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Microbiology - Lecture 10 - Shigella and Salmonella

Make sure you know the learning objectives.
Describe the differences and similarity between Shigella and Salmonella.

Describe the characteristics of invasive diarrhea.

Analyze different cases of infectious diarrhea to make a preliminary diagnosis.

Describe the main features of enteric fever.
What is the gram nature of shigella?
It is a gram negative rod.
Where do you find shigella normally? What is the major pathogen of shigella? What does it usually cause?
Natural habitat is intestinal tract of humans and other primates.

Major pathogen is Shigella dysenteriae, but Shigella sonnei is more common in the U.S.

Causes dysentery, which is an invasive bloody diarrhea.
What is the differential?

S. D. was seen by you in the Emergency Room. 32 y/o man with 2 day h/o bloody diarrhea. Just returned from a trip to India. Sudden onset of fever, crampy abdominal pain, pain with bowel movement.
Diarrhea initially watery but firmer with blood and mucus on second day.
Labs: stool positive for WBCs and RBCs
Stool culture: Gram negative rods (non-lactose fermenter); clear colonies on Hektoen agar.
Shigella sonnei dysentery.
What does shigella look like on a slide?
Slender red bacilli.
What are the three microbiology culture characteristics of shigella?
Facultative anaerobe but grows best aerobically.

Non-lactose fermenter. So will have colorless colonies on MacConkey agar.

Hektoen enteric agar: green colonies.
What is Hektoen enteric agar?
Hektoen enteric agar:

Selective medium: selective growth of Salmonella/Shigella (contains deoxycholate which suppresses other Enterobacteriaceae and Gram positive bacteria).

Differential medium: distinguishes Shigella (green colonies since no H2S production) from Salmonella (black colonies since have H2S production).
What is the motility nature of shigella vs. salmonella?
Shigella: Non-motile.

Salmonella: Motile.
What is the gas production from glucose fermentation of shigella vs. salmonella?
Shigella: No gas.

Salmonella: Gas produced.
What is the H2S production of shigella vs. salmonella?
Shigella: No H2S.

Salmonella: H2S produced.
What is the pathogenesis of Shigella dysenteriae?
Produces heat-labile exotoxin called Shiga toxin.

2 subunit A-B exotoxin that inhibits protein synthesis.

Acts as enterotoxin: produces bloody diarrhea.

Associated with hemolytic uremic syndrome.

Toxin is similar to Shiga toxin of Shiga toxin-producing E. coli (STEC), but is not encoded by a bacteriophage.

Can give antibiotics for treatment.
What invasion do you have with shigella dysenteriae?
There is local invasion of the distal ileum and colon.

Invades intestinal epithelial cells by inducing phagocytosis (Type III secretion system).

Infection almost always limited to the gastrointestinal tract (don't see bacteremia).

Different from STEC and EHEC, which does not invade.
What is the diarrhea progression of shigella dysenteriae?
Early nonbloody, voluminous diarrhea (secretory). Caused by enterotoxin acting in small intestine.

Later, dysentery with blood and pus in stools (inflammatory bloody diarrhea). Caused by/due to invasion of large intestine!!!
What clinical symptoms do you see with Shigella dysenteriae?
Short incubation period: 1-2 days.

Sudden onset of abdominal pain, fever, watery diarrhea. Caused by enterotoxin acting in small intestine.

On second day: stools become firmer and contain blood, mucus, neutrophils. Caused by invasion of large intestine.

Painful bowel movements (tenesmus = rectal spasm) with lower abdominal pain.
What is the epidemiology of dysentery?
Highly communicable - unique to Shigella. Infective dose is 10^2 organisms (much less than Salmonella or Vibrio).

Person to person spread (oral-fecal route).

Less frequently spread by contaminated food or water (more important in developing countries).

Most cases in U.S. are caused by Shigella sonnei, which causes milder disease than Shigella dysenteriae.

Most cases of Shigella will resolve without medical attention.

In 2000 had multi-state outbreak linked to 10-layer mexican dip.
What are the epidemiological control efforts for shigella?
Sanitary conditions (water, food, milk, sewage disposal).

Isolation of patients and disinfection of excreta.

Detection of subclinical cases and carriers, especially food handlers.

