Introduction To Microbes And Normal Flora

Question 1

What was the ancient perspective on what caused disease?

Answer 1

Ancient Greeks thought infectious diseases were due to miasma i.e. poisonous gas from rotting materials e.g. swamps -> malaria

Some diseases were known to be contagious e.g. leprosy + smallpox & humans/animals were known to have visible worm infestations

Question 2

What is the germ theory?

Answer 2

The discovery that diseases were caused by micro-organisms

First came to light in 1674 when Leeuwenhoek discovered micro-organisms + ended with Koch in 1876 proving that a specific microbe caused anthrax

Question 3

What are the 4 points of Koch’s postulates?

Answer 3

1. Micro-organism must be found in abundance in all organisms suffering from disease but not in healthy organisms
2. Micro-organism must be isolated from a diseased organism + grown in pure culture
3. Cultured micro-organism should cause disease when introduced to healthy organism
4. Micro-organism must be re-isolated from inoculated diseased experimental host + identified as being identical to original specific causative agent

Question 4

What are the 5 classes of microbial organisms?

Answer 4

Viruses
Bacteria
Fungi
Protozoa
Prions

Question 5

What are viruses? Give a few examples.

Answer 5

20nm - 1um in size
RNA or DNA
No independent metabolism + no organelles
Simple protein coat + cell membrane from host cell
E.G. influenza, rhinovirus, rotavirus + norovirus

Question 6

What are bacteria?

Answer 6

1-10um in size
DNA as a single chromosome (prokaryotes)
Independent metabolism + no organelles
Cell wall (distinct from cell membrane of host organism)

Question 7

What are fungi?

Answer 7

5-10um in size
DNA as multiple chromosomes in a nucleus (eukaryotes)
Independent metabolism + organelles (no chloroplasts)
Cell wall (distinct from cell membrane of host organism)

Question 8

What are protozoa? Give a few examples.

Answer 8

10-100um in size
DNA as multiple chromosomes in a nucleus (eukaryotes)
Independent metabolism + organelles
Cell membrane similar to host organism
E.G. Entamoeba histolytica + Giardia lamblia

Question 9

What are prions? Give a few examples.

Answer 9

~10nm
No RNA or DNA just a protein enzyme
No independent metabolism + no organelles
No protein coat, cell wall or cell membrane
E.G. Scrapie, Kuru + CJD

Question 10

What are the 2 different types of bacterial cell wall and how would they stain in a Gram stain?

Answer 10

Peptidoglycan = +
Lipopolysaccharide = -

Question 11

What are the 2 ways of classifying bacteria?

Answer 11

Gram stain = + or -

Shape = coccus or bacillus/rod

Question 12

Give 4 sub classes of bacteria and a couple of examples of each.

Answer 12

• Gram + cocci e.g. Staphylococcus + Streptococcus
• Gram + bacilli e.g. Bacillus anthracis + Lactobacilli
• Gram - cocci e.g. Neisseria meningitidis + Haemophilus influenzae
• Gram - bacilli e.g. Escherichia coli + Salmonella

Question 13

What are 2 examples of fungi classes?

Answer 13

Dimorphic fungi e.g. Blastomyces dermatitidis

Opportunistic fungi e.g. Candida Albicans

Question 14

What is a symbiotic relationship?

Answer 14

Close + often long-term interaction between 2 different species

Can be mutualistic, commensal or parasitic

Question 15

What is a commensal relationship?

Answer 15

Symbiotic relationship between 2 different species where 1 derives some benefit + the other is unaffected

Question 16

What is colonisation?

Answer 16

When a microbe grows on or in another organism without causing any disease

Question 17

What is infection?

Answer 17

The invasion + multiplication of microbes in an area of the body where they are not normally present

May be asymptomatic, sub-clinical or cause varying degrees of symptoms being clinically apparent

Question 18

What is a pathogen?

Answer 18

Micro-organism that is able to cause disease

Question 19

What are the 2 types of pathogen?

Answer 19

Obligate: must cause disease to be transmitted from one host to another + must infect a host to survive i.e. cannot survive outside of host

Opportunistic: takes advantage of opportunity not normally available i.e. host with weakened immune system, altered microbiota (e.g. gut flora) or breached barriers

Question 20

What is a parasite?

Answer 20

Old-fashioned term for pathogens that are protozoa or metazoa (worms + ecto-parasites)

Question 21

What are normal commensal bacteria + what do they do?

Answer 21

“Normal flora” or “microbiota”

Occupy majority of available body areas e.g. skin, mouth, upper airways, lower airways, GI tract + genital tract

At least commensal + probably mutualistic in preventing more pathogenic bacteria occupying those areas

Question 22

What is the effect of antibiotic treatment on normal flora?

Answer 22

Antibiotics can eliminate normal flora making infections more likely e.g. Clostridium difficile associated diarrhoea

Hence another reason to limit antibiotic use + consider use of “friendly bacteria” as treatment e.g. faecal microbiota transplant

Question 23

How can you prove the pathogen-disease link?

Answer 23

Find evidence of microbes in various ways in an area of the body where they are not normally present + correlate this with clinical features of the patient to check whether diagnosis is plausible (must correlate with clinical features as microbe identification can be uncertain + take time)

Question 24

List a few key differences between bacteria + human cells.

Answer 24

Human cells have no cell wall
Bacteria (i.e. prokaryotes) have no distinct nucleus
Human cells only undergo asexual reproduction, not sexual too like bacteria
Bacteria survive on their own but human cells are in a group not isolated (dependent on other cells for survival)
Bacteria can move using flagella, human cells normally cant
Human cells bigger

Question 25

List a few key differences between bacteria + viruses.

Answer 25

Viruses have no cell wall but protein coat
Bacteria are unicellular but viruses aren’t really cells or living (depend on host)
DNA/RNA enclosed in protein coat in viruses but free in cytoplasm of bacteria
Bacteria undergo asexual reproduction whilst viruses hijack host cell for replication