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

Question 12

A 30-year-old man eats a meal containing contaminated bean sprouts. Two days later, he develops severe abdominal cramps and diarrhoea.

Which mode of transmission most likely led to his infection?

Answer 12

Faeco-oral route - The faeco-oral route of transmission involves the ingestion of a pathogen through contaminated food or water, often due to poor sanitation or hygiene. Bean sprouts are commonly associated with outbreaks of foodborne illness because of their method of preparation and potential for bacterial contamination with faecal matter.