Lecture 14: Commensal Microbes and Pathogens

Question 1

What has the highest potential impact on overall human health?

Answer 1

Prevention of age-independent causes of death

Question 2

What is a synonym for mutualism?

Answer 2

Symbiotic

Question 3

1) Give 3 examples of parasitic bacteria
2) Give an example of a symbiotic relationship
3) Give an example of a commensal relationship

Answer 3

1) Parasitic: Measles, tuberculosis, typhoid
2) Symbiotic: Microflora inhabiting the rumen of cows
3) Commensalism: Bacteria that live on human skin

Question 4

Define colonization, infection, and disease

Answer 4

1) Colonization: Organism does not interfere with normal physiology of the host. Microbe has established itself and is multiplying on/in the host.
2) Infection: Microbe has a parasitic relationship with host
3) Disease: Presence of organism leads to damage to the human host.

Question 5

What two ways can damage be caused by disease?

Answer 5

1) May arise directly from invading organism (e.g. toxin) 2) From the host immune response to the organism

Question 6

1) Define strict pathogens. Give 2 examples.
2) Define opportunistic pathogens. Give an example.

Answer 6

1) Strict Pathogens: organisms that are always associated with disease
-Example: Mycobacterium tuberculosis or Francisella tularensis
2) Opportunistic Pathogens: Organisms that are typically members of normal microbial flora (e.g. Escherichia coli) but can establish disease when found in unprotected sites within the human host (e.g. bloodstream)

Question 7

What are the 3 potential outcomes to exposure to a microbe?

Answer 7

1) Transient colonization: microbe comes and goes from the host in a short amount of time.
2) Permanent colonization: microbe permanently lives in the host organism.
3) Produce disease: pathogen produces a disease in host organism

Question 8

In most cases, how many different manifestations of a disease can happen from one organism?

Answer 8

Multiple

Question 9

1) Give 2 examples of organisms with single, well-defined disease states
2) Give 1 example of an organism with multiple disease states.
3) Give 1 example of a single disease state produced by many organisms

Answer 9

1) Treponema pallidum produces syphilis; Plasmodium species produces malaria
2) Staphylococcus aureus: endocarditis, pneumonia, wound infections, food poisoning
3) Meningitis can be caused by viruses, bacteria, fungi, and parasites

Question 10

1) Define exogenous infections and give 3 examples
2) Define endogenous infections and give 3 examples

Answer 10

1) Exogenous infections: Disease arising from exposure to external source
-Examples: Diseases caused by influenza virus, Clostridium tetani, Neisseria gonorrhoeae
2) Endogenous infections: Disease arising from normal host microbial flora (more common)
-Examples: Escherichia coli, Candida albicans, Staphylococcus aureus

Question 11

1) Define primary pathogen
2) Define opportunistic pathogen and give an example.
3) Define virulence

Answer 11

1) Primary pathogen: Organisms that cause disease in a healthy host
2) Opportunistic pathogen: Organisms that cause disease only when the host immune system is compromised or if the organism is introduced into an unusual location
-e.g. Pseudomonas, associated with hot tub burns and medical devices
3) Virulence: Measurable degree of pathogenicity

Question 12

1) Define a communicable (contagious) disease
2) Define an infectious dose
3) Define incubation period

Answer 12

1) Communicable (Contagious) disease: Spread easily from one host to another
2) Infectious Dose: Number of microbes required to establish infection
3) Incubation period: Interval of time between introduction/ exposure of an organism to a host and the onset of illness

Question 13

1) Define acute
2) Define chronic
3) Define latent

Answer 13

1) Acute: Rapid onset of symptoms (e.g. strep throat)
2) Chronic: Infection develops slowly and lasts longer (e.g. tuberculosis)
3) Latent: Infection is never completely eliminated

Question 14

What are the 3 terms used to describe the duration of an infection?

Answer 14

Acute, chronic, or latent

Question 15

1) Define convalescence
2) Define horizontal transmission (spread)
3) Define vertical transmission. What type of bacteria do it?

Answer 15

1) Convalescence: Period of recuperation and recovery after illness; infectious agent may still be spread
2) Horizontal Transmission (spread): Microbes are spread to others through infected air, water, food, contact, vectors, etc.
3) Vertical Transmission (spread): Microbes are spread to offspring through ovum, sperm, placenta, milk, contact. Commensal bacteria.

Question 16

List the 6 necessary steps for infectious microorganisms

Answer 16

1) Attachment and/or entry into the body
2) Local or general spread in the body
3) Multiplication
4) Evasion of host defenses
5) Shedding (exit) from the body
6) Cause damage in the host: not obligatory

Question 17

1) What phenomenon is associated with attachment and/ or entry into the body? What is the requirement?
2) What phenomenon is associated with local or general spread in the body? What is the requirement?
3) What phenomenon is associated with multiplication? What is the requirement?

