Envr133_Lecture11

7/30/2019 Envr133_Lecture11

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ENVR 133 Bacterial Pathogens -Overview

Mark D. Sobsey


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Some Important Enteric Bacterial Pathogens and Their Sources

Bacterium/Group Animal Feces Non-fecal sources

Salmonella spp. yes no(except S. typhi) no no

Campylobacterspp. yes yes

Escherichia coli yes no

Helicobacter pylori unknown unknown

Aeromonas hydrophila yes yes

Yersinia enterocolitica yes yes

Listeria monocytogenes

Vibriocholerae yes yes

some otherVibrio sp. yes yes

Shigella spp. no no


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Typical Procaryotic Cell


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Gram Negative Outer Cell Membrane and

Lipopolysaccharide Structure


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Structure of a Gram-Negative Cell Wall


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Porins

Porins are structures in the outer

membranes of Gram-negativebacteria

They allow diffusion of low

molecular weight compounds into

the periplasmic space.

Function as channels for small

hydrophilic molecules.

Porins are beta-barrel proteins in

the outer membranes.

Porins occur as trimers in the

membrane.

In E. coli, porins with different

specificities exist, e.g.. maltoporin

and phosphoporin.


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Shigellosis - Illness

Persistent diarrhea with frequent and painful passage of

stools consisting mostly of blood, mucus and pus accompanied by fever and stomach cramps.

Blood and mucus in stools are likely signs of shigellosis

Shigella infect, invade cells lining the large intestine (colon)

Cause breaks (ulcers) in mucous membrane lining of intestine

Inflammation and tissue damage

Causes painful straining to pass stools; can lead to rectal

prolapse

Ulcers commonly in the rectum

results in increased production of mucus

loss of blood and serum proteins into intestinal cavity

Causes the symptoms of dysentery, which include blood and

mucus in the stool (bloody diarrhea); fever is also common


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Shigella and Shigellosis

Fecal-oral transmission

person-to-person, fomites, food, water, ect.

Waterborne and water-washed

Reservoirs: humans and primates

Infectious dose: low; as few as 10 cells to infect

Incubation period: 1 to 7 days; typically, 1-3 days

Duration of illness: untreated: severe symptoms for about two weeks

Antibiotic treatment shortens illness and prevent spread

to others


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Shigellosis - Epidemiology

Four species ofShigella: flexneri, sonnei, dysenteriae, boydii

Major public health problem in many developing countries causes about 5 to I0% of childhood diarrhoea up to 25% of all diarrhea-related deaths can be associated with Shigella

Developing countries:

Sh. flexneri is endemic (always present) in most communities Sh. dysenteriae type 1 often occurs in an epidemic pattern

organism can be absent for a number of years, then reappear and infect a

large proportion of the population.

These two species of Shigella generally produce the most

severe illness.

Developed countries:

Sh. sonneiis the most common and is the least virulent

Sh. boydii causes disease of intermediate severity

is least common, except in the Indian sub-continent.


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Shigellosis - Epidemiology

Worldwide distribution; infections occur throughout year

Mostly in children aged under five Rates of infection are highest where sanitation is poor

As few as 10 cell can initiate infection

Transmission influenced by:

nutritional status

environmental factors affecting transmission:

rainfall and temperature

Waterwashed as well as waterborne

Incidence highest in dryer climates in hot and dry weather Scarcity of water may limits handwashing and other hygiene

measures that reduce transfer of the very small number of

bacteria needed to cause infection

Incidence in wet climates is often highest in rainy season


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Shigellosis - Complications

severe anorexia (loss of appetite)

hypoproteinaemia (a low concentration of blood

protein)

hyponatraemia (a low concentration of blood sodium)

dilation of the large intestine seizures

anaemia

kidney damage

persistent diarrhoea

weight loss and malnutrition


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Shigellosis - Treatment, Control and Prevention

Treatment:

Continue to eat (feed)

to prevent weight loss and hypoglycemia

include foods rich in potassium (bananas)

Replace fluids

Oral or IV rehydration with fluids and electrolytes

Treat with antibiotics: trimethoprim/ sulfamethoxazole

ampicillin

Prevention and Control:

Handwashing, especially after defacation

Improved sanitation and hygiene

improve water, waste treatment/disposal and food sanitation

reduce overcrowding, etc.

