Microbiology - 1

Question 1

What are the 7 major categories of organizms that cause infectious disease?

Answer 1

1. Arthropods
2. Bacteria
3. Fungi
4. Helminths
5. Prions
6. Protozoa
7. Viruses

Notes:

• identification & classification = essential for correct diagnosis, tx, and control!

Question 2

Compare and contrast prokaryotes & eukaryotes

Answer 2

Bacteria = prokaryotes; All other = eukaryotes

Prokaryote:

• No nucleus
• Single circular DNA (chromosome)
• Plasmids
  
  • extrachromosomal DNA
• Transcription & Translation = simultaneously
• No membrane-bound organelles

Eukaryotes:

• Nucleus (membrane bound)
• Multiple chromosomes
• Transcription –> mRNA in nucleus
• Translation –> ribosomes in cytoplasm
• Membrane bound organelles

Question 3

What are the advantages/disadvantages for intracellular & extracellular microbes?

Answer 3

Intracellular microbes (Microparasites - viruses, bacteria, protozoa, fungi)

• viruses = obligate intracellular
  
  • take genetic machinery from host
  • take nutrients directly from intracell microenvironment
  • produce large # progeny
• protected from:
  
  • host’s immune defenses
  • antibiotics or other drugs
• often live w/in immune cells
• transmission btw host cells involves:
  
  • destruction; contribute to tissue damage

Extracellular microbes (Macroparasites - worms, arthropods):

• take nutrients directly from
  
  • tissue fluids
  • feeding on host cells
  • intestinal contents
• grow &reproduce freely
• move & spread rapidly w/in body tissue
• size –> problems to host’s immune defenses
  
  • worms = several protective external layers & move quickly

Question 4

What are the pros/cons of the human microbiota (normal flora)?

Answer 4

Pros:

• normal flora PREVENTS colonization by potential pathogens
  
  1. skin bacteria
     
     • produce fatty acids –> discourage invasion
  2. gut bacteria
     
     • antimicrobial activity + metabolic waste products –> prevent invasion from other species
     • release organic acids –> metabolic value to host
     • produce B vitamins & vitamin K
  3. vaginal lactobacilli
     
     • maintain acid environment –> supresses growth of other organisms
• antigenic stimulation provided by intestinal flora –> ensures normal development of immune system

Cons:

• spread into previously sterile parts of the body if intestine or skin = perforated
  
  • teeth extration
  • UTI
• normal flora –> causes of hospital–acquired infection
  
  • pts exposed to invasive tx
  • burn victims
• overgrowth by potentially pathogenic members of the normal flore can occur, leading to:
  
  • composition of flora changes (ie after antibiotics)
  • local environment changes (ie increase in stomach or vaginal pH)
  • immune system becomes ineffective (ie AIDS/immunosuppression)
• Potential pathogens take advantage of the opportunity to increase their population size / invade tissues

Notes:

• In health, blood + blood fluids + tissues = sterile

Question 5

What are the differences between the 3 types of symbiosis: commensalism, mutualism, and parasitism?

Answer 5

Commensalism:

• 1 species lives HARMLESSLY in/on the body of a larger species
  
  • smaller species makes use of environment to acquire nutrients
• Majority of these are BACTERIA
  
  • relationsip w/ host may be highly specialized
  • ex: in humans, bacteria ferment and digest food
    
    • harmful if tissue damaged (surgery); flora changes (antibiotics); or immunity reduced

Mutualism:

• reciprocal BENEFITS for BOTH organisms
  
  • relationship is OBLIGATORY for at least 1 member (sometimes both)
• Examples:
  
  • BACTERIA & PROTOZOA living in stomach of domestic ruminants of cattle
    
    • digestion & utilization of cellulose –> make fatty acids
    • host uses those as energy source

Parasitism:

• ALL pathogens are parasites!
  
  • 1 sided relationship –> benefits parasite ONLY!
• Parasites establish innocuous assosciation in natural host, but may be pathogenic in others:
  
  • ie: rabies virus
• Example:
  
  • Entamoeba histolytica - Protozoa in human that feeds on mucosa –> ulcers & dysentery

Question 6

What is the evolution of parasitism?

Answer 6

Bacteria became parasitic by ACCIDENTAL CONTACT

• 1st lived OUTSIDE host cell
  
  • evoluition of INTRACELL HABIT req further modification to allow survival WITHIN host cells
  • survival of microbe DEPENDED on ability to EVADE EUKARYOTE ERADICATION
• May have evolved from symbiotically associated HETEROTROPHIC PURPLE BACTERIA

Evidence of endosymbiont origin of mitochondria:

• EUK cells still have endosymbiotic bacteria
• Ribosomes of mitochondria = more like PROK
• Mitochondira have own DNA = circular
  
  • control their own division
  • gene expression = similar to PROK
• Production of energy associated w/ inner membrane or mitochondria
• Antibiotics that kill bacteria, affect ribosomes of mitochondria

Question 7

What are the 5 major ways microbes can enter the host?

