7 Enteric Gram Negative Bacilli (34)

Question 1

Case
A 45-year-old woman presents with a 2-day history of abdominal cramps and bloody diarrhea. She has just returned from her vacation in India, where she ate many of the local foods. Her blood pressure is 120/80 mmHg, pulse rate is 84/minute, and temperature is 37.8°C. Stool examination reveals many polymorphonuclear leukocytes and aerobic Gram-negative non-motile organisms. From the stool culture, a Gram-negative rod was isolated that forms non-lactose fermenting colorless colonies on eosin-methylene blue (EMB) agar.

What organism is the most likely cause?

1 Shigella dysenteriae
2 Escherichia coli
3 Vibrio cholerae
4 Vibrio parahaemolyticus
5 Campylobacter jejuni

Answer 1

Shigella dysenteriae

Shigella dysenteriae is the causative organism in this case. Shigella are Gram-negative non-motile rods that form non-lactose fermenting (colorless) colonies on MacConkey’s or EMB agar. On TSI agar, they cause alkaline slant and an acid butt, with no gas and no H2S. After an incubation period of 1-4 days, symptoms begin with fever and abdominal cramps, followed by diarrhea, which may be watery at first but later contains blood and mucus. The disease varies from mild to severe depending on 2 major factors: the species of Shigella and the age of the patient. Young children and elderly people are the most severely affected. Treatment includes correction of dehydration, electrolyte imbalance, and antibiotic therapy in severe cases.
Escherichia coli are Gram-negative rods that ferment lactose, a property that distinguishes it from the 2 major intestinal pathogens, Shigella and Salmonella. It produces dark blue-black colonies with a greenish metallic sheen on EMB agar.
Vibrio cholerae is a Gram-negative, highly motile, non-spore forming comma-shaped rod, which causes characteristic watery diarrhea/cholera. It does not ferment lactose.
Vibrio parahaemolyticus is a comma-shaped Gram-negative rod involved in a foodborne illness known as seafood poisoning. It produces watery diarrhea.
Campylobacter jejuni is a comma-shaped Gram-negative rod. It needs to be cultured on a special agar (e.g., Skirrow’s agar) at 42°C in high CO2 and low O2 conditions. It produces enterocolitis.

Question 2

The major property that distinguishes E. coli from the two other intestinal pathogens–Shigella and Salmonella–is that it ferments:

1 Lactose
2 Maltose
3 Fructose
4 Glucose
5 Acetate

Answer 2

Lactose

The major property that distinguishes E. coli form the two other intestinal pathogens Shigella and Salmonella is that it ferments lactose. It does not ferment maltose, fructose, glucose and acetate.

Question 3

Impaired blood supply and the presence of Escherichia coli (E. coli) predisposes to infection by Bacteroides fragilis (B. fragilis) because

1 E. coli is a facultative anaerobe, it uses up oxygen and reduces it to a level so that B. fragilis an anaerobe can grow
2 E. coli and B. fragilis are symbiotic
3 E. coli is an aerobe, it uses up oxygen and reduces it to a level so that B. fragilis an anaerobe can grow
4 B. fragilis is a facultative anaerobe, it uses up oxygen and reduces it to a level so that E. coli an anaerobe can grow
5 B. fragilis is an aerobe, it uses up oxygen and reduces it to a level so that E. coli an anaerobe can grow

Answer 3

E. coli is a facultative anaerobe, it uses up oxygen and reduces it to a level so that B. fragilis an anaerobe can grow

Impaired blood supply and the presence of E. coli predisposes to infection by B. fragilis because E. coli is a facultative anaerobe, it uses up oxygen and reduces it to a level so that B. fragilis an anaerobe can grow.

Question 4

Which of the following group of Escherichia coli is the common cause of diarrhea in infants and is implicated in outbreaks of diarrhea in nurseries?

1 Enterotoxigenic
2 Enteroinvasive
3 Enteropathogenic
4 Enterohemorrhagic
5 Enteroaggregative

Answer 4

Enteropathogenic

Enteropathogenic Escherichia coli (EPEC) is common cause of infantile diarrhea in infants in developing countries. EPEC is associated with outbreaks of diarrhea in nurseries and day care center. EPEC adheres to epithelial cells of the intestine via adhesive factor and causes destruction of microvilli. This results in watery diarrhea and fever.

