65 terms

NGR: Normal Flora vs. Pathogens

OMS 2015, Garcia-Russell, NGR
What is the definition of normal flora?
microorganisms that colonize the body but do not normally produce diseases
Where is the normal flora located?
parts of the body that are exposed or communicate with the outside environment
When does one first acquire his/her normal flora?
First acquired during and shortly after birth; will vary continuously throughout life
What are the three types of symbiosis?
What is commensalism?
Beneficial to one partner while the other is unaffected
What is mutualism?
Interactions between two organisms in which both organisms benefit
What is parasitism?
Beneficial to one partner while harmful to the other
Where do we have normal flora?
Mouth, nose & throat
What are the three advantages of the normal flora?
antigenic stimulation - aids in development of immune system
prevention of colonization by pathogens through MICROBIAL ANTAGONISM
production of vitamins K and B by gut flora
What is microbial antagonism and how does it work?
- competitive exclusion of bad microbes by good microbes
Works by:
- competing with pathogens for nutrients & space
- producing toxic factors or environmental conditions harmful to pathogens
What is the big disadvantage to our normal flora?
It is harmful when overgrown or misplaced
What are two examples of overgrowth/misplacement of natural flora?
Use of broad-spectrum antibiotics
Fecal flora entering urinary tract
What are the two types of bacterial pathogens?
Strict pathogens
Opportunistic pathogens
Define strict pathogens.
- always associated with human diseases
- e.g. Mycobacterium tuberculosis (tuberculosis), Neisseria gonorrhoeae (gonorrhea)...
Define opportunistic pathogens.
- take advantage of preexisting conditions that enhance susceptibility of patient to cause a disease or a more serious disease
- many of them are members of the patient's normal microbial flora - e.g. Pseudomonas aeruginosa, Candida albicans
What are nosocomial infections?
Hospital-acquired infections (≠ community acquired infections)
What are the two causes of the compromised status of a host (which can lead to nosocomial infections)?
- invasive procedures (surgery, anesthesia)
- suppressed immune systems (drugs, radiation)
What are the two transmission routes of nosocomial infection?
- person-to-person
- fomite (i.e. through non-living objects: e.g. needles, respiratory aids...)
Define infection.
- colonization of body by pathogenic microbes
- may exist without symptoms of a disease
What is an etiologic agent?
The cause of disease.
What is pathenogenesis?
How the disease develops.
What are predisposition factors to infection?
Factors that make the host susceptible to disease
e.g. gender, genetic background, geographical location, age, lifestyle, immune status
What is the MOA of bacterial pathogenesis?
1. Enter into the target environment (us!)
2. Colonize - remain in the niche
3. Gain access to food sources
4. Escape clearance by host's protective responses
What is the problem with the process of bacterial pathogenesis?
It causes damage to the human host.
What are the bacterial portals of entry to infection?
Respiratory tract - inhalation
GI tract - ingestion
GU tract - sexual transmission
Skin - trauma; arthropod bite
What are the ways that bacteria can exit the host?
- transmission to a fresh host
- shed in large numbers in secretions and excretions
- available in blood for uptake, e.g. blood-sucking arthropods or needles

(green pathways in graphic = shedding)
What are the host factors that organisms need to overcome to infect the host?
Immune system:
Innate & specific immunity
What are the organism factors that help the organism overcome the host factors?
Dose (# of organisms)
Virulence (ability to cause disease)
To what structures do bacteria adhere during adhesion?
epithelial or endothelial cells of bladder, intestine, respiratory tract, blood vessels...
What are the major mechanisms of adhesion during colonization?
- pili
- adhesin proteins
- teichoic acid
- biofilm
- capsule
What comes after adhesion?

After adhesion, invasive micro-organisms have the ability to penetrate the target tissue and invade a new site
What are the two ways that invasive bacteria gain access to the host?
Normal phagocytic entry mechanisms
Enter host cells that aren't normally phagocytic
How do bacteria enter host cells that aren't naturally phagocytic?
- specific attachment to the host cell
- production of invasins (surface proteins)
- induction of local rearrangements of the cytoskeleton --> formation of pseudopod-like structures which engulf the pathogen into the host cel
How does Legionnaire's disease get into the host cells?
It induces the host to utilize coiling phagocytosis.
The host sends arm outside to grab bacteria and get it inside.
How does salmonella get into the host cell?
Upon adherence, the pathogen triggers surface ruffles in the host membrane that grabs pathogenic bacteria
What are the ruffles made out of?
plasma membrane sites with filamentous actin cytoskeletal rearrangement
What are the three ways that bacteria escape the host defense?
- inactivating or evading the complement system and antibody

