Infectious Disease and Bacterial Characteristics Flashcards
infection remains a major cause of ____
death and disease
some very successful eradication efforts
- polio: worldwide
- smallpox: worldwide
- measles: western hemisphere
many remaining challenges of infectious disease
- emerging diseases
- antimicrobial/antibiotic resistance
- globalization - increasing spread
endemic
- regularly found in the population, stable
- ex: common cold, flu
epidemic
- significant increase in new infections
- AKA “outbreak”
- ex: 2010 cholera outbreak in Haiti - vibrio cholera (contaminated drinking water)
pandemic
- widespread epidemic
- ex: flu pandemic of 1918: ~40 million deaths worldwide
- ex: 2019-2020 SARS-COV-2 pandemic
5 modes of disease transmission
- direct: droplet, direct contact
- indirect: airborne, vectors, vehicles
droplet transmission
- short-range, short-term aerosols with large droplet particles
- coughing, sneezing, and talking
- ex: influenza, pertussis
direct contact transmission
- physical touch
- kissing, sexual intercourse, contaminated soil in an open wound
- ex: HIV, herpes, hookworm
airborne transmission
- longer-term, longer-range aerosols with small particles
- ex: measles, hantavirus (rodent urine)
vector transmission
- organisms that carry infection between hosts
- arthropods and mosquitoes
vehicle transmission
- inanimate objects that harbor the pathogen
- fomites: cover paddle, slit lamp, countertops
- contaminated food or water
entry portal
- route by which infection enters a host
- mucous membranes
- lungs
- skin
- parenteral (IV)
exit portal
- route by which infection leaves a host
- bodily secretions (tears, saliva, mucous, etc.)
- blood
- respiratory droplet
zoonotic transmission
- diseases that move from animal to human
- ex: SARS-COV-2 (bats), anthrax (sheep)
reservoir
- traditional host or source of infection
- environment (soil, air)
- animals
- human carriers (HIV)
- microbiome/microbiota
vertical transmission
- mother to child
- placental: treponema pallidum (syphilis)
- perinatal: HIV
- postnatal: staphylococcus aureus
horizontal transmission
- person to person
- direct or indirect contact
infectivity
- measure of ability to establish an infection
- measured by “ID50” or “infectious dose, 50%”
- dose necessary to infect 50% of exposed individuals
- ex: hepatitis B virus has an ID50 of 10 viral particles
pathogenicity
- measure of ability to produce disease
- if you are infected, will you get sick or remain asymptomatic?
virulence
- measure of disease severity
- if I get sick, how sick?
toxigenicity
- ability of toxins produced by pathogens to cause damage to host cells
- measured by “LD50” or “lethal dose, 50%”
- dose necessary to cause death in 50% of exposed individuals
infectious microorganisms
- bacteria
- viruses
- fungi
- protozoa
- helminths
bacteria characteristics
- prokaryotes
- no membrane-bound organelles
- peptidoglycan cell wall (support and protection)
- reproduce asexually via binary fission
- horizontal gene transfer can occur, and antibiotic resistance can be transferred
bacterial genetics
- circular DNA: genophore found in nucleoid region
- plasmids: separate from genophore, confer additional abilities
horizontal gene transfer
- conjugation: plasmid exchange via pilus
- transformation: uptake of free DNA from the environment
aerobic bacteria
- obligate aerobes
- require oxygen for growth/energy
anaerobic bacteria
- do not need oxygen for growth/energy
- usually found in the GI tract
- 3 types: obligate, aerotolerant, facultative
obligate anaerobes
die with exposure to oxygen
aerotolerant anaerobes
tolerate but do not use oxygen
facultative anaerobes
can use oxygen but don’t need it to survive
bacterial cell wall
- used to classify bacteria
- gram negative: outer membrane + thin cell wall, lose iodine-crystal violet complexes, appear red/pink with a counterstain
- gram positive: thicker cell wall, retains iodine-crystal violet complexes, appear dark blue/purple when decolorized
bacteria morphology
- cocci= spherical
- bacilli= rod-like
- vibrio= comma-shaped
- spirochete= spiral
bacteria categories
- “true” bacteria
- filamentous bacteria
- spirochetes
- mycoplasma
- rickettsia
- chlamydia
“true” bacteria
- most disease causing bacteria
- binary fission
- certain types of morphologies
- ex: staphylococcus aureus
filamentous bacteria
- branching, fungus-like structures
- ex: actinomyces israelii (causes canaliculitis)
spirochetes
- motile, anaerobic, spiral filaments
- treponema pallidum= syphilis
mycoplasma
- lack a cell wall= pleomorphic (many shapes)
- mycoplasma pneumoniae
rickettsia
- pleomorphic, obligate intracellular parasite
- rickettsia rickettsii
chlamydia
- pleomorphic, obligate intracellular parasite
- chlamydia trachomatis (causes chlamydial conjunctivitis and trachoma)
microbiome: normal flora
- 100 trillion microbes
- 10:1 bacteria: human cells
- symbiotic/commensal relationship
- opportunistic pathogens
symbiotic/commensal relationship between humans and microbiome
- secrete digestive enzymes
- metabolite production
- part of the innate immune system
opportunistic pathogens in microbiome
- breakdown in immune/inflammatory system
- injury
- immunocompromised
process of infection: overview
1) colonization
2) invasion and evasion
3) multiplication
4) spread
process of infection: colonization
