Infectious Agents

Question 1

What are the 4 broad biological groups used to classify microbes?

Answer 1

• Viruses
• Bacteria
• Fungi
• Parasites

Question 2

Describe the basic structure of a virus

Answer 2

• Most 20nm to 300nm
• DNA or RNA (not both) core within protein capsid (this is made up of individual capsomeres, which can be helical, cubic or have a more complex arrangement)
• No cytoplasm
• May have an envelope derived from its host cell - this envelope allows the virus to exit the host cell via exocytosis, avoiding cell damage and preventing an immune response
• Contain membrane bound proteins
• Obligate intracellular organisms
• Replicate by a variety of mechanisms

Question 3

Describe the basic structure of bacteria

Answer 3

• Prokaryotic
• Contain haploid DNA which is coiled into a nucleoid
• Plasma membrane - may contain flagella that allow it to become motile, but no all bacteria have these
• Fimbria (pili)- Can allow adherence to cell membrane, some can use these to pass genetic information
• No mitochondria

Question 4

Describe Gram positive bacteria

Answer 4

• Stain blue/purple
• Thick peptidoglycan cell wall consists of chains of hexoses (N-acetylglycosamine and N-acetylmuramic acid), these are cross-linked by peptide side chains, acts as target site for action of penicillin type antibiotics
• Small amount of complex lipteichoic acid traverse wall

Question 5

Describe gram negative bactera

Answer 5

• Stain red
• Outer membrane made of phospholipids, lipopolysaccharides, protein molecules, porins (protein channels)
• Lipopolysachharides composed of lipid A and polysaccharide (O antigen), plays important role as endotoxin
• Between outer membrane and cell membrane there’s periplasmic space, stores enzymes including, in some bacteria, antibiotic inactivating enzymes
• Thin peptidoglycan cell wall
• Outside cell wall, often a capsule and slime layer of polymericpolysaccharides.
• Capsule normally more organised, whilst slime is a polysaccharide mass around the cell. These polysaccharides functions include adherence to other membranes, and prevention of phagocytosis. They are also antigenic

Question 6

What are the growth conditions for bacteria?

Answer 6

• Carbon
• Water
• Nitrogen
• Nutrients + minerals
• Correct pH
• Correct osmotic pressure
• Correct temperature
• Correct atmosphere

Question 7

How do bacteria grow?

Answer 7

Via binary fission, doubling every 20-30 mins (approx)

Question 8

What are common community acquired bacterial infections?

Answer 8

• UTI- Escherichia coli
• Skin and soft tissue infection- Staphylococcus aureus
• Pharyngitis (pharynx inflammation, sore throat) - Stretococcus pyogenes

Question 9

What are common hospital acquired bacterial infections?

Answer 9

• Post-operative wound infection- Staphylococcus aureus (including MRSA)
• Ventilator- associated pneumonia (on ITU) - Pseudomonas aeruginosa

Question 10

What are common viral infections that are community acquired?

Answer 10

• Upper respiratory tract infection - rhinovirus, influenza, RSV, coronavirus
• Gastroenteritis- Norovirus and adenovirus
• Rashes- Varicella zoster virus (chickenpox), measles, meningitis e.g. enterovirus

Question 11

What are common viral infections that are hospital acquired?

Answer 11

• Norovirus
• Influenza
• SARS- CoV-2 - Covid