Antibiotics for infected individuals. Shigella is always a pathogen, not a commensal even in carriers.
What are the diagnostic tests for shigella dysenteriae?
Stool and rectal swabs for Gram stain and culture.

Examine stool for white blood cells (fecal leukocytes) and red blood cells with methylene-blue stain.
What is the immunity associated with shigella dysenteriae?
Circulating antibodies develop after infection but they are not protective.

No vaccine.
What is the treatment for shigella dysenteriae?
Replace fluids and electrolytes.

Antibiotics for dysentery caused by S. dysenteriae:
--Ciprofloxacin or trimethoprim-sulfamethoxazole.
--Shorten duration of symptoms.
--Reduce incidence of hemolytic uremic syndrome.
--Reduce spread to other people.

Antibiotics not necessary for S. sonnei because infection is milder and self-limited. Shigella not activated by bacteriophage so give antibiotic.

Avoid agents that reduce intestinal motility, e.g. opioids - Will prolong symptoms.
What is the reservoir for shigella?
Humans and primates only.
Shigella summary.
Gram negative rod: non-lactose fermenter.

Primary pathogen.

Highly communicable: small infectious dose.

Produces Shiga toxin, an exotoxin that functions as an enterotoxin.

Causes dysentery which is characterized by fever and bloody inflammatory diarrhea.

Infection is LIMITED to the gastrointestinal tract: NO bacteremia.
What is the gram characteristic and shape of salmonella?
Gram negative bacilli.
What is salmonella is the most common cause of?
Most common cause of bacterial foodborne infection in U.S.

Most common infection is gastroenteritis, which is caused by Salmonella Typhimurium and Salmonella Enteritidis.
What is the major salmonella pathogen?
Major pathogen is Salmonella Typhi, which causes enteric fever (typhoid fever).
What is an important treatment issue with salmonella?
Antibiotic resistance has been and continues to be a big issue.
What is the motile nature of salmonella?
Most species are motile with peritrichous flagella.
What is the lactose fermentation nature of salmonella?
Do NOT ferment lactose (colorless colonies on MacConkey agar).
What agar can you use to differentiate Salmonella?
Hektoen enteric agar is a differential medium to distinguish Salmonella from Shigella.

Salmonella will look black due to the production of H2S.
What are the microbiological characteristics of salmonella?
Different strains possess different somatic O antigens and flagellar H antigens.

Loss of O antigen causes a change from smooth to rough colonies.

Some strains have a capsular antigen called the Vi antigen (stands for virulence). =>(Typhi, Paratyphi C and Dublin).
What is the proper classification for salmonella?
Human infections are caused by one species, Salmonella enterica.

Subdivided into > 2,500 serotypes based on differences in surface antigens.

Correct nomenclature:
Salmonella enterica serotype Typhi

But often referred to as Salmonella Typhi or Salmonella typhi.
What are the three main clinical groups of salmonella?
1. Nontyphoidal Salmonella strains, which cause gastroenteritis.

2. Strains that cause enteric fever.

3. Strains that cause bacteremia.
Describe the salmonella group that causes gastroenteritis. What are the serotypes of salmonella for this group?
Animal to human transmission.

Inflammatory diarrhea w/ neutrophils.

Bacteremia is a complication.

Describe the salmonella group that causes enteric fever. What are the serotypes of salmonella for this group?
Human to human transmission.


Constipation or diarrhea, no neutrophils.

Paratyphi A
Paratyphi B
Paratyphi C
Describe the salmonella group that causes bacteremia. What are the serotypes of salmonella for this group?
Animal to human transmission.

Bacteremia and mycotic aneurysm.

What is the dose pathogenesis for salmonella?
Infective dose is relatively high
10^5 - 10^8 organisms.
What host factors contribute to the resistance of salmonella?
-Gastric acidity.

-Normal intestinal flora.

-Local intestinal immunity.

Note: Salmonella contains Type III secretion systems for secreting bacterial effector proteins into host cells.
How does salmonella perform invasion?
Via membrane ruffling of intestinal epithelial cells.

This is why have neutrophils in feces!
In what three bacteria do you find PMNs in the feces?
Shigella, salmonella, and campylobacter.
What is the most common salmonella infection in the US?
What is gastroenteritis from salmonella caused by?
Caused by nontyphoidal Salmonella.

Mostly Salmonella Typhimurium and Salmonella Enteritidis.

But also ≥2500 other serotypes.