Question 5

Which of the following strains of Escherichia coli is commonly implicated in traveler’s diarrhea mediated by heat stable enterotoxin and heat labile exotoxin?

1 Enterotoxigenic
2 Enteroinvasive
3 Enteropathogenic
4 Enterohemorrhagic
5 Enteroaggregative

Answer 5

Enterotoxigenic

Enterotoxigenic Escherichia coli (ETEC) is common cause of traveler’s diarrhea. It is also associated with diarrhea in infants in developing countries. It causes watery diarrhea, abdominal cramps and low grade fever.
The virulence factor implicated are as follows:
Heat stable enterotoxin stimulates fluid secretion by activating guanylate cyclase in enteric epithelial cells.
Heat labile exotoxin binds to the ganglioside at the brush border of epithelial cells of small intestine and leads to activation of adenylate cyclase.
Colonization factors facilitate the attachment of ETEC to epithelial cells of intestine.

Question 6

The virulence factor produced by Escherichia coli includes

1 P pili
2 Aerobactin
3 α-hemolysin
4 K1 capsular polysaccharide
5 All of the above

Answer 6

All of the above

P pili is adhesin and E. coli strains possessing P pili is mostly implicated in urinary tract infections which includes pyelonephritis.
Aerobactin provides the bacterial cell with iron by extracting iron from the host.
α-hemolysin is associated with lysis of red blood cells and tissue injury.
K1 capsular polysaccharide is antiphagocytic and anticomplementary.

Question 7

If a stool specimen containing E. coli Serotype O157:H7 is plated on sorbitol-MacConkey agar and observed at 48 hours. What will happen?

1 Do not ferment sorbitol and appear pink
2 Do not ferment sorbitol and appear colorless
3 Ferment sorbitol and appear pink
4 Ferment sorbitol and appear colorless
5 Do not grow on the medium

Answer 7

Do not ferment sorbitol and appear colorless

Serotype O157:H57 of enterohemorrhagic E. coli (EHEC) strain is associated with colitis, hemorrhagic diarrhea and hemolytic uremic syndrome. When a stool specimen is cultured on MacConkey agar containing sorbitol other E. coli ferments sorbitol and appears pink in color. However, E. coli O157:H7 does not ferment sorbitol and appears colorless. These isolates of E. coli O157:H7 can be tested by latex agglutination test for rapid presumptive identification.

Question 8

The major virulence factor of Enterotoxigenic Escherichia coli (ETEC) includes heat labile toxin and heat stable toxin. The action of heat labile toxin is similar to

1 Exotoxin A of Pseudomonas aeruginosa
2 Vibrio cholerae toxin
3 Diphtheria toxin
4 Botulinum toxin

Answer 8

Vibrio cholerae toxin

Heat labile toxin of Enterotoxigenic Escherichia coli (ETEC) is similar to toxin produced by Vibrio cholerae. It binds to the ganglioside at the brush border of epithelial cells of small intestine and leads to activation of adenylyl cyclase. Adenylate cyclase in turn activates the conversion of adenosine triphosphate to cyclic adenosine monophosphate (cAMP) resulting in cAMP accumulation. This lead to hypersecretion of fluids and electrolytes into the lumen causing watery diarrhea.

Question 9

Salmonella enterocolitis is characterized by

1 Presence of large number of leukocytes in the stool
2 Headache, nausea, vomiting and profuse diarrhea
3 Positive blood cultures always
4 Negative stool cultures throughout the course of disease

Answer 9

Headache, nausea, vomiting and profuse diarrhea

The most common manifestation of Salmonella infection is enterocolitis. Symptoms of enterocolitis appear 8-48 hours after the ingestion of the organism and it includes nausea, headache, vomiting and profuse diarrhea. The stool contains few leukocytes and gives positive culture. It is accompanied by low grade fever and course of disease usually lasts 2-3 days. Mostly blood cultures yield negative results since bacteremia is not commonly seen (except in immunodeficient patients).

Question 10

Stool specimen of a patient with profuse diarrhea was cultured on MacConkey agar and Bismuth sulfite medium. Lactose fermenting colonies and non-lactose fermenting colonies were observed on MacConkey medium. Bismuth sulfite media grow black colonies. The isolate fermented glucose and mannose but not lactose and sucrose. What is the most probable causative agent of diarrhea?

