Bugs In The System And Viruses

Question 0

What causes infection?

Answer 0

Pathogenic microbes (Koch’s postulates)

Question 1

What is infection?

Answer 1

Multiplication of a pathogenic microbe on or in a susceptible host with associated dysfunction or damage

Question 2

Why do particular individuals get particular infections?

Answer 2

Weakened immune system, poor nutrition, poor living circumstances, no access to healthcare (vaccinations), genetic predisposition, lifestyle (e.g. STDs)

Question 3

Explain Koch’s postulates

Answer 3

The organism occurs in every case of the disease
It occurs in no other disease
On removal from the body and growing in pure culture it can induce the disease anew
1. Isolate the organisms from every case
2. Propagate in pure culture in vivo
3. Re-inoculate and produce disease
4. Re-isolate

Question 4

Compare and contrast the key properties of bacteria and viruses as distinct from eukaryotic cells

Answer 4

Bacteria - no membrane bound organelles, one chromosomes, introns absent, transcription and translation are coupled, smaller ribosomes (30S+50S=70S), peptidoglycan cell wall
Viruses - have RNA or DNA (not both), no small ions or polysaccharides, may (enveloped) or may not (non-enveloped) contain lipids, protein outer coat (capsid)
Eukaryotic cells - membrane bound organelles, many chromosomes, introns present, transcription and translation are compartmentalised, bigger ribosomes (40S+60S=80S), no peptidoglycan cell wall

Question 5

Explain the significance of the Gram and acid fast stains for the classification and detection of bacteria

Answer 5

Gram stain - positively charged crystal violet binds to negatively charged cell components, iodine forms large molecular complexes with crystal violet, acetone or methanol extract the complexes through the gram-negative but not through the gram-positive bacterial cell wall, red dye stains now unstained gram-negative cells
(Gram-negative - red, gram-positive - blue)
Acid fast - detect bacterial causes of TB and leprosy (mycobacteria), positive acid fast bacilli = infection

Question 6

Explain the terms ‘pathogen’ and ‘non-pathogen’

Answer 6

Pathogen - organisms (bacteria, viruses) that causes disease

Non-pathogen - bacterium that don’t cause disease, many perform essential ecological roles

Question 7

Explain the general significance of genome composition, envelope and replication strategy for the classification and detection of viruses

Answer 7

Genome composition - RNA viruses more likely to mutate (RNA less stable)
Envelope - more easily sterilised (if holes are punched in, virus cannot survive)
Replication strategy - RNA viruses use reverse transcriptase (HIV), DNA viruses use cell machinery (HPV), can carry its own replication enzymes

Question 8

Outline the biochemical and genetic basis of bacterial susceptibility and resistance to antibiotics

Answer 8

Penicillin, amoxicillin, flucloxacillin, cephalexin have a B-lactam group and target cell wall. Vancomycin has a glycopeptide group and targets cell wall.
Gentamicin has an aminoglycoside group and targets protein synthesis, erythromycin has a macrolide group and targets protein synthesis, tetracycline has a polyketide group and targets protein synthesis
Rifampicin, trimethoprim, metronidazole are chemotherapeutic and target anti-folates

Question 9

What influences the outcome of infection?

Answer 9

Host’s immune system, affected body systems, drugs used

Question 10

Explain basic features concerning the epidemiology of infection by understanding the different habitats that may be occupied by microbes

Answer 10

Air, soil, bodily fluids, animals, plants, humans

Question 11

Understand the terms ‘reservoir’, ‘source’ and ‘mode of transmission’ in the context of infection

Answer 11

Reservoir - any person, plant, animal, soil, substance in which an infectious agent lives/multiples
Source - readily available form of infectious agent
Mode of transmission - ingestion, inhalation, physical contact, inoculation, sexual transmission

Question 12

Udders tamed the terms ‘carriage’, ‘normal flora (microbiota)’ and ‘commensal’

Answer 12

Carriage - passage taken by microorganism e.g. nasal, throat
Normal flora (microbiota) - non-pathogenic bacteria naturally found in the body e.g. in the gut
Commensal - symbiotic relationship where one species drives benefit (microbe) and the other is unaffected (host). The microbe may be replicating or waiting for the immune system to become compromised