Introduction To Microbes And Normal Flora Flashcards
What was the ancient perspective on what caused disease?
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
What is the germ theory?
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
What are the 4 points of Koch’s postulates?
- Micro-organism must be found in abundance in all organisms suffering from disease but not in healthy organisms
- Micro-organism must be isolated from a diseased organism + grown in pure culture
- Cultured micro-organism should cause disease when introduced to healthy organism
- Micro-organism must be re-isolated from inoculated diseased experimental host + identified as being identical to original specific causative agent
What are the 5 classes of microbial organisms?
Viruses Bacteria Fungi Protozoa Prions
What are viruses? Give a few examples.
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
What are bacteria?
1-10um in size
DNA as a single chromosome (prokaryotes)
Independent metabolism + no organelles
Cell wall (distinct from cell membrane of host organism)
What are fungi?
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)
What are protozoa? Give a few examples.
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
What are prions? Give a few examples.
~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
What are the 2 different types of bacterial cell wall and how would they stain in a Gram stain?
Peptidoglycan = + Lipopolysaccharide = -
What are the 2 ways of classifying bacteria?
Gram stain = + or -
Shape = coccus or bacillus/rod
Give 4 sub classes of bacteria and a couple of examples of each.
- 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
What are 2 examples of fungi classes?
Dimorphic fungi e.g. Blastomyces dermatitidis
Opportunistic fungi e.g. Candida Albicans
What is a symbiotic relationship?
Close + often long-term interaction between 2 different species
Can be mutualistic, commensal or parasitic
What is a commensal relationship?
Symbiotic relationship between 2 different species where 1 derives some benefit + the other is unaffected
What is colonisation?
When a microbe grows on or in another organism without causing any disease
What is infection?
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
What is a pathogen?
Micro-organism that is able to cause disease
What are the 2 types of pathogen?
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
What is a parasite?
Old-fashioned term for pathogens that are protozoa or metazoa (worms + ecto-parasites)
What are normal commensal bacteria + what do they do?
“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
What is the effect of antibiotic treatment on normal flora?
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
How can you prove the pathogen-disease link?
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)
List a few key differences between bacteria + human cells.
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
List a few key differences between bacteria + viruses.
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