1 Escherichia coli
2 Enterobacter aerogenes
3 Shigella boydii
4 Proteus mirabilis
5 Salmonella typhi

Answer 10

Salmonella typhi

Bismuth sulfite is a selective medium and is used for rapid detection of Salmonella typhi. The colonies appear black due to production of H2S. It is non-lactose fermenter and appears clear on MacConkey medium.
Escherichia coli and Enterobacter aerogenes are lactose fermenters and are normal flora of intestine.
Shigella boydii and Proteus mirabilis are non-lactose fermenters and do not produce H2S.

Question 11

Treatment of chronic carriers of Salmonella typhi with gall stones includes

1 Ampicillin
2 Fluoroquinolones
3 Cholecystectomy
4 A and B
5 A and C

Answer 11

A and C

Salmonella typhi is carried in gall bladder and biliary tract in most carriers. The organism persists in the gall bladder especially if the person has gall stones. In these cases the effective treatment includes combination of ampicillin and cholecystectomy.

Question 12

Which of the following species of Shigella is a lactose fermenter?

1 Shigella boydii
2 Shigella dysenteriae
3 Shigella flexneri
4 Shigella sonnie

Answer 12

Shigella sonnie

Shigella sonnie type D is only lactose fermenting species of Shigella, as shown in the following:
Group and Type Glucose Lactose Mannitol Ornithine
Decarboxylase
S. boydii C + - + -
S. dysenteriae A + - - -
S. flexneri B + - + -
S. sonnei D + + + +

Question 13

The urine specimen of a patient admitted in the hospital grew large, mucoid, pink colony on MacConkey agar. The isolate was oxidase negative, non motile gram negative rod. Which of the following organism you will suspect?

1 Proteus mirabilis
2 Klebsiella pneumoniae
3 Hemophilus ducreyi
4 Salmonella typhi
5 Streptococcus agalactiae

Answer 13

Klebsiella pneumoniae

Klebsiella pneumoniae contains large polysaccharide capsule contributing to its mucoid growth.
It is gram negative rod that is non motile and ferments lactose.
Proteus mirabilis is
Gram negative rod
Motile
Non lactose fermenter
Exhibits swarming on the medium
Hemophilus ducreyi is
Gram negative coccobacillus
Oxidase positive
Non lactose fermenter
Salmonella typhi
Gram negative rod
Motile
Non lactose fermenter
Streptococcus agalactiae
Gram positive cocci
Non motile

Question 14

Klebsiella oxytoca is

1 Non lactose fermenter
2 Gram positive rod
3 Spore former
4 Capsulated
5 Motile

Answer 14

Capsulated

Klebsiella oxytoca is part of the family Enterobacteriaceae. It has the following characteristics:
Gram negative rod
Non motile
Capsulated
Non spore former
Lactose fermenter

Question 15

The urine specimen of a 55-year-old AIDS patient yields non-lactose fermenting colonies on MacConkey agar. Typical swarming of the colonies was observed. The organism isolated was a gram-negative rod and gave a negative oxidase and positive urease test. Which of the following is the most probable organism isolated?

1 Escherichia coli
2 Klebsiella pneumoniae
3 Hemophilus ducreyi
4 Proteus mirabilis
5 Pseudomonas aeruginosa

Answer 15

Proteus mirabilis

Proteus mirabilis species is associated with urinary tract infections, bacteremias, and pneumonia in debilitated hosts and individuals receiving antibiotic treatment.
Klebsiella pneumoniae is
Gram-negative rod
Non-motile
Capsulated
Lactose fermenter
Mucoid colony on MacConkey medium
Escherichia coli is
Gram-negative rod
Motile
Lactose fermenter
Hemophilus ducreyi and Pseudomonas aeruginosa are oxidase positive.

Question 16

Case
A 15-year-old boy was admitted to a healthcare facility with a 2-day history of nausea, vomiting, diarrhea, headache, and a continued fever with 39.5°C (103.1°F) temperature. The patient was found to have rose-colored spots on his abdomen. The physician suspected it to be a case of Salmonella infection based on the symptoms. Microscopic examination of the stool revealed Gram-negative bacilli. The organism did not show any motility and produced mauve colonies on CHROMagar Salmonella (CAS) media and black transparent colonies on Hektoen enteric agar (HEA), Salmonella-shigella (SS) agar, and xylose-lysine-desoxycholate agar (XLD). On XLD agar, yellow-colored zones surrounded the colonies.

