Medical Microbiology I - Lecture12

MEDICAL MICROBIOLOGY I

Lesson 12 Lesson 12

Enterobacteriaceae Part II

Salmonella

Gram negative rods, motile, and facultatively

anaerobic

Oxidase negative, catalase positive, KCN

negative, usually citrate positive and H2S negative, usually citrate positive and H2S

positive, ferments carbohydrates

Most salmonellae are parasites of man, animals

(pigs, cows, goats, etc) and birds (hens, duck,

etc)

S. typhi and S. paratyphi are parasites of man

only

Salmonella

Salmonella on CLED agar

Pathogenesis and Immunity

After ingestion and passage through the

stomach, salmonellae are able to invade and

replicate in the M (microfold) cells located in

Peyers patches of the terminal portion of the Peyers patches of the terminal portion of the

small intestine

These cells typically transport foreign antigens

to the underlying macrophages for clearance


Two separate type III secretion systems

mediate the initial invasion into the intestinal

mucosa (Salmonella pathogenicity island 1

[SPI-1] and subsequent systemic disease [SPI-[SPI-1] and subsequent systemic disease [SPI-

2])

Binding to M cells is mediated by species-

specific fimbriae


The SPI-1 secretion system then introduces

Salmonella-secreted invasion proteins (Sips or

Ssps) into the M cells, resulting in

rearrangement of the host cell actin with rearrangement of the host cell actin with

subsequent membrane ruffling

The ruffled membrane surround and engulf

salmonellae, leading to intracellular replication

in the phagosome with subsequent host cell

death and spread to adjacent epithelial cells

and lymphoid tissue


The inflammatory response confines the infection to the GIT, mediates the release of prostaglandins, and stimulates cAMP and active fluid secretion

Salmonella species are also protected from stomach acid and the acid pH of the phagosome by an acid tolerance response (ATR) gene

Catalase and superoxide dismutase are other factors that protect the bacteria from intracellular killing

Epidemiology

Salmonella can colonise virtually all animals including poultry, reptiles, livestock, rodents, domestic animals, birds, and humans

Animal-animal spread and the use of Animal-animal spread and the use of Salmonella-contaminated animal feeds maintain an animal reservoir

S. typhi and S. paratyphi are highly adapted to humans and do not cause disease in non-human hosts

Epidemiology

Transmission: contaminated food products

(e.g. poultry, eggs, dairy products and foods

prepared on contaminated work surfaces),

faecal-oral route (especially in children)faecal-oral route (especially in children)

The infectious dose for S. typhi infections is

low, so person-person spread is common

Large inoculum is required for symptomatic

disease to develop with other Salmonella sp.

Epidemiology

The infectious dose is lower for people at high

risk for disease because of age,

immunosuppression or underlying disease

(e.g. leukaemia, lymphoma, sickle cell (e.g. leukaemia, lymphoma, sickle cell

disease), or reduced gastric acidity

Clinical Diseases

1. Enteritis

The most common form of salmonellosis

Symptoms generally appear 6 - 48 hours after the consumption of contaminated food or the consumption of contaminated food or water, with the initial presentation consisting of nausea, vomiting, and non-bloody diarrhoea

Fever, abdominal cramps, myalgias, and headache are also common

Symptoms can persist from 2 days to 1 week before spontaneous resolution

Clinical Diseases

2. Septicaemia

All Salmonella species can cause bacteraemia, although infections with S. cholerasuis, S. paratyphi and S. typhi more commonly lead to bacteraemia phasebacteraemia phase

The risk of Salmonella bacteraemia is higher in paediatric and geriatric patients as well as patients with AIDS

Symptoms: Gram negative bacteraemias; 10% localised suppurative infections, such as osteomyelitis, endocarditis, and arthritis

Clinical Diseases

3. Enteric fever

S. typhi produce febrile illness - typhoid fever

Mild: S. paratyphi A, S. schottimuelleri, and S.

hirschfeldii - paratyphoid feverhirschfeldii - paratyphoid fever

Bacteria pass through the cell lining the

intestines and are engulfed by macrophages -

replicated (liver, spleen, and bone marrow)

