Normal microbiota Flashcards
How many microbial cells does a normal human body contain?
10^14.
-10 times the number of human cells
Where in the body is the density of microorganisms greatest?
Oral cavities and colon (10^10).
-moist areas
What is normal flora?
Organisms found in/on the body that don’t cause disease.
What is symbiosis?
Two or more organisms co-existing in close physical association.
-e.g. human host and normal flora
What are the different types of symbiosis? (4)
- Mutualism
- Neutralism
- Commensalism
- Parasitism
What is mutualism?
Both organisms benefit from symbiosis.
-e.g. N-fixing bacteria
What is neutralism?
Neither organism derives benefit or harm.
What is commensalism?
One organism benefits, the other derives neither benefit nor harm.
What is parasitism?
One organism (parasite) benefits at the expense of the other (host).
What is the difference between sterile and non-sterile sites?
Non-sterile sites have normal flora, sterile sites have no normal flora.
What is a feature of non-sterile sites?
They are exposed to the environment (directly/indirectly).
When does the acquisition of normal flora begin?
At birth.
-sterility is maintained until birth
Give some examples of non-sterile sites.
Skin
Nasopharynx
GI tract
Vagina
What is the difference between the flora present in babies who are breast fed and bottle fed?
BREAST-FED; mainly bifidobacteria and lactobacillus.
BOTTLE-FED; more enterobacteriacea.
How is sterility maintained at sterile sites?
- SURFACE CLEANING (open to environment)
- BARRIERS that allow unidirectional flow (next to non-sterile sites)
- PHYSICAL SEPARATION (cavities)
Give an example of a sterile site maintained by surface cleaning.
Lower respiratory tract.
- cleaning by ciliated epithelium
- bacteria removed by coughing/sneezing
Give an example of a sterile site maintained by barriers that allow unidirectional flow.
- Upper genital tract (cervix)
- Urinary tract (urethra)
- Middle ear (auditory tube)
Give an example of a sterile site maintained by physical separation.
- Plural cavity
- Peritoneal cavity
- Spinal cord and meninges
What are microenvironments?
Different sites within sites.
What is tissue tropism?
How well a particular organism grows in a particular habitat.
How does H. pylori survive in the stomach?
In releases ammonia to increase the pH.
What sort of bacteria tend to colonise moist areas?
Gram -ve.
Where are most bacteria located on the skin?
In sweat glands.
What is a big difference between the environments of the skin and the gingival crevice (space between tooth and gum)?
- Skin has variable temperature, and is an aerobic, dry and nutrient-poor environment
- Gigival crevice has a constant temperature and is an anaerobic, moist and nutrient-rich environment
What bacteria make up the skin flora? (3)
- Staphylococcus epidermis
- Staphylococcus aureus
- Propionibacterium acnes
What bacteria make up the mouth flora? (2)
- Viridans/oral streptococci
- Anaerobes
How are mouth flora beneficial?
Synthesise essential vitamins.
What bacteria make up the nasopharyngeal flora?
NOSTRILS - S. aureus
PHARYNX - streptococcus pyogenes, haemophilus influenzae, Streptococcus pneumoniae, etc
What bacteria make up vaginal flora pre-puberty?
- Skin flora
- Lower GI flora (mainly E. coli)
What bacteria make up vaginal flora post-puberty?
- Lactobacillus spp
- Skin flora
- Some Candida albicans
Why does the composition of vaginal flora change after puberty?
Glycogen is produced due to circulating oestrogens.
-Lactobacillus spp. ferment glycogen to maintain pH
Why is a low pH in the vagina beneficial?
Prevents infection by opportunistic pathogens.
-e.g. C. albicans overgrowth
What is an opportunistic infection?
An infection caused by pathogens that take advantage of a host with a weakened immune system or altered microbiota.
What feature of the stomach helps to inhibits bacterial growth?
Low gastric pH.
What organisms are found in the stomach and small intestine? (2)
Predominantly aerobic bacteria:
-Lactobacilli (acid-tolerant)
-H. pylori
Increase in numbers distally
How did Barry Marshall prove that there is a link between H. pylori and stomach ulcers?
Drank cultures of H. pylori»_space; stomach ulcer.
How can H. pylori survive in very acidic environments?
Produces urease»_space; convert urea to ammonia and CO2.
What proportion of the population is H pylori present in?
30-50%.
How many organisms are present in the large intestine?
10^10 organism/gram.
-400+ species
What proportion of the species in the large intestine are anaerobes?
95-99%.
- Clotridium spp
- Bacteriodes spp.
Which aerobic bacteria are present in the large intestine?
Enteric gram -ve bacilli.
-E coli, Enterobacter, etc
What prevents overgrowth of pathogens in the large intestine?
Colonisation resistance by normal gut microflora.
What are the main benefits of normal flora?
- Synthesis of vitamins (e.g. K and B12 by enteric bacteria)
- Colonisation resistance
- Induction of cross-reactive antibodies
What does E coli produce in the colon?
Vitamin K.
-essential for blood clotting, bone metabolism
What are the main methods of colonisation resistance?
- Environment manipulation (e.g. decrease pH)
- Antibacterial agents (e.g. FAs, bacteriocins)
What is the main risk factor for C. difficile?
Antibiotic treatment.
- disruption of normal flora»_space; bacteria overgrowth
- HCAI
What are faecal transplants used to treat?
C. difficile.
- increases the diversity of microfora
- better recovery than just vancomycin
How can normal flora lead to disease? (3)
- Overgrowth
- Translocation
- Cross-infection
How does translocation of normal flora lead to disease?
Presence at the wrong site.
-e.g. conjunctivitis; H. influenzae in eye
Give an example of a disease caused by overgrowth of normal flora.
Vaginal thrush.
-often due to broad spectrum antibiotics
Give an example of a disease caused by cross-infection of normal flora.
MRSA.
What proportion of normal flora is culturable?
1%.