What is the causative organism in this patient?

1 Salmonella enteritidis
2 Salmonella gallinarum
3 Salmonella paratyphi
4 Salmonella typhi
5 Salmonella typhimurium

Answer 16

Salmonella typhi

The patient has been infected by Salmonella typhi. Salmonella species are a group of non-spore forming, facultative anaerobic, Gram-negative bacilli that are predominantly motile; they are found in the intestinal tracts of humans and other warm-blooded animals. They are responsible for the outbreak of food poisoning instances. S. typhi infection causes typhoid with symptoms like abdominal pain, headache, loss of appetite, and in some cases appearance of rose-colored spots across the abdomen. Diagnostic test for all the species of Salmonella involve microscopic examination of stool for the detection of organisms, and later biochemical tests are used to identify the species. All species of Salmonella produce mauve colonies on CAS medium. S. typhi and paratyphi produce black-colored colonies on XLD, SS, and HEA media as a result of hydrogen sulfide (H2S) production, but only S. typhi produces yellow-colored halos around its colonies on XLD media due to fermentation of the sugar xylose present in it.
The best way to prevent Salmonella infections is by properly cooking food, good hygiene practices, and heating food to appropriate temperatures or prompt refrigeration.
Salmonella enteritidis is transmitted to humans from infected hens via raw or undercooked eggs and causes gastroenteritis. These organisms contaminate the eggs before the shells are formed in the infected hens, resulting in transmission of the organisms to humans. Infection by S. enteritidis results in diarrhea and fever. These organisms form mauve colonies on CAS, but serological tests confirm the infection by S. enteritidis. Preventive measures include eating well-boiled eggs and avoiding consumption of cracked eggs.
Salmonella gallinarum is found to infect only chickens, turkeys, and other animals. This organism causes fowl typhoid, resulting in mortality or drop in egg production by the birds. These organisms have not been found to infect humans.
Salmonella paratyphi infection results in symptoms similar to typhoid. The infected person develops rose-colored spots on the chest. The organism is transmitted by direct or indirect contact with feces of the infected person or by contaminated food and water. Differential tests include growing the organism on CAS and XLD media. S. paratyphi do not produce yellow halos around colonies on XLD media. Preventive measures involve hygienic practices when handling food.
Salmonella typhimurium are generally found to infect mice but can be transmitted to humans from contaminated foods of animal origin and cause salmonellosis. Symptoms include diarrhea, fever, headache, abdominal pain, and vomiting. In rare cases, blood is seen in stools. Symptoms are evident 12-72 hours after ingestion of the organism and may last for 4-7 days. Diagnosis involves microscopic examination of stool and serological tests. Preventive measures include proper handling of food.

Question 17

A 12-year-old girl is taken to the emergency room with a fever (104°F), abdominal cramps, fatigue, muscle aches, and headache. She was on vacation in Venezuela 1 week ago. A blood culture showed Salmonella typhi. Which is the reservoir for these bacteria?

1 Chicken
2 Deer
3 Human
4 Rabbit
5 Rat

Answer 17

Human

Salmonella Typhi is a Gram negative rod, non-lactose fermenter and motile, that lives only in humans. It produces typhoid fever which is more common in developing countries. It is found in bloodstream and intestinal tract, is transmitted by eating or drinking food contaminated with feces. The symptoms are non-specific, fever (103°F), abdominal pain, headache, fatigue. If salmonella is suspected, the bacteria should be found in blood or stool to confirm the diagnosis. A Widal test is one of the methods used to make the diagnosis The test detects H and O agglutinins, but in endemic areas many false positive and false negative results can be found. Mortality is between 12 and 30%. Treatment is with antibiotics such as ciprofloxacin, but resistance to flouroquinolones has increased.

Question 18

Which of the following groups of Escherichia coli is a common cause of diarrhea in infants and is implicated in outbreaks of diarrhea in nurseries?

1 Enterotoxigenic
2 Enteroinvasive
3 Enteropathogenic
4 Enterohemorrhagic
5 Enteroaggregative

Answer 18

Enteropathogenic

Enteropathogenic Escherichia coli (EPEC) is common cause of infantile diarrhea in infants in developing countries. EPEC is associated with outbreaks of diarrhea in nurseries and day care center. EPEC adheres to epithelial cells of the intestine via adhesive factor and causes destruction of microvilli. This results in watery diarrhea and fever.

