Enterobacteriaceae

Question 1

three groups

Answer 1

• large number of genera
• major facultative anaerobes in large intestine but small numbers compared to Bacteroides.
• depends on major anatomic disease location

(1) pathogens both
within & outside enteric tract

(2) pathogens primarily
within enteric tract

(3) pathogens outside enteric tract.

Salmonella, Shigella, & Campylobactera frequent pathogens in gastrointestinal tract; Escherichia, Vibrio, & Yersinia less so.

Question 2

4 common metabolic processes:

Answer 2

(1) facultative anaerobes
(2) ferment glucose
(3) none have cytochrome oxidase (oxidase-negative)
(4) reduce nitrates to nitrites as
part of their energy-generating processes.

• used to distinguish Enterobacteriaceae from nonfermenting gram-negative rods-> Pseudomonas aeruginosa-> UTI & sepsis in hospitalized patients, does not
  ferment glucose or reduce nitrates, strict aerobe
  & derives energy from oxidation, not fermentation.

Question 3

Pathogenesis

Answer 3

• gramnegative,
  contain endotoxin in cell walls & several exotoxins produced.
• E. coli O157
  produces Shiga toxin that causes hemolytic-uremic
  syndrome (HUS).

Question 4

3 surface
antigens

Answer 4

(1) O antigen: cell wall or somatic

(2) H antigen: flagellar protein

(3) K antigen: capsular

Question 5

(1) O antigen: cell wall or somatic

Answer 5

• outer polysaccharide portion of lipopolysaccharide. - repeating oligosaccharides consisting of 3 or 4 sugars repeated 15 or 20 times, basis for serologic typing of many enteric rods.
• number of different O antigens is very large.

Question 6

(2) H antigen: flagellar protein

Answer 6

• Escherichia & Salmonella-> motile
• Salmonella-> reversibly alternate between 2 types of H antigens: phase 1
  & 2-> antigenicity
  to evade immune response.

Question 7

(3) K antigen: capsular

Answer 7

• heavily encapsulated organisms ->
  Klebsiella.
• quellung reaction in presence of specific antisera
• serotype E. coli & S. typhi for epidemiologic purposes.

Question 8

Lab diagnosis

Answer 8

two media: blood agar plate & selective differential medium: MacConkey’s agar or eosin–methylene blue (EMB) agar [differential ability based on lactose fermentation-> identification].

• non–lactose fermenters (Salmonella
  & Shigella) form colorless colonies; lactose fermenters (E. coli) form colored colonies.

EMB agar-> E. coli colonies: green sheen.

• selective effect of media suppressing unwanted gram-positive organisms exerted by bile salts or bacteriostatic
  dyes in agar.
• triple sugar iron (TSI) agar & urea agar->prior to definitive identification procedures.
• array of 20 or more biochemical tests required to identify species.
• Motility->presence of flagella.
• differentiation
  of Enterobacter cloacae (motile); K. pneumoniae (nonmotile).
• Salmonella or Shigella strain presence-> agglutination test-> identify genus of organism & determine member of group A, B, C, D.

Question 9

Prevention

Answer 9

• Contamination of public water supply system by sewage
  is detected by presence of coliforms in water.
• only E. coli exclusively in large intestine organism->indicator of fecal
  contamination.
• In water quality testing, E. coli identified: ferment lactose with production of acid
  & gas, ability to grow at 44.5°C, colony type on EMB agar.
• above
  4/dL: unacceptable
  fecal contamination.
• E. coli & enteric pathogens killed by chlorination of drinking water.

Question 10

Antibiotic Therapy

Answer 10

• Tailored to antibiotic sensitivity of organism.
• Wide range of antimicrobial
  agents effective
• Isolates of highly antibiotic resistant: production of β-lactamases.
• undergo conjugation frequently->acquire plasmids (R factors), mediate multiple drug resistance.