No effective vaccine


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Escherichia - Taxonomy and Biochemistry

Five accepted species:

E. coli, E. blattae, E. fergusonii, E. hermaniiand E. vunerisCan be differentiated on the basis of the following reactions:

Indole, citrate, lysine decarboxylase, growth in KCN and

malonate utilizationBiochemistry:

frequently produce indole (except E. blattae and E.vulneris)

ferment glucose by mixed acid fermentation

do not produce H2S, phenylalaninedeaminase or urease, do notutilize citrate as sole carbon source (except some strains of E.

blattae , and E. fergusonii. )

Most are motile, ferment a variety of carbohydrates and

decarboxylae arginine, lysine and/or ornithine.


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E. coliGenetics and Serology

Genetics:

Single, circular DNA molecule, ~4 x 106 base pairs Molecular weight of 4 x 109

Total length of about 1.4mm.

Two strains completely sequenced and genomic

organization is now being characterized many of the genes have been mapped.

Serology:

E. colican be subdivided by somatic (cell-wall) or O

antigens and flagellar or H antigens.

>160 recognized O types and 55 recognized H types

over 8000 possible OH serotypes.

also capsular (K) and fimbrial antigens.


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Virulence Properties of E. coli

Enterotoxins:

at least two types: Heat Stable (ST) and Heat Labile (LT) Verotoxins or Shiga-like toxins (interchangeable terms):

Verotoxin term is based on the reactions of toxins on Vero cells

at least two families of these toxins:

VT1 (SLT I): similar to Siga-toxin (produced by some strains of

Shigella dysenteriae)

VT2 (SLT II) which is only about 50% realted Shiga toxin.

Other Toxins: Cytolethal distending toxin (CLDT), VirCytotoxin, Cytotoxic necrotising

factors (CNF), a possible Enteropathogenic E. coli EPEC) enterotoxin anda possible E. coli Sudden Infant Death Syndrome (SIDS)-toxin.

Haemolysins: extracellular haemolysin known as alpha-haemolysin (many strains)

cell-associated haemolysin, beta-haemolysin, (some strains)

enterohaemolysin: extracellular; Enterohaemorrhagic E. coli (EHEC)


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Virulence Properties of E. coli

Fimbriae: CFAI/CFAII, Type 1 fimbriae, P fimbriae, S fimbriae

most important: K88, K99 and CFA fimbriae associated withenterotoxigenic E. coli(ETEC). They have differing species

specificities.

The p-fimbriae: associated with urinary tract pathogens.

E. colialso produce common fimbriae not associated withvirulence.

Adhesins:

Intimin: non-fimbrial adhesin; causes the intimate association

with target cells in enteropathogenic and

enterohaemorrhagic E. coli.

Associated with the 'attachment and effacement' phenomenon

Causes destruction of the intestinal surface cells.

Other outer membrane proteins can act as adhesins.


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Iron and E. colivirulence

Enterobactin (Enterochelin)

A siderophore: low-molecular weight, high-affinity iron-bindingcompounds

In vitro conditions are iron-limitated; E. coliproduces the

phenolate iron chelator enterobactin, which is also known as

enterochelin.

cyclic triester of 2,3-dihydroxy-N-benzoyl-L-serine and removes

iron from iron-binding proteins and transports it into the bacterial

cell..

Seven chromosomal genes needed to synthesise enterobactin

Used once, because the molecule is cleaved within the E. colicell

and the remnants discarded for the iron to become available. It is

thus a very energy expensive process.


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Iron and E. colivirulence

Aerobactin

Another siderophore

plasmid mediated

associated with virulence in E. coli causing extraintestinalinfections.

A conjugate of 6-(N-acetyl-N-hydroxylamine)-2-

aminohexanoic acid and citric acid.

Forms an octahedral complex with ferric iron.

Can be recycled (unlike enterobactin)

Not bound by serum albumin.


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Iron and E. colivirulenceNon-native Siderophore Use by E. coli:

fungal siderophores such as ferrichrome

coprogen rhodotorulic acid

citrate (inefficient)

Outer Membrane Proteins (OMP)

For iron-loaded siderophores to be adsorbed on the cell surface Also function as bacteriophage and/or colicin receptors.

Other proteins needed by these receptors to move iron into cells.

act as 'gated porins'

Iron from Heme Compounds E. colitakes up iron from haemoglobin preferentially to by

siderophores,

E. colihemolysins may help make heme and hemoglobin iron

available.