Question 19

Which of the following strain of Escherichia coli is commonly implicated in traveler’s diarrhea mediated by heat stable enterotoxin and heat labile exotoxin?

1 Enterotoxigenic
2 Enteroinvasive
3 Enteropathogenic
4 Enterohemorrhagic
5 Enteroaggregative

Answer 19

Enterotoxigenic

Enterotoxigenic Escherichia coli (ETEC) is common cause of traveler’s diarrhea. It is also associated with diarrhea in infants in developing countries. It causes watery diarrhea, abdominal cramps and low grade fever.
The virulence factor implicated are as follows:
Heat stable enterotoxin stimulates fluid secretion by activating guanylate cyclase in enteric epithelial cells.
Heat labile exotoxin binds to the ganglioside at the brush border or epithelial cells of small intestine and leads to activation of adenylate cyclase.
Colonization factors facilitate the attachment of ETEC to epithelial cells of intestine.

Question 20

If stool specimen containing E. coli Serotype O157:H7 is plated on sorbitol-MacConkey agar and observed at 48 hours, the isolates

1 Do not ferment sorbitol and appear pink
2 Do not ferment sorbitol and appear colorless
3 Ferment sorbitol and appear pink
4 Ferment sorbitol and appear colorless
5 Do not grow on the medium

Answer 20

Do not ferment sorbitol and appear colorless

Serotype O157:H57 of enterohemorrhagic E. coli (EHEC) strain is associated with colitis, hemorrhagic diarrhea and hemolytic uremic syndrome. When stool specimen is cultured on MacConkey agar containing sorbitol other E. coli ferments sorbitol and appears pink in color. However, E. coli O157:H7 does not ferment sorbitol and appears colorless. This isolates of E. coli O157:H7 can be tested by latex agglutination test for rapid presumptive identification.

Question 21

The major virulence factor of Enterotoxigenic Escherichia coli (ETEC) includes heat labile toxin and heat stable toxin. The action of heat labile toxin is similar to

1 Exotoxin A of Pseudomonas aeruginosa
2 Vibrio cholerae toxin
3 Diphtheria toxin
4 Botulinum toxin

Answer 21

Vibrio cholerae toxin

Heat labile toxin of Enterotoxigenic Escherichia coli (ETEC) is similar to toxin produced by Vibrio cholerae. It binds to the ganglioside at the brush border of epithelial cells of small intestine and leads to activation of adenylyl cyclase. Adenylate cyclase in turn activates the conversion of adenosine triphosphate to cyclic adenosine monophosphate (cAMP) resulting in cAMP accumulation. This lead to hypersecretion of fluids and electrolytes into the lumen causing watery diarrhea.

Question 22

Salmonella enterocolitis is characterized by

1 Presence of large number of leukocytes in the stool
2 Headache, nausea, vomiting and profuse diarrhea
3 Positive blood cultures always
4 Negative stool cultures throughout the course of disease

Answer 22

Headache, nausea, vomiting and profuse diarrhea

The most common manifestation of Salmonella infection is enterocolitis. Symptoms of enterocolitis appear 8-48 hours after the ingestion of the organism and it includes nausea, headache, vomiting and profuse diarrhea. The stool contains few leukocytes and gives positive culture. It is accompanied by low grade fever and course of disease usually lasts 2-3 days. Mostly blood cultures yield negative results since bacteremia is not commonly seen (except in immunodeficient patients).

Question 23

Stool specimen of a patient with profuse diarrhea was cultured on MacConkey agar and Bismuth sulfite medium. Lactose fermenting colonies and non lactose fermenting colonies were observed on MacConkey medium. Bismuth sulfite media grow black colonies. The isolate fermented glucose and mannose but not lactose and sucrose. The most probable causative agent of diarrhea is:

1 Escherichia coli.
2 Enterobacter aerogenes.
3 Shigella boydii.
4 Proteus mirabilis.
5 Salmonella typhi.

Answer 23

Salmonella typhi.

Bismuth sulfite is a selective medium and is used for rapid detection of Salmonella typhi. The colonies appear black due to production of H2S. It is non lactose fermenter and appears clear on MacConkey medium.
Escherichia coli and Enterobacter aerogenes are lactose fermenters and are normal flora of intestine.
Shigella boydii and Proteus mirabilis are non lactose fermenters and do not produce H2S

Question 24

Which of the following species of Shigella is a lactose fermenter?

