5.2 pathogens

primary pathogens

make healthy people ill

opportunistic pathogens e.g., Pseudomonas aeruginosa in burns, Staphylococcus epidermidis on vascular catheters).

exploit breaks in barriers or lowered immunity

Some bacteria switch roles depending on location

for instance, E. coli is beneficial in the colon but pathogenic in the urinary tract.

virulence factors

factors than envoke disease

virulence factor examples

• surface receptors that bind to host cells]

• surface coats that inhibit immune responses like phagocytosis and toxins

virulence factors allow bacteria

to colonise invade and replicate withing an immune competent host

bacterial toxins manipulate the funcitons of host cell functions

resulting in the hijacking of vital processes in order to favour microbial infection

this interference with host cellular processes often involves

toxins secreted across their outer membrane through different secretion systems or by direct injection into the host cell

bactieral toxins have 2 classifications

1. exotoxins

2. endotoxins

exotoxins

able to damage the host through either causing cellular damage/ destruction or by disrupting cellular metabolism

exotoxins are highly potent

and can cause major damage to the host

Major GI relevant exotoxin diseases

botulism,

diphtheria,

Dysentery  

and Cholera.

endotoxins are a type of LPS.

Soluble endotoxin are released when the bacteria are destroyed, and also released physiologically as outer membrane vesicles. 

exotoxin explained

1. exotoxins are proteins

2. produced inside pathogenic gram positive bacteria, part of their grown and metabolism

3. rereleases into the surrounding medium following lysis

endotoxin explained

• endotoxins are  lipid proportions of lipopolysaccharides LPSs

• part of the outer membrane of the cell wall of gram negative bacteria

• liberated when bacteria die and the cell wall breaks apart

gram stian process

a differential staining technique used to categorise bacteria into gram positive/ gram negative groups based on cell wall structure

gram negative bacteria

thin wall of peptinoglycan + outer membrane

gram positive

thick wall of peptinoglycan

gram staining chemicals required

1. crystall violet- pass through the cell wall

2. iodine- interacts with crystal violet ions that trap it-

3. alcohol- washes out crystal violet-iodine complexes (in gram negatives)

4. counterstain- taken up by both, recolours gram negative

gram positive appear purple

gram negatives appear red/pink

more difficult to treat gram negative bacteira

due to extra outter membrane

Gram‑positive bacteria (purple)

• Thick peptidoglycan retains crystal violet‑iodine complex during alcohol wash.

• Usually more susceptible to β‑lactam antibiotics because the wall is exposed.

• Examples include Staphylococcus aureus (skin/soft‑tissue infections), Streptococcus pyogenes (pharyngitis), Enterococcus faecalis (UTI).

Gram‑negative bacteria (pink/red)

• Thin peptidoglycan plus outer membrane.  Alcohol easily leaches out the primary stain; cells take up safranin counterstain.

• Outer membrane (LPS) restricts large antibiotics, so many Gram‑negatives resist penicillins without β‑lactamase inhibitors.

• Representative species: Escherichia coli, Neisseria gonorrhoeae, Pseudomonas aeruginosa.