Bacterial Morphology and Growth

Question 1

List some structural components of bacteria.

Answer 1

• haploid (single chromosome) DNA (circular)
• ribosomes in the cytoplasm
• DNA in nucleoid region (NOT bound in a nucleus)
• peptidoglycan membrane (a target for antibiotics)
• no mitochondria
• no membrane-bound organelles
• may have a capsule
• may have pilli/ flagella
• may have spores

Question 2

Describe the Gram stain, and how to apply it to bacteria.

Answer 2

It is a differential CELL WALL stain dividing bacteria into:
GRAM-POSITIVE - blue/purple
GRAM-NEGATIVE - pink

First, you fix the bacteria. Then, you add the crystal violet dye, then you add iodine. You then add alcohol (this will remove traces of the dye from Gram-negative bacteria), then you add another dye, such as Carbol Fuschin or Safranin.

The Gram-positive bacteria have thick cell walls which are able to hold on to the original dye.
The Gram-negative has a thinner cell wall, so the alcohol step damages the outer membrane and allows the blue/purple Crystal Violet dye out. The second Safranin/Carbol Fuschin dye then stains the cell wall red/pink.

Question 3

Describe the Acid Fast stain.

Answer 3

It is used on bacteria with waxy cell walls because the Gram stain won’t stick to them.

First, you fix the bacteria. Then you add Carbol Fuschin or Auramine. The, you add an acid or an alcohol (which will remove the dye from non-acid-fast bacteria), then you add a background stain.

If a bacterium is non-acid-fast, they do not retain (keep fast) the Carbol Fuschin/Auramine stain on their cell surface when washed with acid.

Question 4

Give an example of Gram-positive cocci in pairs.

Answer 4

Streptococcus pneumoniae (pneumonia)

Question 5

Give an example of Gram-positive cocci in chains.

Answer 5

Streptococcus pyogenes (pharyngitis)

Question 6

Give an example of Gram-positive cocci in clusters.

Answer 6

Staphylococcus aureus (toxic shock syndrome)

Question 7

Give an example of Gram-positive rods (bacillus).

Answer 7

Corynebacterium diptheriae (diptheria).

Question 8

Give some examples of Gram-positive rods with spores.

Answer 8

• Clostridium tetani (tetanus)
• Clostridium perfringens (gangrene)
• Bacillus anthrax (anthrax)

Question 9

Give some examples of Gram-negative rods (bacilli).

Answer 9

• Escherichia coli (colitis)

- Salmonella typhi (typhoid fever)

Question 10

Give some examples of Gram-negative cocci (diplococci).

Answer 10

• Neisseria meningitidis (meningitis)

- Neisseria gonorrhoea (gonorrhoea)

Question 11

Give some examples of spiral (helical) shaped bacteria.

Answer 11

• Treponema pallidum (syphilis)
• Helicobacter pylori (stomach ulcers)
• Vibrio cholerae (cholera)

Question 12

Give an example of acid-fast staining bacteria.

Answer 12

Mycobacterium tubercolosis (tuberocolosis)

Question 13

What bacterium is an exception to the cell wall staining tests, and why?

Answer 13

Mycoplasma pneumoniae (atypical pneumonia)

It has no cell wall (and doesn’t have peptidoglycan), thus it cannot be stained. It only has a lipoprotein outer coat.
These types tend to be intracellular.

Question 14

What are the classifications of bacteria based on temperature requirements?

Answer 14

• PSYCHROPHILES (-20°C to 20°C)
  eg. Camplyobacter jejuni (food poisoning)
• MESOPHILES (2°C to 45°C)
  most animal pathogens
• THERMOPHILES (42°c TO 80°C)
  eg. Bacillus stearothermophilus (used for sterilisation strips)
• EXTREME (HYPER) THERMOPHILES (60°C TO 250°C)
  eg. Thermus aquaticus (source of Taq for PCR)

Question 15

What are the classifications of bacteria based on pH requirements?

Answer 15

• ACIDOPHILE: thrives in acidic conditions
• NEUTROPHILE: thrives in neutral conditions
• ALKALIPHILE: thrives in alkaline conditions

Most human pathogens are neutrophiles.