Answer 7

1. Skin
   
   • hair follicles
   • sebaceous glands
     
     • skin bacteria can enter –> cause styes & boils
   • wounds/abrasions/burns
   • biting arthropods
     
     • penetrate skin via feeding
   • Defenses:
     
     • fatty acids inactivate microbes
     • pH ~5.5
     • normal flora
2. Conjunctiva
   
   • specialized area of skin cleaned w/ tears & lids
   • contaminated w/ fingers/towels/flies
   • Defense:
     
     • antimicrobial tears
     • flushing of eyelids
3. Respiratory Tract
   
   • air contains suspended particles (smoke/dust/microorganisms)
   • some microorganisms can overome cleansing mechnanisms
   • Defense:
     
     • mucous has antimicrobial properties
     • alveolar macrophages remove foreign bodies
4. GI Tract
   
   • some microorganisms can survive the intestine’s defenses of acid, mucus, and enzymes
   • Defense:
     
     • no particular cleansing mechanism besides flow (peristalsis)
     • saliva w/ antimicrobials
     • mucous lined epithelial cells
     • digestive enzymes
     • secretory IgA
5. Urogenital Tract
   
   • from perineum –> urethra
     
     • intestinal microbes
   • foreskin & fecal bacteria
   • Defense:
     
     • low pH f ~5.0
     • bladder = protective mucous layer + secretion of Abs & immune cells
     • flushing action of urine
     • secretory IgA

Question 8

What are the factors that can affect transmission of microbes?

Answer 8

Nearly all SHED from body surfaces

• Extracted by vectors:
  
  • blood sucking arthropods
    
    • transmit yellow fever, malaria, filarial worms

Successful Transmission depends on:

1. # of microorganisms shed
2. microorganisms stability in environment
3. # of microorganisms needed to infect fresh host
   • efficiency of infection
4. Genetic factors

Question 9

What are the mechanisms for transmission between humans?

Answer 9

Microorganisms can be transmitted to humans via:

1. humans
   
   • most effective
2. vertebrates
   
   • zoonoses
3. biting arthropods

Most common infections spread by humans:

• skin
  
  • shed/direct contact
• respiratory
  
  • droplets
  • poor ventilation
  • air conditioning
    
    • not readily controllable
• fecal-oral
  
  • saliva
  • food/water
  • contaminated hands
    
    • controllable by public health
• venereal routes (urogenital tract)
  
  • contact w/ discharge
  • mucosal contact w/ semen & vaginal secretions
  • perinatal
    
    • difficult to control b/c social factors

Note:

• Infections acquired from other species = not transmitted well from human –> human

Question 10

What are the mechanisms for transmission from animals to humans?

Answer 10

Transmission from Animals:

• domestic pets = reservoirs of infection
  
  • pathogens spread via:
    
    • contact (bites, scractches)
      
      • rabies
    • vectors
      
      • ticks/sandflies
    • contamination w/ fecal material
      
      • hookworm/toxocarisis/plasmosis

Major infections transmitted this way:

1. TOXOCARIASIS = dogs
2. TOXOPLAMOSIS = cats

Question 11

What is the difference btw surface and systemic infections?

Answer 11

Basics:

Surface infection

• faiure to spread to deeper tissue
  
  • influenza virus in respiratory epithelium

Systemic Infection

• invasion of deeper tissue
  
  • herpes simplex virus

Details:

• microorganisms –> spread systemically b/c they FAIL TO SPREAD/MULTIPLE at the site of initial infection/body surface
  
  • next to no replication at site of entry
    
    • measles/typhpoid
  • only after spreading thru body systemically –> large number of microorganisms delivered back to surface to multiply & shed
• microorganisms –> spread systemically b/c they COMMIT THEMSELVES TO INFECTION BY 1 ROUTE
  
  • shedding/replication happens at another site
    
    • mumps/hep A = respiratory/mouth routes
      
      • invade & multiple in salivary glands & liver

Question 12

What are the mechanisms for the spread of infections through the
body?

Answer 12

Once microbe enters body via epithelium & basement membrane:

1. tissue fluid has antimicrobial substances (Ab + complement)
2. local macrophages (histocytes)
   
   • subcutaneous & submucosal
3. physical barrer of local tissue structure
   
   • hydrated gel matrix
   • virus spread via step-wise invasion of cells
4. lymphatic system
   
   1. infection can multiply locally or in lymph nodes
   2. evading phagocytosis –> reach bloodstream
5. blood stream
   
   1. organisms free in blood = exposed to defenses such as Abs & phagocytes

Spread & replication affected by:

• genetics of host
• genetics of microbe
• other host factors:
  
  • pregnancy
  • age
  • malnutrition
  • stress/hormones
  • pollution

Question 13

How can the genetic determinants of the host and the invading
microbe affect the spread of infection?

Answer 13

Genetics of the microbe & host determine spread & replication

• Virulence = determined by:
  
  • adhesion, penetration into cells, antiphagocytic activity, production of toxins, interaction w/ immune system
• different microbial genes & gene produces are involved in diff stages of pathogenesis
  
  • some pathogens infect same species or related primate
    
    • measles, trachoma, typhoid, HBV, warts
  • some pathogens infect a wide range of hosts
    
    • rabies, anthrax
• Genetic determinants of host –> susceptibility