1 Shigella boydii
2 Shigella dysenteriae
3 Shigella flexneri
4 Shigella sonnei

Answer 24

Shigella sonnei

Shigella sonnei type D is only lactose fermenting species of Shigella as shown:
Group
and
Type Glucose Lactose Mannitol Ornithine
Decarboxylase
S. boydii C + - + -
S. dysenteriae A + - - -
S. flexneri B + - + -
S. sonnei D + + + +

Question 25

Urine specimen of a patient admitted in the hospital grew large, mucoid, pink colony on MacConkey agar. The isolate was oxidase negative, non motile gram negative rod. Which of the following organisms you will suspect?

1 Proteus mirabilis
2 Klebsiella pneumoniae
3 Hemophilus ducreyi
4 Salmonella typhi
5 Streptococcus agalactiae

Answer 25

Klebsiella pneumoniae

Klebsiella pneumoniae contains large polysaccharide capsule contributing to its mucoid growth.
It is gram negative rod that is non motile and ferments lactose.
Proteus mirabilis is
- Gram negative rod
- Motile
- Non lactose fermenter
- Exhibits swarming on the medium
Hemophilus ducreyi is
- Gram negative coccobacillus
- Oxidase positive
- Non lactose fermenter
Salmonella typhi is
- Gram negative rod
- Motile
- Non lactose fermenter
Streptococcus agalactiae is
- Gram positive cocci
- Non motile

Question 26

Klebsiella oxytoca is

1 Non lactose fermenter
2 Gram positive rod
3 Spore former
4 Capsulated
5 Motile

Answer 26

Capsulated

Klebsiella oxytoca is part of family Enterobacteriaceae. It has the following characteristics
Gram negative rod
Non motile
Capsulated
Non spore former
Lactose fermenter

Question 27

Urine specimen of a 55-year AIDS patient yields non lactose fermenting colonies on MacConkey agar. Typical swarming of the colonies was observed. The organism isolated was a gram negative rod and gave a negative oxidase and positive urease test. Which of the following is the most probable organism isolated?

1 Escherichia coli
2 Klebsiella pneumoniae
3 Hemophilus ducreyi
4 Proteus mirabilis
5 Pseudomonas aeruginosa

Answer 27

Proteus mirabilis

Proteus species is associated with urinary tract infections, bacteremias and pneumonia in debilitated host and individuals receiving antibiotic reatment. Klebsiella pneumoniae is Gram negative rod Non motile Capsulated Lactose fermenter Mucoid colony on MacConkey medium Escherichia coli is Gram negative rod Motile Lactose fermenter Hemophilus ducreyi and Pseudomonas aeruginosa are oxidase positive.

Question 28

Plesiomonas belongs to family

1 Aeromonadaceae
2 Pseudomonadaceae
3 Pasteurellaceae
4 Enterobacteriaceae

Answer 28

Enterobacteriaceae

Plesiomonas is classified in the family Enterobacteriaceae wherein it is the only oxidase-positive member, and only and contains only one species. Plesiomonas shigelloides and has following characteristics:
Gram negative bacillus
Oxidase positive facultative anaerobe
Motile with multiple polar flagella
Found in brackish water
Infection is associated with ingestion of uncooked shellfish

Question 29

Plesiomonas shares the antigenic and biochemical features with

1 Salmonella
2 Shigella
3 Streptococcus
4 Pseudomonas

Answer 29

Shigella

Plesiomonas shares antigenic and biochemical features with Shigella. It contains only species Plesiomonas shigelloides that often cross-agglutinate with Shigella species, but is less virulent than Shigella. Plesiomonas is associated with gastroenteritis and other extraintestinal infections.