***The key here is nontyphoidal.
What is the clinical presentation of gastroenteritis from salmonella?
Presentation: Acute onset of fever, abdominal cramps, diarrhea.

Inflammatory diarrhea (neutrophils in stools) like Shigella and Campylobacter.

Diarrhea is sometimes bloody (if very bloody, think of Shigella or Shiga Toxin-producing E. coli).

Diarrhea can be severe.

Incubation period: 6-72 hours after eating contaminated food.

Usually resolves spontaneously in 4-7 days
But prolonged fecal shedding: ~ 1 month in adults and 7 weeks in children < 5 years.

May be more severe in the elderly and infants which leads to bacteremia.

Stool cultures positive and fecal leukocytes present.

Blood cultures usually negative.
What happens if you give a person with salmonella antibiotics?
Give antibiotics it increases shedding and makes person carrier of Salmonella.
What is the differential?

S. T. M. was seen by you in the Emergency Room. 25 y/o man with 1 day h/o fever and diarrhea. c/o fever, headache, nausea, vomiting and profuse diarrhea.
Exam: 38ºC
Labs: stool: fecal WBCs
Stool cultures: Gram negative rods (non-lactose fermenter); black colonies on Hektoen agar.
Blood cultures: negative.
Patient's roommate is sick with same symptoms and they wondered whether it was from some chicken that they had cooked.
Salmonella Typhimurium gastroenteritis.
What was the largest food borne salmonella outbreak in the US in the past decade?
Had salmonella outbreak in April-June 2008 due to contaminated Jalapeno peppers.

Side note: large egg recall in August 2010 due to salmonella.
What is a current salmonella outbreak?
Have multistate outbreak of Salmonella Typhimurium infections linked to cantaloupes.
MMR for salmonella.
From eating group beef.

From chicks and ducklings.

From rodents.

From african pygmy hedgehog.

Unpasteurized milk.

Pet turtles.

Aquatic frogs.
When do you see salmonella bacteremia?
5% of gastroenteritis cases have bacteremia.

More likely in immunocompromised individuals.

Risk of developing focal infection.
What are two other diseases that can be causes my salmonella?
1. Infective endarteritis. Especially involving abdominal aorta.

2. Osteomyelitis. More common with sickle cell anemia (S/S hemoglobin).
What diseases is salmonella bacteremia associated with?
Nontyphoidal Salmonella is common cause of bacteremia in HIV and malaria patients in Africa, and the leading cause of death.

May be recurrent.

Can be life-threatening.
The type of Salmonella usually associated with infections in humans is nontyphoidal Salmonella.
What is the treatment for salmonella gastroenteritis?
Replace fluids and electrolytes.

Antibiotics not routinely recommended for mild to moderate salmonellosis.
-May prolong fecal shedding by suppressing protective effect of endogenous GI bacteria.

Antibiotics reserved for certain circumstances:
-Severe cases (invasive or focal disease suspected or likely).
-Patients with risk factors (e.g. > 50 years old with atherosclerotic lesion that can be seeded by bacteremia).
How are antibiotics are problem for salmonella gastroenteritis?
Antibiotic resistance is a problem. Can no longer use conventional antibiotics (ampicillin, chloramphenicol, SMX-TMP) for empiric therapy because of high resistance rates.

First-line agents:
-Fluoroquinolone (ciprofloxacin).
-3rd generation cephalosporin.
What type of fever can salmonella cause?
Enteric fever.

Called typhoid fever if caused by Salmonella Typhi.

Can also be caused by Salmonella Paratyphi A, B, C
-Cause milder disease and are less common than S. Typhi.

Not common in U.S.
<1% of Salmonella infections in U.S.
Most cases acquired abroad (Indian subcontinent)

Still a big problem worldwide:
-200,000 deaths/year
-Mainly in infants, children and adolescents
Describe the pathogenesis of enteric fever.
Infection always spreads beyond the GI tract:
-Bacteria invade intestinal epithelial cells.
-Multiply in intestinal lymphoid tissue (Peyer's patches).
-Spread to liver and spleen inside macrophages: Ability to survive inside macrophages is important for pathogenesis!!!
What are the clinical symptoms of enteric fever?
High fever and abdominal symptoms (abdominal pain, enlarged spleen and liver).

Constipation more common that diarrhea (1/3 pt).