Answer 17

1) Attachment’s phenomenon: Infection (early)
Requirement: Evade hosts’ natural protective and cleansing mechanisms
2) Spread’s phenomenon: Local events, spread
Requirement: Evade immediate local defenses, and the natural barriers to spread
3) Multiplications’ phenomenon: Multiplication
Requirement: Multiply; many offspring will die in host or en route to a fresh host

Question 18

1) What phenomenon is associated with evasion of host defenses? What is the requirement?
2) What phenomenon is associated with shedding (exit) from the body? What is the requirement?
3) What phenomenon is associated with causing damage to the host? What is the requirement?

Answer 18

1) Evasion’s phenomenon: Microbial answer to host defenses
Requirement: Evade phagocytic and immune defenses long enough for full cycle in host to be completed.
2) Shedding’s phenomenon: Transmission
Requirement: Leave body at the site and on a scale that ensures spread to a fresh host
3) Damage’s phenomenon: Pathology, disease
Requirement: Not strictly necessary but often occurs

Question 19

Only a small proportion of microorganisms associated with humans gives rise to disease or causes pathological changes in spite of cell and tissue invasion. Why does this make sense evolutionarily?

Answer 19

Get what you need and move on. The goal is to be a successful parasitic microbe, not necessarily cause disease.

Question 20

1) How is polio transmitted?
2) How is polio shed?
3) In what percent of cases does polio cause a subclinical infection?
4) In what percent of cases does polio spread, and if it does, what part of the body does it invade?

Answer 20

1) Transmitted by fecal-oral route
2) Virus is shed in the feces of an infected person
3) 95% of cases only cause a subclinic intestinal infection
4) In 5% of cases the virus spreads and invades the CNS, causing meningitis and sometimes paralysis

Question 21

1) What must a pathogen do to cause disease?
2) What percentage of infections begin on mucous membranes? What 3 tracts can these membranes be found in?
3) What percentage of infections begin from vector bites? Give examples of vector bites.

Answer 21

1) A pathogen must attach and enter the host
2) 95% of infections begin on mucous membranes of the respiratory, GI, and urogenital tracts.
3) 5% from vector bites (conjunctiva, scratch, injury, arthropod vector)

Question 22

Name and define the 4 distinguishable types of microbial infection methods

Answer 22

1) Microbial attachment/ penetration mechanism: A strict (primary) pathogen is attached
2) Biting arthropod: A biting insect opens the skin to allow the pathogen entry
3) Skin wound/ animal bite: Skin barrier is broken due to an injury
4) Antimicrobial Defenses impaired: The host cell/ person’s immune defenses are impaired, which allows for infection.

Question 23

1) In a healthy host, what tissues are free of microorganisms?
2) In a healthy host, what tissues have microorganisms? Why?
3) Define normal flora

Answer 23

1) The internal tissues (e.g. blood, brain, muscle, etc) are normally free of microorganisms
2) Surface tissues (e.g. skin, mucous membranes) are constantly in contact with environmental organisms and become readily colonized by certain microbial species
3) Normal flora: The mixture of organisms regularly found at any anatomical site

Question 24

Are normal flora good or bad? Explain

Answer 24

-In exchange for room and board and a steady thermostat, our microbial flora (with few exceptions) work tirelessly on our behalf, supplying us with key nutrients and keeping infections and invaders at bay
-You would not be alive if you did not have normal flora!

Question 25

Colonization by ___________ microorganisms is an unavoidable consequence of all normal individuals

Answer 25

commensal

Question 26

Are there microbes in the womb? If not, when is an infant first exposed to mirobes?

Answer 26

The normal intrauterine environment is sterile, therefore the newborn infant has no bacteria (approximately ‘germ free’).
During birth, the infant is exposed to a variety of microbes that take up residence.

Question 27

What are the 3 benefits of normal flora?

Answer 27

1) Supply of certain vitamins (B12, K, folate, biotin)
2) Keeps the immune system ‘revved up’
3) Ecological stability: excludes potential pathogens

Question 28

1) How many species of bacteria are in the normal flora?
2) What factors does microbial flora make depend on?
3) What microbes can be found in microbial flora? Which is most numerous?

Answer 28

1) Highly complex consisting of more than 200 species of bacteria
2) Makeup depends on various factors of the host: genetics, age, sex, stress, nutrition, diet , etc.
3) Normal flora of humans consists of few eukaryotic fungi and protists, some methanogenic Archaea, but bacteria are certainly the most numerous

Question 29

1) How many cells are in normal flora?
2) How many cells make up the human body?
3) Is the normal flora population transient, resident, or both?
4) Why are the normal flora located at particular anatomical sites?

Answer 29

1) ~10^14 individual normal flora
2) ~10^13 body cells in a human
3) Normal flora population is both “transient” and resident
4) Tissue tropism: each pathogen has a certain cell type it’s good at invading.

Question 30

What are the 3 main sites of microbial colonization on the skin?

Answer 30

1) Primarily dermis
2) Hair follicles
3) Sebaceous glands

Question 31

1) What is the largest organ?
2) What type of bacteria are found on the largest organ the most? Why?
3) What happens when microorganisms other than commensals get on this organ?

