Microbiology: Lower GI infections

Question 1

What are characteristics of picornaviruses?

Answer 1

non-enveloped, icosahedral capsid
(2, 3, 5-fold symmetry)

+ssRNA

RNA is infectious

Question 2

What is the picornavirus replication cycle?

Answer 2

1. virus binds to cellular receptor and genome is uncoated
2. RNA is translated in cytoplasm. Virus RNA is a single strand producing a polyprotein that is self-cleaving, and self-assembling.
3. Viral +RNA is copied to form -RNA, which are copied again to make replications of +RNA (by viral RNA pol)
4. Newly synthesized virus particles are assembled and released from cell by lysis

Question 3

What is the Cytopathic Effect?

Answer 3

It is the in vitro cellular response to enterovirus infection

–> infected cells shrivel up and die

Question 4

What are characteristics of poliovirus?

Answer 4

it is a picornavirus

• 3 serotypes (1, 2, and 3) - no common antigen
• Identical physical properties
• Humans are only susceptible hosts

Question 5

What are the options for poliovirus vaccination?

Answer 5

**Salk:** Inactivated polio vaccine
 formalin inactivated (killed)

Sabin: Oral polio vaccine
attenuated (live)

Question 6

Why is poliovirus 3 risky to vaccinate against wth live attentuated virus?

Answer 6

It is at risk of reversion and infecting host with virus

Question 7

What are characteristics of Clostridia?

Answer 7

Anaerobic

Spore-forming

Gram + rods

Agents of:
Botulism
Tetanus
Gas Gangrene,
Food poisoning,
pseudomembranous colitis

Question 8

What are the virulence factors of C. difficile?

Answer 8

Toxin A: an enterotoxin

• *Toxin B**: potent, necrotizing cytotoxin
• believed to be responsible for clinical manifestations

Question 9

What is C. difficile-associated disease?

Answer 9

Associated with pseudomembranous enterocolitis

-> ranges frmo milid diarrhea to a severe, necrotizing process involving the large intestine

–> occurs as a complication of antibiotic therapy

Question 10

What are treatments for C. difficile infection?

Answer 10

Stop antibiotics that lead to C. diff overgrowth

Give oral metronidazole (or vancomycin if necessary)
–> remember, Abx not useful against spores and recurrence is possible

Fecal transplants for recurring illness

Question 11

What lab tests are used to Dx C. difficile?

Answer 11

Stool culture for C. diff

Toxin testing for C. diff

Molecular PCR tests

Question 12

What are the main types of foodborne illness?

Answer 12

1. Intoxications caused by toxin pre-formed in food
   - short incubation period (3-6hrs)
   - C. botulinum, S. Aureus, B. cereus

• *2. Intoxications by toxins manufactured in body**
• Live organisms ingested
• C. perfringens, ETEC, V. cholerae
• *3. Intestinal invasive diseases**
• Salmonella, Campylobacter

Question 13

What are three common pre-formed toxins that cause food poisoning?

Answer 13

1. Botulinum Toxin: Produced by C. botulinum
in smoked, vacuum-packed, or canned foods
Spores germinate and produce toxin
Ingested toxins produce flaccid paralysis

2. Staphylococcal Enterotoxin: Superantigens produced by S. aureus that stimulate excessive amounts of cytokines –> Nausea, vomiting

3. Bacillus cereus toxin: Emetic toxin often occurs after eating fried rice

Question 14

What are two possible results of Bacillus cereus infection?

Answer 14

• Emetic form: Intoxication
  (occurs w/fried rice)
• Diarrheal form: Infection
  Ingestion of bacteria from food
  Bacteria make heat labile enterotoxin in gut, producing diarrhea

Question 15

What are characteristics of Clostridium perfringens food poisoning?

Answer 15

• Common in USA, freq. with cooked beef or other meat
• Occurs after large number of vegetative cells are ingested and sporulate in the gut –> enterotoxin produced
• Incubation period 8-12hrs, usuallly a self-limiting disease

Question 16

What is the only entamoebiasis infection considered to be pathogenic?

Answer 16

Entamoeba histolytica is only pathogenic amoebae

• However, presence of non-pathogenic amoebae indicates fecal contamination and where commensal non-pathogenic amoebaes are found, pathogenic are likely to be as well

Question 17

What is amebiasis?