Symptoms: fever, headache, myalgias, malaise,

and anorexia, persist for a week or longer

Clinical Diseases

4. Asymptomatic colonisation

The species of Salmonella responsible for

causing typhoid and paratyphoid fevers are

maintained by human colonisationmaintained by human colonisation

Chronic colonisation for more than 1 year after

symptomatic disease develops in 1 - 5% of

patients, the gall bladder being the reservoir in

most patients

Shigella

Gram negative rods, aerobic and facultatively

anaerobic, non-motile, mostly catalase

negative, oxidase negative and ferment

carbohydrates mostly without gas, citrate and carbohydrates mostly without gas, citrate and

KCN negative

Shigellae are found in the intestinal tract of

man

They are strict human parasites

Shigella

Shigella Infection


Shigella cause disease by invading and replicating in cells lining the colonic mucosa

Structural gene proteins mediate the adherence of the organisms to the cells as well adherence of the organisms to the cells as well as their invasion, intracellular replication, and cell-cell spread

Shigella species appear unable to attach to differentiated mucosa cells; rather, they first attach to and invade the M cells located in Peyers patches


The type III secretion system mediates

secretion of four proteins (IpaA, IpaB, IpaC,

IpaD) into epithelial cells and macrophages

Shigella are able to lyse the phagocytic Shigella are able to lyse the phagocytic

vacuole and replicate in the host cell

cytoplasm; unlike Salmonella, which replicate

in the vacuole


With the rearrangement of actin filaments in

the host cells, the bacteria are propelled

through the cytoplasm to adjacent cells, where

cell-cell passage occurs - protected from cell-cell passage occurs - protected from

immune-mediated clearance

Shigella survive phagocytosis by inducing

apoptosis - release of interleukin-1, resulting

in the attraction of polymorphonuclear

leukocytes into the infected cells - destabilises

the integrity of the intestinal wall


S. dysenteriae produce an exotoxin, Shiga toxin

Like the toxin produced by EHEC, the Shiga toxin has one A subunit and five B subunits

The B subunits bind to a host cell glycolipid The B subunits bind to a host cell glycolipid (Gb3) and facilitate transfer of the A subunit into the cell

The A subunit cleaves 28s rRNA in the 60s ribosomal subunit, thereby preventing the binding of aminoacyl-transfer RNA and disrupting protein synthesis

Epidemiology

4 species consisting of more than 45 O

antigen-based serogroups have been

described: S. dysenteriae, S. flexneri, S. boydii,

and S. sonneiand S. sonnei

S. sonnei is the most common cause of

shigellosis in the industrial world, and S.

flexneri is the most common cause in

developing countries

Epidemiology

Shigellosis is primarily a paediatric disease; 70% of all infections occur in children younger than 15 years

Endemic disease in adults is common in male Endemic disease in adults is common in male homosexuals and in household contacts of infected children

Epidemic outbreaks of disease occur in daycare centers, nurseries, and custodial institutions

Epidemiology

Transmission: faecal-oral routes, primarily by

people with contaminated hands and less

commonly in water or food

Because as few as 200 bacilli can establish Because as few as 200 bacilli can establish

disease, shigellosis spreads rapidly in

communities where sanitary standards and

the level of personal hygiene are low

Clinical Diseases

Symptoms: abdominal cramps, diarrhoea,

fever, and bloody stools

Clinical signs and symptoms of the disease

appear 1 - 3 days after bacilli are ingested appear 1 - 3 days after bacilli are ingested