Question 30

Which of the following are non-motile enteric gram-negative bacteria with a very distinct polysaccharide capsule? This capsule is thought to interfere with immunologic reactivity of the organism. These bacteria are commonly associated with lobular pneumonia and can be highly resistant to antimicrobial agents

1 Escherichia coli
2 Klebsiella pneumoniae
3 Neisseria meningitidis
4 Shigella sonnei

Answer 30

Klebsiella pneumoniae

Klebsiella pneumoniae is a gram-negative rod in the Enterobacteriaceae group that is non-motile and is characteristically associated with a thick polysaccharide capsule. This structure results in a mucoid appearance of colonies on agar and is the basis of more than 70 serotypes. This organism causes “classic” lobular pneumonia and has become the most antibiotic resistant bacteria in the enteric group.
Escherichia coli and Shigella sonnei are both members of the Enterobacteriaceae as well, but lack such distinctive capsules as seen in Klebsiella. The species Neisseria meningitidis is a gram-negative diplococci.

Question 31

Which of the following organisms is the most frequently isolated member of the Enterobacteriaceae group and is known to be among the most penicillin sensitive species in this group? This species is associated with extreme motility often referred to as swarming on plates. Urease production is used in the identification of this gram-negative rod

1 Shigella sonnei
2 Providencia
3 Escherichia coli
4 Proteus mirabilis

Answer 31

Proteus mirabilis

Proteus mirabilis is a member of the Enterobacteriaceae which displays a characteristic motility described as swarming and also produces urease. These characteristics are used in the identification of the organism. This organism is also known to be one of the most sensitive of all Enterobacteriaceae to penicillin. Proteus species are opportunistic pathogens which cause wound infections and problems in immuno-compromised hosts. Escherichia coli, Shigella sonnei and Providencia are also members of the Enterobacteriaceae however Shigella are non-motile and Providencia do not produce urease, while Escherichia coli produces indole.

Question 32

An 11-year-old African American female diagnosed as having sickle cell disease comes to the hospital complaining of pain in her right arm. An X-ray film of her arm confirms the clinical diagnosis of osteomyelitis. Which of the following is the most likely causal organism?

1 Clostridium septicum
2 Listeria monocytogenes
3 Proteus mirabilis
4 Pseudomonas aeruginosa
5 Salmonella enteritidis

Answer 32

Salmonella enteritidis

Sickle cell anemia is an autosomal recessive disorder wherein the hemoglobin molecule has a defective beta chain. The substitution of glutamine at the sixth position results in an abnormal molecule of hemoglobin designated as Hb-S. In the deoxy- form, Hb-S tends to polymerize and thereby damage the RBC membrane. RBC that undergoes repeated sickling is damaged beyond repair.
The rate of sickling is increased by
The concentration of Hb-S (most important)
RBC dehydration
Acidosis
Hypoxemia
As Hb-F cannot polymerize, symptoms arise only after the first 6 months when the levels of Hb-F drop. The presence of Hb-F retards sickling.
Clinical features include:
Chronic hemolytic anemia
Anemia
Jaundice
Pigment gallstones
Poor healing ulcers
Enlarged spleen
Aplastic crises
Due to infections and folate deficiency
Acute painful crises
Precipitated by:
Hypoxia
Infection
Dehydration
Causes:
Bone pain
Stroke
Priapism
Repeated vascular occlusion
Results in
Retinopathy
Infarction of the renal papillae
Splenic infarction
Ischemic necrosis of bone which renders the bone susceptible to osteomyelitis due to staphylococcus or salmonella
Salmonellae cause many cases of bacteremia associated with sickle cell disease. Among patients with hemoglobinopathies, osteomyelitis caused by salmonella is 10 times more frequent than that caused by other pyogenic bacteria.
In otherwise healthy patients, it is a rare complication with infection of bone occurring in less than 1% of patients with typhoid. Bone lesions due to salmonellosis in these patients tend to be solitary.
After a splenectomy patients are found to be more susceptible to infections by encapsulated organisms such as pneumococci.

Question 33

A 30-year-old man comes into your office because of diarrhea that began on the airplane while he was returning from a vacation in the Middle East. He had multiple episodes of watery, non-bloody diarrhea and little vomiting. BP 130/85 mm Hg, P 76, T 36,8°C. You make a stain for fecal leukocytes, which reveals no PMNs. Test for occult blood is negative. Culture of the stool reveals pigmented colonies on eosin-methylene blue (EMB) agar. Which one of the following organisms is the most likely cause?

