Enterobacteriaceae

Question 1

General characteristics of Enterobacteriaceae

Answer 1

1. gram negative bacilli
2. non-spore forming
3. grow on MacConkey
4. catalase positive
5. oxidase negative
6. facultative anaerobes/ ferment glucose
7. reduce nitrate to nitrite

Question 2

Natural habitat for Enterobacteriaceae

Answer 2

• Most are commensal flora of the human and animal GI tract
  -opportunistic pathogens in various clinical settings: urinary tract, resp tract, wound infection
  -some enterics are definitive pathogens in the GI tract or possess virulence factors that lead to life-threatening illness

Question 3

Growth in culture: Enterobacteriaceae

Answer 3

-Grow on routine media (BAP and CHOC) at 35 degrees in CO2 or non CO2 usually large wet colonies, white or grey
-Grow on MacConkey
-a variety of selective/differential media to screen for specific over pathogens: EMB, XLD, HEK, SS agar, MAC w/ sorbitol, CIN

Question 4

What are key tests to narrow the ID into defined families of Enterobacteriaceae?

Answer 4

1. Indole
2. MR
3. VP
4. Citrate
   -More than 50 tests are available in tubes, only state labs and reference labs keep them on hand
   -presumptive ID’s often sufficient for nonsterile specimens

Question 5

API - Test

Answer 5

• API 20e: a strip of microwells for GNR ID
• performs 20 tests on an unknown with a single inoculum
• Tests are interpreted to generate a 7-digit bio-code after overnight incubation
  -Compare bio-code to a database complied from known organism IDs to obtain the ID of an unknown

Question 6

Automated ID systems: VITEK/Phoenix

Answer 6

-VITEK/PHX: Automated inoculation, incubation, and reading of many tests
-generates bio-code for ID within hours
-also automates ABx. susceptibility testing is still used for this

Question 7

VITEK MS Biomerieux

Answer 7

-Matrix-assisted laser desorbtion ionization: Time of flight (MALDI-TOF)
-detects differences in ribosomal cytoplasmic proteins as the basis for ID within minutes
-matrix binds to proteins and donates protons
-Laser provides energy to bind protons and to form positively charged fragments of various sizes
-magnetic field causes ionized proteins to move toward the detection plate at speeds dependent on molecular weight
- Graphs of peaks in arrival times are compared to a database of peaks to generate ID.