Week 23 - microbe-host interactions: microbiota and pathogens Flashcards
Symbiotic relationships
Microorganisms have distinct
relationships (symbiosis) with humans
Symbiosis
Close interaction between two different species
Three symbiotic relationships
-Mutualism
-Commensalism
-Parasitism
Mutualism
Both species benefit from their interactions
E.g., Bacterial species living in the gut (gut microbiota/flora)
The human gut harbours trillions of microbes
Benefit to the bacteria -> nutrients availability, physical requirements for their growth
(anaerobic conditions, pH suitability)
Benefits to the human -> bacteria aid digestion, breaking down food that the host
cannot normally digest and producing vitamins (such as B and K)
Commensalism
One partner in the relationship benefits -> the other neither benefits nor is harmed
E.g., Commensal bacteria colonise epithelial surfaces of skin
Benefit to the bacteria -> acquire nutrients, ability to grow, colonising niches, without causing neither harm nor help
Commensal bacteria may become opportunistic pathogenic (e.g. via broken skin)
and cause disease
Parasitism
One partner, the pathogen, harms the host, causing infectious disease
E.g., Influenza viruses infects human cells of the respiratory system, causing COVID-19
Benefit to the virus -> virus takes advantage of the translational machinery of the cell to replicate (multiply) virus particles. Viruses are defined as obligate intracellular parasites
Harm for the human cells -> viral infections lead to the death of the cells, tissue damage and inflammation
Human microbiota
Microbiota - all the microorganisms that live in and on an organism
-Approximately 10^11 organisms
-1-3% total body mass
-Generally non-pathogenic
-Symbiotic with host
Early microbiota colonisation
-Microbiota begins developing at birth
->Breastfeeding affects microbiota
Bifidobacteria are important coloniser of the gut
Can ferment sugars found in human breast milk provides the infant with calories and lowers the gut pH, limiting growth of pathogens
Dynamic and diverse microbiota composition
Microbiota reaches an adult-
like composition by age 3
->Not only bacteria
Relatively stable in adults -> it changes with altering physiological states or lifestyle
e.g., diet, stress, antibiotic therapy
Variable composition from person to person, at different sites, over time -> reduced variability in the elderly
Human microbiota body sites
Distribution and composition of normal microbiota are determined by many factors
-Nutrients
-Physical and chemical factors
-Host defenses
-Mechanical factors
Internal organs and tissues (brain, blood, cerebrospinal fluid, muscles) are normally free of microorganisms
Different microbiota composition in distinct body sites
Microbiomes vary by body site
Microbiota species can be:
Resident flora – permanently
colonising host
Transient flora – temporarily
present with limited presence
Human microbiota functions
->Protection against pathogens / infections
-Competing for nutrients
-Exclusion of binding sites, preventing pathogens attachment/colonisation
-Production or stimulation of antimicrobial molecules
-Immune system stimulation / maturation
->Regulate inflammation
->Stimulate tissue development
->Dietary fibre fermentation (undigested) into short chain fatty acids
->Synthesise vitamins
->Modulate and affect the central nervous system (Gut-Brain Axis)
Dysbiosis
Dysbiosis -> imbalance of microbial composition
Results in reduction in microbial diversity
Can be caused by dietary changes, psychological and physical stress / including oral broad-spectrum antibiotics use
Dysbiosis can lead to a variety of diseases that involve inflammation
Opportunistic infections
Infections caused by commensals (normal microbiota) that do not usually harm the host in a healthy individuals but in some cases can become opportunistic pathogens:
Dysbiosis – altered microbiota composition -> opportunistic pathogens can outgrow beyond their niches
Immunocompromised patients – Permanent or temporary weakened immunity
-After treatments (anti-cancer chemotherapy)
-After surgery
-Disease (HIV infections)
-Malnutrition – alcoholism – drug abuse – genetic defects
Probiotics
Live microorganisms to restore the normal balance of microbiota (gut and genital tract) and related functions, conferring a health benefit
Prebiotics
Non-digestible fibre compound(s) to stimulate the growth and the colonization of probiotic microbes within a microbiota -> probiotic food
Synbiotics
Supplements that combine both prebiotics and a probiotics
Pathogen
Any microorganism that causes disease in a defined host
Opportunistic pathogen
Component of normal microbiota that can cause disease when the host is immunocompromised or when they have chance to outgrowth
Pathogenicity
Ability of a pathogen to cause disease
Virulence
Degree of harm (pathogenicity) inflicted on its host
Steps in pathogenesis of bacterial infections
- Entry of pathogens into the body -> any organism that causes disease according to the transmission routes (e,g.
penetration, inhalation, ingestion and introduction into the blood) - Attachment of the pathogen to some tissues
- Multiplication / colonisation
- Invasion / spread of the pathogen
- Evasion if the host defences / immunity
- Damage to the host tissue(s)
1) Entry
Entry into the host, portals of entry:
eyes, nose, mouth, ears, broken skin (blood), needles (blood), bites (blood), skin, anus, urethra, vagina, placenta, mammary glands
2) Attachment / adherence
Attachment of microbe to specific target cells (highly
specific and permanent or nonspecific and reversible)
->Adherence structures:
-Pili / Fimbriae
-Glycocalyx (Capsule)
3) Colonisation
Establish a site of microbial replication on or within host
4) Invasion
Invasiveness - ability to spread to adjacent/deeper tissues
virulence factors –> adaptations used by microbes to invade and
caused the host – causing tissue damage
Invasion (active or passive)
Active: high virulent pathogens breaching tissue barriers
and disrupt the tissue integrity through secretion of:
-Toxins - poisonous for host tissues
-Proteolytic enzymes – to dissolve tissue components
Passive: exploiting pre-existing tissue alterations -> skin lesions, insect bites, wounds
5) Overcoming host defences
Successful pathogens elude initial host responses during entry as well as the adaptive immune system during infection:
-Find shelter to avoid recognition by immune cells (surviving and replicating inside or between host cells, within biofilms)
-Avoid phagocytosis (capsule)
-Produce enzymes that inactivate innate immunity
-Mutate and / or reduce cell surface proteins detected by immune cells
6) Damage to the host tissue(s)
->Secreting proteolytic enzymes that degrade host cell for nutrients
->replicating inside the host cells and inducing apoptosis
->Toxins – disrupting the normal structure and function of host cells/tissues
-Exotoxins
-Endotoxins
->Inducing hypersensitivity reactions - excessive release of cytokines by immune cells and exacerbating inflammatory responses, even life threating
shock syndrome
Types of toxins
-Exotoxins
-Endotoxins
Exotoxins
Produced mainly inside gram+ bacteria as part of their growth / metabolism -> they are then released into the surrounding medium
Endotoxins
Part of the outer portion of the cell wall of gram- bacteria
They are liberated when the bacteria die and the cell wall breaks apart