1 Escherichia coli (enterotoxigenic strains)
2 Escherichia coli (enteropathogenic strains)
3 Escherichia coli O157: H7
4 S. typhimurium
5 S. dysenteriae

Answer 33

Escherichia coli (enterotoxigenic strains)

Escherichia coli is a gram-negative rod that ferments lactose, a property that distinguishes it from the two major intestinal pathogens, Shigella and Salmonella. E. coli forms pigmented colonies, whereas lactose-negative organisms are colorless. On EMB agar, E. coli colonies have a characteristic green sheen.
The enterotoxin-producing strains do not cause inflammation, do not invade intestinal mucosa and cause a watery, non-bloody diarrhea. The disease is self-limited and of short duration (1-3 days). It is frequently associated with travel (traveler’s diarrhea or “turista”).
Certain strains of E. coli are enteropathic (enteroinvasive) and cause disease by invasion of the epithelium of the large intestine. The result is dysentery-like syndrome characterized by bloody diarrhea, abdominal cramps and fever, accompanied by neutrophils in the stool.
Certain enterohemorrhagic strains of E. coli, i.e. those with the O157: H7 serotype, also cause bloody diarrhea but do not cause inflammation; therefore no neutrophils are found in the stool. Some patients with bloody diarrhea caused by O157: H7 strains also have a life-threatening complication called hemolytic-uremic syndrome. Refer to the table for additional information.
Gram-negative rods causing diarrhea
Species Fever PMNs in stool Blood in stool Typical bacteriologic or epidemiologic findings
Enterotoxin-mediated 1. Escherichia coli 2. Vibrio cholerae - - - - - - Ferments lactose Comma-shaped bacteria
Invasive-inflammatory 1. Salmonella, e.g. S. typhimurium 2. Shigella, e.g. S. dysenteriae 3. Campylobacter jejuni 4. Escherichia coli (enteropathogenic strains) 5. Escherichia coli O157:H7 + + + + + + + + + - +/- + + + + Does not ferment lactose Does not ferment lactose Comma- or S-shaped bacteria; growth at 42° C Causes hemolytic-uremic syndrome
Mechanism uncertain 1. Vibrio parahaemolyticus 2. Yersinia enterocolitica + + + + - + Transmitted by seafood Usually transmitted from pets

Question 34

A 39-year-old woman comes into your office, because she has had severe diarrhea for the past 2 days, accompanied by fever and a shaking chill. She uses antacids. Otherwise, she is healthy. A stain for fecal leukocytes reveals many PMNs. Test for occult blood was positive. From the stool culture a gram-negative rod was isolated that does not ferment lactose. Which one of the following organisms is the most likely cause?

1 Salmonella typhimurium
2 Escherichia coli
3 Escherichia coli O157:H7
4 Vibrio parahaemolyticus
5 Vibrio cholerae

Answer 34

Salmonella typhimurium

Salmonellae are gram-negative rods that form nonlactose-fermenting (colorless) colonies on MacConkey’s or EMB agar. On TSI agar, an alkaline slant and an acid butt, frequently with both gas and H2S are produced.
After an incubation period of 6-48 hours, enterocolitis begins with nausea and vomiting, and then progresses to abdominal pain and diarrhea, which can vary from mild to severe, with or without blood. Usually the disease lasts a few days, is self limited, causes non-bloody diarrhea, and does not require medical care, except in the very young and very old. The most common cause of enterocolitis is Salmonella typhimurium, but virtually every species has been implicated. Gastric acid is an important host defense; gastrectomy or use of antacids lowers the infectious dose significantly.
If PMNs are present in a fecal sample, an invasive organism such as Shigella, Salmonella or Campylobacter is involved rather than a toxin-producing organism such as V. cholerae, E. coli or Clostridium perfringens. Refer to the table for additional information.
Gram-negative Rods Causing Diarrhea
Species Fever PMNs in Stool Blood in Stool Typical Bacteriologic or Epidemiologic Findings
Enterotoxin-mediated
Escherichia coli - - - Ferments lactose
Vibrio Cholerae - - - Comma-Shaped bacteria
Invasive-Inflammatory
Salmonella (S. typhimurium) + + + Does not ferment lactose
Shigella (S. dysenteriae) + + + Does not ferment lactose
Campylobacter jejuni + + + Comma- or S-shaped bacteria
Escherichia coli (enteropathogenic strains) - +/- + Growth at 42°C
Escherichia coli O157:H7 + + + Causes heloytic-uremic syndrome
Mechanism Uncertain
Vibrio parahaemolyticus + + + Transmitted by seafood
Yersinia enterocolitica + - + Usually transmitted from pets