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Pathogenic E. coli

Enteric Infections:

Enteroadherent E. coli(EAEC)

Enteroaggregative E. coli(EAggEC)

Enterohaemorrhagic E. coli(EHEC)

Enteroinvasive E.coli(EIEC)

Enteropathogenic E. coli(EPEC)

Enterotoxigenic E. coli(ETEC)

Extraintestinal Infections:

Uropathogenic E. coli(UPEC): urinary tract infections

Neonatal Menigitis E. coli(NMEC).


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Enterohemorrhagic E. coli

Harbor genes for one or more of the virulence attributes known

to associated with the EHEC. shiga toxin(s)

adherence factor(s)

enterohaemolysin

somatic antigens characteristic of many EHEC serogroups,such as O111 or O157.

An E. coli must cell carry a sufficient number of such genes

to cause disease.

Magnitude of exposure or size of infectious dose is alsoimportant.

Dose is very small in comparison with those for most other

enteric pathogens; a few bacteria per dose may cause

infection and illness


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Enteropathogenic E. coli

Cause infantile gastroenteritis.

Certain serotypes are associated with infantile diarrhea. Infantile gastroenteritis with dehydration is an important problem.

Serogroups O26, O55, O111, O119, O125, O126, O127, and O128

are most commonly isolated.

EPEC have declined in the developed world as major causes ofinfantile diarrhea, but remaining very important in the developing

world.

EPEC adhere to the intestinal mucosa to produce a characteristic

"attaching and effacing" lesion in the brush border microvillousmembrane.

EPEC trains belonging to serogroups O26, O111 and O128 have

recently emerged as enterohaemorrhagic E. coli (EHEC).


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Enterotoxigenic E. coli

Major cause of travellers' diarrhea (Montezumas revenge,

Delhi belly, Aztec two-step, etc.) and of diarrhea in childrenin the developing world

Produce an enterotoxin similar to cholera toxin.

They are involved with a condition known as "Non-Vibrio cholerae

cholera-like diarrhoea".

Produce one or both of two enterotoxins:

heat stable enterotoxin (ST) : survives boiling for 30 minures

heat labile enterotoxin (LT): does not survive such boiling

LT response is sensitive to acid pH; ST was not.

LT is closely related to choleragen (CT) the enterotoxin of V.cholerae.

Most ETEC isolated from humans produce colonization

factor antigens which are human specific fimbrial antigens

also a common cause of diarrhea in young animals.


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Enteroinvasive E. coli(EIEC)

Cause positive reaction in the Sereny test (ability to cause

keratoconjunctivitis in guinea pig eyes). a characteristic shared with strains of Shigella

By DNA probes the invasiveness plasmids of both E. coliand

Shigella are identical.

120-140 mD invasiveness plasmids encode all the genesnecessary for the virulence of the EIEC.

Many EHEC are non-motile and anaerogenic.

Account for only a small proportion of diarrhea in non-tropical

countries but, cause high proportions of illness and death, mostly in

warmer seasons

Also important causes of dysentery-like diarrhea in tropical

countries.


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Enteroahherent E. coli(EAEC)

Different patterns of adherence to cultured epithelial

cells.

Localized adherent E. coli(LAEC) by some serotypes

form adherent microcolonies on HEp-2 cells

associated with acute, non-bloody diarrhoea in

children.

Diffusely adhering E. coli(DAEC)

a cause of diarrhea in some studies


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Enteroaggregative E. coli (EAggEC)

3rd type of adherence: enteroaggregative adherence

Bacteria align in parallel rows to cells or glass ('stacked

brick-like).

persistent childhood diarrhoea in South America and India

Illness lasts >14 days. Produce a heat-labile toxin antigenically related hemolysin

but not hemolytic

Produce plasmid encoded heat stable toxin (EAST1)

unrelated to the heat stable enterotoxin of ETEC. Form a distinct scum on the surface of Mueller-Hinton

broth

possibly use as a rapid screening method for EAggEC.


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Uropathogenic E. coli(UPEC) Common cause of urinary tract infections (UTI).

Severity: from asymtomatic to bacteriuria, cystitis and pyelonephritis.

Women more frequently affected than men.

Same serotypes are found in feces and urine of patients, but, UPEC

have virulence factors which enhance their ability to cause infection.

Only some O groups cause UTI: O1, O2, O4, O6, O7, O18 and O75.

