organs can be seeded and there can be metastatic

disease including osteomyelitis, pneumonia, and meningitis.

3. Enteric fevers (Typhoid Fever and Paratyphoid Fever)- Systemic Infections

Initiated in the Gastrointestinal Tract (See Case 3, “Ms. J”, p. 192-193)

a. Etiological Agents

1. S. typhi – typhoid fever

2. S. paratyphi types A, B, C – paratyphoid fever

b. Reservoir – only humans: spread from person to person via contaminated

food or water.

c. Chronic carriage – after infection, people carry the organism for months or

years, providing a continuous source of infection (Typhoid Mary).

d. Symptoms – Enteric fever is a multiorgan system infection characterized by

prolonged fever, sustained bacteremia, and extensive involvement of the

RES, particularly the mesenteric lymph nodes, liver and spleen.

e. Pathogenesis of Typhoid Fever (see Text Fig. 17.4, p. 195)

1. Ingestion of S. typhi

2. Mean incubation period of 13 days. During this time:

a. Organisms invade across the M cells of the small intestines

and reach the Peyer's patches. The organisms are ingested

by macrophages but not killed.

b. The bacteria proliferate and they are carried to the mesenteric

lymph nodes.

c. They reach the bloodstream via the thoracic duct.

d. There is transient bacteremia followed by multiplication in the

macrophages of the liver, spleen and bone marrow. This is

when the signs and symptoms begin; fever begins to rise and

stays elevated for 4-8 weeks (if untreated)

. 3. Progression to:

a. Septicemia and infection of other organs

b. Cholecystitis – infection of the gallbladder especially with

gallstones (chronic carriage). Organisms survive in bile.

c. Late infection of the small intestines via bile with

ulceration of Peyer's patches leading to diarrhea,

hemorrhage and perforation of the bowel. (Early in

the infection, constipation rather than diarrhea is

seen.)

4. Virulence Factors?

a. LPS accounts for the systemic symptoms of fever

and triggers the inflammatory damage.

b. The long O side chains help S. typhi resist lysis by

complement – causing the MAC to form far

enough away from the cell surface that the cells

are not lysed.

c. The ability to survive and multiply inside phagocytes.

d. Why is S. typhi so invasive???

Little conclusive information. The most virulent

strains produce the Vi antigen, a capsular

polysaccharide. (Since capsular polysaccharides

are produced by bacteria trying to prevent

phagocytosis, and S. typhi can survive inside

phagocytes, how does the Vi antigen benefit the

bacterium? Unknown. )

3. Campylobacter jejuni – The Leading Cause of Bacterial Diarrhea in the United

States

Read: Reading Assignment (3) pp. 231-232; Case studies: Case 22 (Reading Assignment

(4); Case 79 in Reading Assignment (5) and India Jones and the Temple of Porcelain;

1. Unique cell morphology.

2. Microaerophilic growth pattern

3. Pathogenesis poorly understood

a. Small infectious dose

b. Invasive disease – similar to Salmonella enterocolitis; many cases feature such

severe abdominal pain that the patient is assumed to have appendicitis and surgery is

performed.

c. Epidemiology similar to non-typhoidal Salmonella spp.

4. Clostridum difficile and Pseudomembranous Colitis – Read Case 26, in Reading Assignment (4).

See Self Study Assignment on the Genus Clostridium. Be able to describe the pathogenesis of this

disease.

TABLE 18-8. COMPARISON OF IMPORTANT FEATURES OF SALMONELLA AND SHIGELLA

Table Taken From: Levinson, W., and E. Jawetz. 1998. Medical Microbiology and Immunology-

Examination and Board Review. (5^th ed.) Appleton and Lange, Stamford, CT.