The bacilli initially colonise the small intestine

and begin to multiply within the first 12 hours

Abundant pus, neutrophils, erythrocytes, and

mucus in stool

Clinical Diseases

Infection is generally self-limited, although

antibiotic treatment is recommended to

reduce the risk of secondary spread to family

members and other contactsmembers and other contacts

Asymptomatic colonisation of the organism in

the colon develops in a small number of

patients and represents a persistent reservoir

for infection

Other Enterobacteriaceae

1. Klebsiella

Members of the genus Klebsiella have a

prominent capsule that is responsible for the

mucoid appearance of isolated colonies and mucoid appearance of isolated colonies and

the enhanced virulence of the organisms in

vivo

K. pneumoniae can cause community-

acquired primary lobar pneumonia


Alcoholics and people with compromised pulmonary function are at increased risk for pneumonia because of their inability to clear aspirated oral secretions from the lower respiratory tractrespiratory tract

Pneumonia due to Klebsiella species frequently involves necrotic destruction of alveolar spaces, formation of cavities, and the production of blood-tinged sputum


2. Proteus

Infection of the urinary tract with Proteusmirabilis is the most common disease produced by this genusproduced by this genus

P. mirabilis produces large quantities of ureases, which splits urea into carbon dioxide and ammonia

This process raises the urine pH and facilitates the formation of renal stones


The increased alkalinity of the urine is also

toxic to uroepithelium

Despite the serologic diversity of these

organisms, infection has not been associated organisms, infection has not been associated

with any specific serogroup

The pili on P. mirabilis may decrease its

virulence by enhancing the phagocytosis of

the bacilli, unlike E. coli

Laboratory Diagnosis

Specimens: sterile specimens such as CSF and tissue collected at surgery should be cultured on blood agar (non-selective media)

Specimens: contaminated specimens such as Specimens: contaminated specimens such as sputum and faeces should be cultured on MacConkey and eosin methylene blue (EMB) agar

This is to separate lactose fermenting from non-lactose fermenting strains


Highly selective or organism-specific media are

useful for the recovery of organisms in stool,

whereas abundance of normal flora can

obscure the presence of these important obscure the presence of these important

pathogens

Cold enrichment permits the growth of

Yersinia but inhibits or kills other organisms


Biochemical identification

Triple sugar iron agar, indole, methyl red, Voges-Poskauer, citrate, malonate, urease,

pheulalanine deaminase, orthonitrophenyl--D-galactopyranoside test ( -galactosidase test), galactopyranoside test ( -galactosidase test), arabinose fermentation

Serologic classification

Serotyping specific pathogenic strains: usefulness of this procedure is limited

Treatment, Prevention and Control

Antibiotic therapy must be guided by in vitro susceptibility test results and clinical experience

Some organisms, such as E. coli and P. Some organisms, such as E. coli and P. mirabilis are susceptible to many antibiotics, other can be highly resistant

Susceptible organisms exposed to sub-therapeutic concentrations of antibiotics in a hospital setting can rapidly develop resistance


Symptomatic relief, but not antibiotic

treatment, is usually recommended for

patients with E. coli or Salmonella

gastroenteritis because antibiotics can prolong gastroenteritis because antibiotics can prolong

the faecal carriage of these organisms or

increase the risk of secondary complications

(e.g. HUS with EHEC infections in children)


Avoid risk factors:

The unrestricted use of antibiotics that can select

for resistant bacteria

The performance of procedures that traumatise The performance of procedures that traumatise

mucosal barriers without prophylactic antibiotic

coverage

The use of urinary catheters

Exogenous infection with Enterobacteriaceae

is theoretically easier to control


These bacteria are ubiquitous in poultry and eggs

Shigella organisms are predominantly transmitted in young children, but it is difficult to interrupt the faecal-hand-mouth to interrupt the faecal-hand-mouth transmission responsible for spreading the infection in this population

Prevention and control: education and introduction of appropriate infection-control procedure (e.g. hand washing, proper disposal of soiled diapers and linens)


Vaccination with formalin-killed Yersinia pestis has proved effective for people at high risk

Chemoprophylaxis with tetracycline has also proved useful for people in close contact with a patient with pneumonic plaguepatient with pneumonic plague

Improvement in the live, attenuated S. typhi vaccines gives significant protection, persist for up to 5 years

Vaccination with purified Vi antigen (the polysaccharide) capsular antigen of S. typhi associated with virulence), are also protective

Medical Microbiology I - Lecture12

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