- originate in reservoir
- transmission (direct or indirect)
- adherence
bacterial adherence
- adhesion molecules and receptors - protects against mechanical barriers of the innate immune system, ex: coughing
- tissue tropism - due to specificity of adherence
- glycocalyx (glycoprotein coat) - sticky “capsule”, blocks interaction of immune cells
- fimbriae/pili - hairlike structures on the surface of the bacteria, stick to the host tissue (like Velcro)
- flagella (also motility)
- other various specific adhesion molecules
process of infection: invasion and evasion
- confrontation of host defense mechanisms
- penetrate tissues - surrounding tissues or elsewhere
- evade host immunity - specific and non-specific (innate barriers)
evasion of host immune response
- encapsulation
- coat themselves in “self-protein”
- degrade immune molecules (IgA)
- adaptive immune suppression via toxins
- block complement cascade products
- antigenic variation
evasion of host immune response: encapsulation
- polysaccharide coat + some specific proteins
- prevents complement activation and inhibits phagocytosis
evasion of host immune response: coat themselves in “self-protein”
- host antibodies
- fibrin coat
- capsule antigen mimicry (hyaluronic acid)
evasion of host immune response: antigenic variation
- mutation: changes genes that code for surface molecules of bacteria; can happen quickly, even during an infection
- transformation: picking up DNA from the environment
- gene switching: different genes turn on and off at different times; bacteria express different surface antigens
process of infection: multiplication
- host= warm, nutrient rich environment –> rapid growth; tissue damage, disease symptoms
- obligate intracellular
- extracellular bacteria growth can produce biofilm
biofilm
- complex aggregation of bacteria
- glycocalyx holds the cells together and blocks immune system
- slows pathogen metabolism and growth (weaker immune response)
- resistant to antibiotics and host defenses
- grows on solid substrates: body (tooth plaque, heart valves (endocarditis)); medical devices (catheters, artificial valves, contact lenses)
process of infection: spread
- not required for disease (ex: cholera)
- within body or between people
- relies on a variety of virulence factors: adhesion molecules, toxins they produce aid in movement, protection against host immune system, opportunistic infections
- some highly invasive (enter lymphatics, blood (sepsis))
process of infection: multiplication and spread
- rapid division (bacteria reproduce much faster than host can respond)
- bacteremia or septicemia (presence of bacteria in blood, usually gram negative*)
- secrete enzymes that allow spread (hyaluronidase, streptokinase/fibrinolysin, collagenase)
clinical stages of infection
- incubation
- prodromal
- invasion (illness)
- convalescence
clinical stages of infection: incubation
-period between initial exposure and onset of first symptom
clinical stages of infection: prodromal
-onset of initial symptoms - vague (malaise)
clinical stages of infection: invasion (illness)
- period of active immune response
- major symptoms experienced
clinical stages of infection: convalescence
- period of recovery and decline of symptoms after removal
- infection goes dormant
- alternative: death
common symptoms: direct or indirect cause
- fatigue
- malaise
- weakness
- lack of concentration
- generalized aching
- loss of appetite (anorexia)
hallmark of infection
fever
fever is caused by ____
- exogenous pyrogens: microbial origin
- endogenous pyrogens: immune cell origin (IL-1, TNF-alpha)
- pyrogens act on the hypothalamus
endotoxin/lipopolysaccharide (LPS)
- in outer membrane of gram negative bacteria
- released upon death of bacteria
- lipid A and polysaccharide chain
- activates plasma protein systems (initiates widespread inflammatory response)
- causes the release of inflammatory mediators (TNF-a, interferon, IL-1)
- fever and shock may result (pyrogenic bacteria)
exotoxins
- enzymes produced while bacteria is alive
- released during bacterial cell growth
- cause major damage to host directly (disrupt plasma membranes, disrupt intracellular signaling, inhibit protein synthesis, weaken epithelial adhesions)
- elicit production of antibodies (immunogenic)- we can develop immunity and vaccines against these exotoxins (ex: tetanus, pertussis, diphtheria)
botulinum toxin (clostridium botulinum)
- neurotoxin
- blocks release of ACh at NMJ= no action
- Botox: treatment for blepharospasm
diphtheria toxin (corynebacterium diphtheria)
- blocks protein synthesis within target cells
- causes necrosis of heart and liver
cholera toxin
- changes electrolytic balance across intestinal cell membrane
- causes massive outflow of water into intestinal lumen
- can cause death due to dehydration
superantigens
- exotoxins (some)
- binds MHC II and TCR regardless of specificity (cause activation of T-cells w/o being specific for that T-cell)
- activates many more Helper T-cells (widespread activation)
- induce excessive cytokine production (fever, low blood pressure, shock)
endospores
- produced by some gram positive bacteria
- formed under harsh environments
- metabolically inactive
- resistant to environmental extremes (heat, dryness, radiation, acids and other chemicals)
- bacillus anthracis and clostridium botulism