- evading recognition and killing by phagocytes

- growing inside cells to hide from these host responses
What are the 5 ways that bacteria evade complement or antibody response?
- Intracellular growth
- Antigenic variation/mimicry/masking
- Inactivation of antibody or complement
- Capsule/biofilm (prevent complement & antibody from reaching the bacteria)
- Protection of the membrane from attack (LPS O antigen)
What are the three ways that bacteria evade phagocytosis?
- Preventing contact with phagocyte (e.g. capsule)
- Preventing opsonization
- Production of enzymes capable of lysing phagocytes
How does bacteria evade phagocytic killing?
Escape from phagosome
Inhibition of phagolysosome fusion
Resistance to lysosome products
What are the four types of damage done to the host by pathogens?
By-products of bacterial growth - tissue destruction (acids or gas from fermentation)

Degradative enzymes - break down host tissues & facilitate development of infection and spread into the tissue

Toxins - directly harm tissue or trigger destructive biologic activities

Induction of hypersensitivity reactions
What are endotoxins? Name three examples.
Cell wall components freed when cells die and break up.

- gram(-) lipopolysaccharide = endotoxin
- in gram(+): teichoic and lipoteichoic acids - endotoxin-like effects
- peptidoglycan and its breakdown product
What are the effects of endotoxin-mediated toxicity?
What enzyme activates almost every immune mechansim, including the clotting pathway?
Lipopolysaccharide (LPS)
What are endotoxins?
Proteins released by living bacterial cells
Where are endotoxin genes located?
on the plasmid
on lysenogenic phages
What are cytolysis?
Extracellular-acting toxins
How do cytolysins act?
Enzymatic lysis of the cell membrane, which leads to host cell death

Pore formation, causing loss of nutrients and host cell death
What are the three ways that intracellular-acting toxins access their substrates inside the host cells?
AB toxins
Direct injection of toxins by flagella or pilus
Direct entry of toxins through the plasma membrane
What are AB toxins? How do they work?
Intracellular-acting extratoxin that is released from the pathogen. The B subunit binds to the target host receptor and induces AB endocytosis into the host cell. Upon entering the cytosol, the A subnit is released into the cytosol to find its target, and modify the target's activity (ribosome, transport mechanisms, intracellular signaling)
What type of secretion system uses direct injection via flagella or pilus to access substrate inside host cells?
How does the diphtheria toxin work?
AB toxin, where A stops translation, protein synthesis & causes cell death
What is the pathenogenesis of dipththeria?
Infection of mucosa of oropharynx
Multiplication of bacteria and production of toxin
Exotoxin activity --> death of surrounding cells

Inflammatory reaction leads to collection of a gray exudate which evolves into thick pseudomembrane -- SUFFOCATION/OBSTRUCTION
What is the intracellular action of cholera?
AB toxin
A subunit HYPERACTIVATES cAMP, causing diarrhea and shrinking of cells (less water)
What is the intracellular action of tetanus toxin?
AB toxin
blocks release of INHIBITORY neurotransmitters, causing unopposed firing of motor neurons (TETANY)
What is the intracellular action of botulinum toxin?
AB toxin
blocks release of acetylcholine from peripheral nerves, preventing contraction (FLACCID PARALYSIS)
How do superantigens work?
They remove the specificity of the TCR - MHCII interaction such that ANY TCR will bind to ANY MHCII
Causes a massive release of cytokines
Name two superantigenic pathogens
Scarlet fever
Toxic shock syndrome
What is the MOA and what are the clinical features of TSS?
S. aureus where the toxin (NOT the organism!) disseminates throughout the body

Clinical features: Sudden onset: fever, vomiting, diarrhea
Then: red rash resembling a sunburn and desquamation 1-2 wks later
Hypotensive shock, impaired renal and hepatic functions, occasional deaths
When you think colonization, what two concepts should you think of?
What should you think of when you think of pathogenic damage?
By-products of growth (gas, acid)
Degradative enzymes
Cytotoxic proteins
What should you think of when you hear pathogenic evasion of host defense?
Evasion of phagocytic and immune clearance
Capsule / Biofilm
Intracellular growth
What are epidemic diseases?
Infectious diseases that attack many people at the same time in the same geographical area.
What are endemic diseases?
Diseases that constantly present in a local population, e.g. Malaria
What are pandemic diseases?
Worldwide occurence