Answer 17

1. Amebiasis is infection with Entamoeba histolytica
2. Invasion of trophozoites into intestinal or extra-intestinal tissue

Question 18

What is a Cyst Passer?

Answer 18

A patient infected with Entamoeba histolytica, who presents without symptoms

Question 19

What is the pathogenesis of amebiasis?

Answer 19

E. histolytica secretes enzymes that lyse tissue

- Small ulcers form –> ulcers deepen –> capillaries rupture –> parasite feeds on tissue and blood and multiplies

Ulcers present as “flask shaped” uclers
–> start in sigmoid colon and move deeper into colon as disease progresses

Question 20

What are symptoms of intestinal amebiasis?

Answer 20

Incubation period (1-4months) is variable

Symptoms first occur with diarrhea leading to amoebic dysentery with mucoid, bloody stools

Lactose Intolerance (may be permanent)

Acute:
weight loos
Abdominal pain
tenesmus
fever

Chronic:
constipation alternating with diarrhea

Question 21

What is pathogenesis of extra-intestinal amebiasis?

Answer 21

• Ulcers perforate bowel
• Trophozoites may be carried to:
  Liver
  Brain
  kidney
  skin
  others
• Causes ulcerations and abcesses in these organs
• Amebic abcesses are sterile with no cysts present
  Trophozoites found in periphery of ulcer

Question 22

What are symptoms of extra-intestinal amebiasis?

Answer 22

• Depends on organ infected

Liver: RUQ pain, fever, hepatomegaly, elevated diaphragm, mas in RUQ

Lung: cough, bloody sputum, possibly trophozoites

Question 23

How is amebiasis diagnosed?

Answer 23

Intestinal:
Direct microscopy of mucus sample
- trophozoites can be seen
–> sometimes ingesting RBCs

Extra-intestinal:
Needle aspiration of abscess
- red/white, pinkish color
- Examine pus for motile trophozoites
- Gram stain pus for bacteria (should be sterile)

Serology: (best for extra-intestinal)
Gel diffusion
Hemagglutination
ELISA

Question 24

What is treatment for amebiasis?

Answer 24

Metronidazole

Other options:
Tinidazole
Paromomycin

Question 25

What is the pathogenesis of giardia lamblia?

Answer 25

Fecal to oral transmission, often due to drinking infected water

Cycsts can survive form months (even in chlorinated water)

Once ingested, can form a physical obstruction oto absorption by adhering to epithlia of small intestine

Question 26

What are symptoms of Giradia infection?

Answer 26

Range from none to acute, explosive diarrhea to malabsorption

Incubation period: 1-14 days

Acute:
Diarrhea
Abdominal Pain
Nausea and vomiting
bloating
Flatulence
Steatorrhea

Chronic:
malabsorption
weight loss
debilitation

Question 27

How is Cryptosporidium parvum contracted?

Answer 27

Waterborne spores

Found in swimming pools and unpasteurized apple cider

Question 28

What are clinical features of cryptosporidium parvum?

Answer 28

Asymptomatic in healthy people

Life-threatening in immunocompromised

• -> watery diarrhea, vomiting, abdominal pain
• *Chronic wasting disease** that can be assoc. with pulmonary disease

Question 29

How is Cryptosporidiosis diagnosed and treated?

Answer 29

Dx:
Acid fast staining of stool
Immunofluorescenct antibody staining is best

Treatment:
Nitazoxanide may help
otherwise, no established therapy

Question 30

How is Cyclospora cayetanensis contracted?

Answer 30

Transmission through water and fresh produce

Most common in tropical and subtropical areas
(Nepal)

Question 31

What are clinical features of Cyclospora infection? Treatment?

Answer 31

Watery diarrhea - WBC negative
(can be severe)

Incubation period of ~1week

Untreated infections can last 10-12 weeks

Treat with Trimehtroprim/sulfa

Question 33

What are causes of Bloody Diarrhea?

Answer 33

IBD (Ulcerative Colitis)

Infectious (EHEC, E. histolytia, yersinia, shigella)

Ischemic colitis

Radiation proctitis