Diversity of bacterial pathogens

Question 1

How do pathogenic microbes cause infection?

Answer 1

By gaining foothold to a particular niche.

Question 2

What does pathogenicity mean?

Answer 2

ability of a pathogen to inflict damage on the host

Question 3

What does colonisation mean?

Answer 3

growth of a microbe after gaining access to host tissue.

Question 4

what does virulence mean?

Answer 4

the degree of pathogenicity of an infection pathogen. So, pathogenicity is the bacteria’s ability to inflict damage, and virulence is the level of damage caused. For example, a bacteria may have a good ability to inflict damage (pathogenicity), but the level of damage may be low (virulence).

Question 5

What is the process of an infection:

Answer 5

Exposure to pathogens
Adherence to skin or mucosa
Invasion through the epithelium
Colonisation and growth (production of virulence factors)
This can lead to either:
- Toxicity - toxin effects local or systemic
- Invasiveness - further growth at original and distant sites
Both of which cause tissue damage and disease.

Question 6

respiratory infections:

Answer 6

Bacterial or viral. Upper respiratory tract has an abundant microbiome which is often asymptomatic due to colonisation resistance. Lower respiratory tract is devoid of a microbiome. Infections will cause air sacs to inflame and leak fluid, which is bad for transmission as it will be spread when coughing.

Question 7

What is mycobacterium tuberculosis (respiratory infection)?

Answer 7

• 10 mill deaths annually
• mortality rate of 2/3 people
• carried in 1/4 population, but not always activated (wont replicate)
• intracellular pathogen - survives within immune cells
• rising drug resistance

Question 8

What is Legionella Pneumophila (respiratory infection)?

Answer 8

• 10 cases per 1 mil people.
• grows in stagnant water (35dC)
• high morality - targets vulnerable groups like elderly and immunocompromised
• survives within immune cells through creation of a vacuole

Question 9

What is E.coli?

Answer 9

• gram -ve rod bacterium and has many pathotypes
• intestinal enterohaemorrhagic E.coli is known as the burger bug. O157:H7 being the most common stereotype. It encodes a shiga toxin which is phage encoded, it is released after antibiotic exposure, moving to kidneys and causing failure. This is why drugs are not used to treat it, and instead rehydration therapy is used.
• E.coli can cause disease outside of the gut too when they evolve, such as - UTI and meningitis.

Question 10

What is vibrio cholerae?

Answer 10

• 4mil cases and 100,000 deaths annually
• gram -ve motile, commonly associated with aquatic reservoirs
• a phage-encoded CTX results in massive loss of fluid. The toxin does this by invading epithelium cells and interrupting ion balance, causing mass deflux of ions out of the cell, causing water to follow - thus diarrhoea.

Question 11

What is helicobacter pylori?

Answer 11

• gram -ve motile that spirals
• very common, affecting 40% of people, mostly asymptomatic
• can cause stomach pain and an agent of gastric (80%) and peptic (95%) ulcers.
• survives at a low pH and hence is associated with the stomach, unlike most GIT pathogens.

Question 12

What are UTI and bloodstream infections?

Answer 12

UTI - caused by E.coli that has adapted to survive outside the gut, and instead in the urinary tract. They enter the bladder and form biofilms along bladder epithelium. This also allows them to hide from immune cells. UTIs are often redcurrant and drug resistant.
Bloodstream infections - lead to sepsis

Question 13

What are skin infections?

Answer 13

• often gram+ve cocci e.g. staphylococcus and streptococcus.
  The skin is constantly exposed to bacteria. Immuno-challenged people allow pathogens to colonise and form a new niche, these pathogens are called opportunistic pathogens as in a healthy person they wouldn’t cause disease, but will colonise when the opportunity arises.
  They cause skin leisons, inflammation, but can also disseminate to infection in the throat, ear, blood and lungs.

Question 14

What is immunity and inflammation?

Answer 14

Immunity = bodies defence system. Can be innate which is immediate rapid response, or adaptive which is targeted defence and built up by exposure.
Inflammation = nonspecific reaction driven by neutrophil accumulation.

Question 15

what causes rubor (redness) and calor (heat) during the body defence against a pathogen?

Answer 15

Both caused by increased blood flow due to vasodilation.

Question 16

What causes tumor (edema/ swelling) and dalor (pain) during the bodies response to a pathogen?

Answer 16

Tumor = increased movement of plasma from blood to tissue (capillary level permeability)
Dalor = due to localised swelling from tumor formation. Fluid will stretch the nerves and pain receptors. This can cause a change in function, meaning the change in tissue structure can lead to, for example, reduced mobility.

Question 17

What is the difference between humoral and cellular immunity?

Answer 17

Humoral - B cells produce antibodies to attack free antigens.
Cell-mediated - T cells attack intracellular pathogens.

Question 18

What is the timeline to s. enterica respone?

Answer 18

0m - ingest contaminated food
15m - invasion of epithelial cells
1h - neutrophils appear
3h - neutrophils induce tissue damage and inflammatory induced fluid accumulation
8h - mass effusion of neutrophils and fluid in intestinal lumen
24-48h - tissue damage, diarrhoea, more fluid accumulation

Question 19

What is localised vs systemic infection?

Answer 19

Localised is within one tissue, most gut infection are self-limiting and they mainly remain local to the gut. however, if they get into the blood stream they can move about the body and become systemic. Bacteraemia = the presence of bacteria in the bloodstream, leading to mass organ dissemination. Sepsis = extreme, system-wide inflammatory response to blood poisoning. Systemic shock = systemic drop in blood pressure leading to mass organ failure.

Question 20

What are four types of culture diagnostics:

Answer 20

• General purpose (non-selective, establishes growth)
• Enrichment (selective FOR certain species)
• Selective (grows ONLY certain bacteria)
• Differential (selects BETWEEN 2 bacterial species)

Question 21

what is general purpose media?

Answer 21

Uses LB (Luria-Bertani) agar, generally containing yeast extracts, salts, water and agar.
It can grow fastidious (fussy/needy) and non fastidious bacteria such as yeast and fungi.
Its often a starting point to look for unexpected organisms; and it allows the build up of number of bacteria to do further diagnostics.

Question 22

what is enrichment media?

Answer 22

When testing fastidious bacteria, the agar media is replaced with sheep’s blood agar - allowing the organism to grow faster. But it is dependant on the organism.
This is used when we are trying to identify the organism based off what WE KNOW about the organism. For example, streptococcal strains produce enzymes called haemolyses that lyse RBCs, so by culturing this on blood we can see if any colonies seem to be lysing the blood, therefore proving it is strep.

Question 23

what is selective media?

Answer 23

Used on a mixed bacterial population sample, and often performed on blood based agar containing antimicrobials. The sample is incubated at 42dC in microaerobic (low O2) conditions - inhibiting the growth of most enteric bacteria and allowing growth of campylobacter.
It not only uses media, but also growth conditions as selected e.g. antibiotic selection.
Therefore, this media will only permit the growth of organisms you are looking for, e.g. temperature restricted growth. Antibiotics will repress the growth of everything on the plate except campylobacter.

Question 24

what is differential media?

Answer 24

General purpose but with a chromogenic substrate additive, testing for the presence or absence of enzymes = diagnosis.
Organisms such as E.coli, enterococcus and salmonella look similar on a growth plate and have similar growth requirements, by understanding the enzymes they produce, we can develop different diagnostic tools to help us identify them.
the media will contain dyes that are activated when the specific enzyme is present. E.coli produces beta galactosidase.