Only some K antigens (K1, K2, K3, K5, K12 and K13) cause UPEC. Certain pili (fimbrae), the p-pili, are an important virulence factor

bind to the P-antigen, a blood grouping antigen

bind more to uroepithelial cells of persons with the P or P2 phenotype

other adhesins may also be involved. Production of alpha-haemolysin by UPEC

Production of aerobactin

Several virulence enhance an E. coli's ability to cause UTI

an infecting strain may only express some of them to cause infection

.


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Neonatal Meningitis E. coli(NMEC)

Neonatal menigitis in about 1/2500 live births.

Up to 80% of cases of neonatal menigitis are due to E. coli.

~80% of the isolates possess the K1 capsular antigen.

a 2.8 a-linked homopolymer of N-acetylneuraminic acid (sialic acid)

chemically and immunologically identical to the group B acidic

polysaccharide of Neisseria menigitidis. masks the underlying structures of the bacterial cell surface, preventing

specific antibody responses and the activation of the alternate complement

system from being activated.\

poor immunogen, maybe due to it resembling extracellular matrix proteins.

may also reduce the serum sensitivity of E. coli.

serotypes isolated from meningitis cases often found in maternal faeces;

source; infection occurs at birth.

Reasons for susceptibility of neonates to NMEC is not known.


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Salmonella and Salmonellosis

Belong to Enterobacteriaceae family

Gram-negative bacilli; facultative and flagellated (motile). 3 major antigens:

"H" or flagellar antigen (phase 1 & 2)

"O" or somatic antigen (part of the LPS moiety)

"Vi" or capsular antigen (called "K" in other

Enterobacteriaceae).

Posess LPS endotoxin characteristic of Gram-negative bacteria

composed of an "O polysaccharide ("O" antigen)

"R" core

endotoxic inner "Lipid A".

Endotoxins evoke fever and can activate complement, kinin

and clotting factors.


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Salmonella Illness

Gastroenteritis or salmonellosis

contaminated food or water. Most commonly,

often from contaminated poultry (turkeys and chickens).

S. enteritidis orS. choleraesuis: most commonly isolated

speciesEnteric fever:

transmitted from person to person and involves

S. typhi(no animal reservoirs) reservoirs:

Contaminated food or water with human feces

asymptomatic human carriers


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Salmonella enteritidis


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Salmonella typhi


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Salmonella typhimurium DT 104 A commonly reported phage type ofS. typhimurium

Resistant to many antibiotics: ampicillin, chloramphenicol,

streptomycin, sulphonamides, and tetracyclines. also some additionally resistant to trimethoprim and to

quinolone antibiotics such as ciprofloxacin

Antibiotic resistance emerged as a result of widespread use

both prophylactically and therapeutically. effective antibiotics to control its spread is severely limited.

Associated with cattle, pigs and chickens and other animals.

May infect animals without showing any signs of illness.

Spreads from farm to farm by raw water and other sources Once a farm becomes contaminated it is hard to eradicate;

it survives in dry as well as in wet environments

Illness from S. typhimurium DT 104 is more difficult to treat than

illness from other salmonellae; also has a higher mortality rate.


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Campylobacters

Gram-negative

Curved rod

about 1.5-3 microns

motile via polar

flagella

Microaerophilic

Prefer high CO2


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Campylobacters

Due to Campylobacter jejunior sometimes C. coli.

Normal intestinal flora of many warm-blooded animals chickens and turkeys; also in raw water and raw milk.

Illness in sheep (abortion), dogs and cats (gastroenteritis)

Causes illness in humans

Cause 5-11% of all diarrhea cases in the United States. Symptoms: from mild to severe:

bloody diarrhoea is the most characteristic symptom)

also fever, nausea, abdominal cramps and (seldom) vomiting

duration of illness usually 2-10 days abdominal cramps, may recur for up to 3 months after infection

Complications such as septicaemia, may arise.

The infectious dose may be very low, i.e. 100s of cells

Infants, young children and debilitated people at highest risk


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Complications and Sequelae of

Campylobacteriosis; Guillain-Barre Syndrome

Develop a rare disease of the nervous system beginning

several weeks after the diarrheal illness.

Called Guillain-Barr syndrome

Person's immune system is "triggered" to attack the body'sown nerves

can lead to paralysis lasting several weeks; usually requires

intensive care

About 1 per 1000 reported campylobacteriosis cases leads to

Guillain-Barr syndrome.

Perhaps 40% of Guillain-Barr syndrome cases in this

t b t i d b l b t i i

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