Rash in some patients: Rose spots on skin of abdomen and chest.

Positive blood cultures (Salmonella is in macrophages) and stool cultures.
---This is how to diagnosis since clinical doesn't tell much.

Carriers ("Typhoid Mary") can harbor salmonellae in gall bladder and shed the organism for years.
-Cholecystectomy for chronic carrier.
-Carriers should not be food handlers.
What unique clinical symptoms will you see in enteric fever?
Rose spots. These are little red dots.
What happens if you have enteric fever and ulceration of Peyer's patches?
Clinical manifestation: Intestinal hemorrhage and perforation.

- Main cause of death if not treated.
What is the treatment for enteric fever?
Multidrug resistance is widespread for S. Typhi.

Antibiotic choices are similar to Salmonella gastroenteritis.

Ciprofloxacin is widely used as the first-line agent.

Alternatives: Third generation cephalosporin or azithromycin.
What is the differential?

T. F. has just returned from a family wedding in India. 27 y/o woman with 10 day h/o fever. Gradual onset of malaise and headache. Also had fever, faint macular rash on trunk
Initially had constipation, then diarrhea.
Immunizations: traveling companion, who isn't sick, received typhoid vaccine (patient didn't).
Exam: 40ºC, HR 60, slightly enlarged liver & spleen.
Labs: Blood and stool cultures: Gram negative rods (non-lactose fermenter).
Salmonella typhi: Enteric Fever.
What is the epidemiology of salmonella?
Animals are the major reservoir for most Salmonella.
-Poultry: fecal contamination during processing.
-Eggs: transovarial transmission in chicken
-Less frequently: dairy products, meat, fresh produce, contaminated water, pets (reptiles).

But Salmonella Typhi, and Salmonella Paratyphi A, B, C only colonize humans.
-Infection acquired through fecal contamination from individual who has enteric fever or is a chronic carrier.

Transmission by oral route from contaminated food and drink (fecal contamination).

Control effort:
-Thoroughly cook meat (especially poultry) and eggs.
What is the immunity that we have for salmonella?
Some degree of natural immunity after Salmonella Typhi and Salmonella Paratyphi infection.
-Due to antibodies to O and Vi antigens.
Can you be vaccinated against salmonella gastroenteritis?
There is no vaccine against Salmonella gastroenteritis.
What vaccines for typhoid are available?
Two typhoid vaccines available (against Salmonella Typhi, not Paratyphi).
-Mainly for travelers to endemic countries.
-Oral: live attenuated vaccine (every 5y).
-Intramuscular injection: purified Vi capsule (every 2y).
-Killed vaccine is discontinued.

Pro Tip: If patients get typhoid fever vaccine they can still get enteric fever.
Salmonella summary.
Gram negative rod: non-lactose fermenter.

Primary pathogen.

Salmonella Typhimurium and Salmonella Enteritidis cause gastroenteritis, a self-limited illness localized to the gastrointestinal tract.

Salmonella Typhi (human host reservoir) and Salmonella Paratyphi cause enteric fever, a systemic illness that spreads from the intestine via the bloodstream to the liver and spleen.

Transmission of Salmonella is by oral route from contaminated food and drink.
Shigella and Salmonella Concepts.
1. Fecal leukoyctes
WBCs in stool = inflammatory diarrhea.

In general, non-inflammatory diarrhea is caused by enterotoxins or viral causes.

Inflammatory diarrhea is caused by local invasion of intestinal epithelial cells.

2. Facultative intracellular bacteria
Able to survive and replicate inside host cells, but also able to survive as free-living bacteria.

Invasion: bacteria have evolved mechanisms for binding and entry into the host cell.

Host cells: intestinal epithelial cells.

3. Non-invasive vs. locally invasive vs. systemic infection.

Non-invasive: Cholera (Vibrio cholerae)
- Disease caused by enterotoxin. Doesn't activate immune system.

Locally invasive: Dysentery (Shigella dysenteriae). Salmonella gastroenteritis
- Local invasion of intestinal epithelial cells

Systemic: Typhoid fever (Salmonella Typhi)
- Spread via lymphatics and bloodstream

4. Carrier
Salmonella Typhi:

Individual can be a carrier of an infectious organism for years without having symptoms.

May have previously been infected and then recovered, or may never have had symptoms.

Can spread infection to others.