Definitions + tests

Question 1

Definition of a pathogen?

Answer 1

a bacterium, virus or microorganism that can cause disease

Question 2

What is gram staining?

Answer 2

A technique used to differentiate bacteria into two large groups (gram -ve/+ve), based on the physical and chemical properties of their cell walls (thickness of layer of peptidoglycan in cell wall)

Question 3

Why do gram positive bacteria stain purple?

Answer 3

Thick peptidoglycan layer which retains the violet crystal stain well when washed with alcohol.

Question 4

What does Ziehl-Neelson stain identify and what colour are the results?

Answer 4

Identifies acid-fast organisms, mainly mycobacteria (notably TB - see picture)
acid-fast = RED
non acid-fast = BLUE

Question 5

How is the catalase test performed and what does it test for?

Answer 5

Hydrogen peroxide placed on microscope slide.
Bacterial added using swab.
Bubbles / froth = +ve (staph)
No bubbles = -ve (strep)
If positive, catalase in the bacteria is present.

Question 6

Difference in structure between gram +ve and -ve bacteria?

Answer 6

gram positive has much thicker cell wall
gram positive has much thicker layer of peptidoglycan
gram negative has two cell membranes
gram negative has porins and lipopolysaccharides