Commensal Bacteria vs Pathogens

What is the microbiome

The bacteria on us, in us, and around us, maintaining a balanced microbial community that can shift to a disease state (dysbiosis) if disrupted.

What are virulence factors

Properties / Mechanisms that allow microorganisms to become pathogenic and cause harm to the host.

How can microorganisms be used to treat diseases

"Microorganisms, such as probiotics, can treat diseases by maintaining the gut microbiota, helping restore balance and prevent dysbiosis."

What is mutualism in symbiotic associations

"Mutualism is a symbiotic relationship where both organisms rely on and are dependent on each other for their metabolic needs."

What is commensalism in symbiotic associations

"Commensalism is a symbiotic relationship where the host provides food or shelter, benefiting another organism, which may return the favor without dependency."

What is parasitism in symbiotic associations

"Parasitism is a symbiotic relationship where one organism harms the host through its presence and activity; these relationships can dynamically switch to other types."

What is the progression from asymptomatic carriage to disease

1. Asymptomatic Carriage: (organisms present without symptoms) →

2. Colonisation: (organisms establish on host) →

3. Infection: (organisms invade) →

4. Disease (harmful symptoms manifest).

Why is the skin an inhospitable microbiome for organisms

Skin makes it challenging for microbial growth:

• Dry

• Sheds hair

• Has an acidic pH

• Lacks food (only keratinized regions provide nutrients)

How can Staphylococcus epidermidis shift from commensal to parasite

"Staphylococcus epidermidis shifts from commensal to parasitic when it enters a surgical wound, changing its anatomical positioning and causing disease."

What is the microbiome of the upper respiratory tract

"The upper respiratory tract (nose, oropharynx) has microflora;

• Neisseria meningitidis can colonize,

• Saliva with lysozymes removes organisms

• Virulence factors enable attachment to gingivitis gaps and biofilm formation.

How do phagocytes protect the lungs in the respiratory microbiome

"Phagocytes ENGULFS organisms entering the lungs, while mucus clears them, preventing infection in the lower respiratory tract."

How does pH affect the microbiome of the GI tract

• The acidic pH of the stomach kills most bacteria

• As pH rises along the GI tract, more bacteria grow, with the highest numbers in the large intestine and colon

• Microbes can migrate to the appendix causing appendicitis.

What is the role of bacteria in the GU tract microbiome

In the GU [Genitourinary] tract (urethra, rectum, vagina):

• Lactobacilli in the vagina feed on glycogen

• Produces lactic acid to lower pH, creating a safe environment for childbirth

• Prevents uterine infections that could lead to premature birth or miscarriage.

What are Koch's postulates

FOUR criteria to establish a causal relationship between a specific microbe and a specific disease.

1. The organism must be found in all hosts with the disease.

2. It must be isolated in pure culture.

3. It must produce the same disease when reintroduced into a healthy host.

4. It must be re-isolated in pure culture.

What are the problems with Koch's postulates?

• Organisms may only grow in lab conditions (unrealistic)

• Genetic diversity

• Shifts from virulent (disease-causing) to avirulent (non-disease-causing) state

• Ethical issues with deliberately causing disease in humans.

What is the molecular version of Koch's postulates

1. Genes (or products) have pathogenic potential.

2. Genes are in pathogenic strains, not avirulent ones.

3. Disrupting the gene reduces virulence.

4. Cloning the gene transforms avirulent strains to virulent.

5. The gene is expressed during infection.

6. The gene product elicits an immune response.

What are the methods of pathogen transmissibility

• Direct airborne contact (e.g., coughing)

• Indirect airborne contact (e.g., wind), direct contact (e.g., touching)

• Contact via inanimate objects (e.g., handrails, contaminated food)

• Vectors (e.g., mosquitoes causing malaria)

How do pathogens adhere to and colonize the host

"Pathogens use pili for transient adhesion to membranes, easily breaking off to evade immune response, and produce mucous slime to attach and grow on surfaces."

What occurs during colonization of mucosal surfaces

"Enzymes break down epithelial tissues, aiding pathogen colonization on mucosal surfaces."

How do pathogens invade the host

• Hiding from immune detection

• Rearranging the host cell cytoskeleton

• Forcing uptake into the cell

How do pathogens grow and multiply in the host

Iron is vital for DNA synthesis, respiration, and metabolism, but the host’s immune system limits its availability to hinder pathogen growth via Nutritional Immunity.

Pathogens use:

• Siderophores (catechols/hydroxamates) to chelate free iron (scarce in natural environments)

• Exotoxins (secreted proteins from vegetative cells) to release bound iron by breaking down tissues.

How do pathogens evade the complement pathway

"Pathogens evade the complement pathway leading to opsonisation (marking pathogens for destruction), inflammation, and direct pathogen lysis as part of the immune response, using anti-phagocytic strategies to avoid destruction."

What is opsonization in immune evasion

• Opsonization tags pathogens for phagocytosis, being engulfed and destroyed.

• Pathogens employ anti-phagocytic strategies.

Involves the coating of the pathogen's surface with molecules called opsonins (e.g., antibodies or complement proteins like C3b).

What are nosocomial infections

"Nosocomial infections are hospital-acquired diseases caused by pathogens present in healthcare settings."