Answer 31

1) The skin
2) Gram positive bacteria (Salt inhibits gram negatives)
3) Microorganisms other than commensals (resident flora) are soon inactivated

Question 32

Name how many bacteria are on the:
1) Scalp
2) Axilla
3) Abdomen
4) Forearm

Answer 32

1) Scalp: 1 million (per cm^2)
2) Axilla: >500K
3) Abdomen: 40K
4) Forearm: 11K

Question 33

What 7 factors are hostile to potential pathogens and maintain ecological balance between intact skin and its resident microflora?

Answer 33

1) Shedding of skin cells (desquamation), 5 x 10^8 squames per day
2) Dryness
3) High salt (so Gram-negatives die)
4) Presence of toxic fatty acids, etc
5) Acidic pH (~5.5)
6) General lack of nutrients
7) Normal microbial flora

Question 34

List the 4 protective mechanisms of the mouth, oropharynx, and nasopharynx against pathogens

Answer 34

1) Hairs of the nares act as a filter
2) Lysozyme in secretions in Nasopharynx
3) Lysozyme in sIgA in saliva
4) Desquamation

Question 35

1) What type of bacteria is the upper respiratory tract heavily colonized with?
2) Are most of these virulent or avirulent? If avirulent, when can they become virulent?
3) Give an example of an avirulent microbe found in the upper RI

Answer 35

1) Normal flora with at least a factor of 10 more anaerobes than aerobes
2) Most of these are avirulent unless introduced into sterile sites such as the sinus cavity, middle ear, or brain
3) Ex: Sinus infection with Streptococcus pneumoniae

Question 36

What organ system has numerous formidable defenses against microbes?

Answer 36

The respiratory tract

Question 37

1) Are the bacteria of the respiratory tract mostly pathogenic or non-pathogenic? If non-pathogenic, when can they become pathogenic?
2) What part of the respiratory tract is generally sterile? Why?

Answer 37

1) Organisms of upper respiratory tract are relatively non-pathogenic unless introduced to what is normally a sterile site (e.g. sinuses, middle ear, and brain).
2) Lower respiratory tract; glottis protects against aspiration of infected secretions and a mucociliary blanket covers most of its surface

Question 38

1) What can physically carry debris out of the respiratory tract?
2) What has a similar lining?

Answer 38

1) Ciliated cells, goblet cells (secrete mucus), and subepithelial mucus–secreting glands known as the mucociliary escalator carries debris (microbes) as far as the pharynx.
2) Nasal cavity has similar mucociliary lining

Question 39

1) What are the terminal air spaces of the respiratory tract called?
2) What protection do they have against microbes?

Answer 39

1) The alveoli
2) No cilia or mucus, but lined by alveolar macrophages

Question 40

GI Tract:
1) What are most organisms in the esophagus?
2) Most bacteria are suppressed in what part of the GI tract? Why?
3) As you go further down the lower GI tract, will you find more bacteria or less bacteria as you get lower?

Answer 40

1) Transient colonizers
2) In the stomach; gastric acid suppresses most bacteria (because most bacteria are acid sensitive and prefer slightly alkaline growth conditions)
3) More; 10^8 to 10^10 bacteria per gram in terminal ileum 10^11/ gram in the colon and rectum

Question 41

In what two ways does the intestinal tract differ from the respiratory tract?

Answer 41

1) Always in motion (peristalsis)
2) Changing surface contours (villi, crypts, etc).

Question 42

Bacteria normally compose about ______ of total fecal mass

Answer 42

1/4

Question 43

In what 4 ways does the GI tract protect against microorganisms?

Answer 43

1) Mucus barrier
2) Secretory IgA antibodies
3) Bile
4) Antibacterial pancreatic enzymes

Question 44

In what 4 ways does normal flora protect the GI tract from foreign bacteria? Which way is specific and which way is nonspecific?

Answer 44

1) Production of bacteriocins (specific)
2) Production of toxic short chain fatty acids (non-specific)
3) Competition for attachment sites and/ or nutrients
4) Stimulate production of cross-reactive antibiotics

Question 45

What are the 5 disadvantages of normal flora? (particularly gut flora)

Answer 45

1) Suggested role in colon and gastric cancers; peptic ulcer disease; IBD
2) Toxic compounds produced by anaerobic microbes
3) If normal balance mechanisms are upset, may become pathogenic
4) Normal fecal bacteria can infect UT (e.g catheters)
5) Modern medical care requires lower host immune response (organ transplant or Dx of cancer); these patients will often die of infections from commensals.

Question 46

1) What part of the genitourinary system is sterile?
2) What part of the genitourinary system is colonized?
3) What part of the genitourinary system has considerable flora? What is the most common bacteria here?

Answer 46

1) The bladder, uterus, and usually the cervix
2) Anterior urethra and vagina
3) The vagina; lactobacilli

Question 47

Genitourinary system: Give 3 reasons why the bladder is sterile

Answer 47

1) Protected by flushing
2) Length of urethra (20 cm in males, 5 cm in females)
3) Hypertonicity of the kidney is an unfavorable environment

Question 48

What protection does the vagina have against microorganisms? How?

Answer 48

No special cleansing mechanisms, but there is acid pH protection:
-At puberty, vaginal epithelium contains glycogen (via estrogen stimulation) which is metabolized by lactobacilli to lactic acid, offering pH protection