Gram Negative Bacteria

Question 1

What can we categorise Gram negative bacilli into? (3)

Answer 1

glucose fermenters
non fermentors
‘curved’ rods

Question 2

What can gram negative glucose fermenters be further categorised as?

Answer 2

oxidase positive & oxidase negative (enterobactales)

Question 3

What agar do Enterobacterales grow best on?

Answer 3

MacConkey agar because they’re commensals of the GIT & facultative anaerobes

Question 4

Enterobactales can be ____________ or ___________ .
Give an example of each.

Answer 4

lactose fermentors e.g. E.Coli (harmless commensals of the gut)
non lactose fermenters e.g. salmonella (pathogens of the gut)

Question 5

How do we test for E.Coli?

Answer 5

MALDI-TOF which is a mass spectrometry profile that produces a protein fingerprint for each species

Question 6

What diseases can E.Coli cause?

HINT- what happens when E.Coli attaches close to the lumen

Answer 6

disease when prescence is found in a new site e.g. UTI’s, mastitis
or attaching E.Coli can attach closer to the lumen of the intestine and cause diarrhoea, scours etc.

Question 7

What two categories can we classify E.Coli into?

Answer 7

[Kauffman white scheme]
O antigens (have a lipopolysaccharide layer)
H antigens (have flagellae)

Question 8

What medium can we grow non lactose fermentors such as Salmonella bacteria on?

Answer 8

overnight in enriched selenite broth

Question 9

How do we serotype salmonella?

Answer 9

Using the Kaufmann-White scheme (O= lipopolysaccharide antigen, H= flagellum) & Monovalent test

Question 10

Give an example of a curved rod gram negative bacilli?

Answer 10

Campylobacter (!)
Helicobacter

Question 11

What medium do curved rods of gram negative bacilli grow on?

Answer 11

selective medium- charcoal agar & antimicrobials, 42 degrees for 2-3 days

Question 12

Why is Campylobacter a public health concern?

Answer 12

Common in chickens therefore factory processing causes carcass contamination & freezing allows survival inside carcass

Question 13

What is the purpose of an oxidase test and how does it work?

Answer 13

detects colonies that produce cytochrome oxidase (oxidase positive vs negative)
- Cytochrome oxidase is part of the bacteria respiratory system that catalyses the reduction of oxygen to water- not all bacteria respire this way though so helps differentiate

Question 14

So we know Enterobactales are oxidase negative, glucose fermenting gram negative bacilli- Give an example of an oxidase positive, glucose fermenting gram negative bacilli?

Answer 14

Pasteurella spp.
Actinobacillus

Question 15

What differs Pasteurella spp from other gram negative bacteria?

Answer 15

It’s susceptible to penicillin
[also has a capsule which makes it look like self bacteria but so do other gram -ve’s)

Question 16

What agar is best for growing Actinobacillus?

Answer 16

Some species require NAD supplementation (found in chocolate agar which has lysed red blood cells- nutrients are exposed)

Question 17

Give three examples of oxidase positive, non fermenting gram negative bacilli?

Answer 17

Pseudomonas
Bordetella
Aeromonas
Brucella
Taylorella

Question 18

Pseudomonas spp secretes what? Why is this significant in its identification?

Answer 18

Secretes pyocyanin’s which have a metallic appearance & characteristic smell

Question 19

What does Bordetella bacteria secrete? (2)

Answer 19

leukotoxins- destroys neutrophils
dermonecrotic toxin- damages host epithelium

Question 20

How can we control Bordetella spp?

Answer 20

Vaccination - traditionally intra nasal (Live vaccine)
Injectable is also available - fimbriae antigen

Question 21

Can we grow Brucella spp in any lab? Why?

Answer 21

It’s zoonotic & a category 3 (notifiable!) pathogen so specialist labs needed to grow it