Human Microbe Interactions

Question 1

What is a parasitic interaction?

Answer 1

• Where one partner is harmed while the other may, or may not, derive benefit
• Infectious disease
  ○ Host harm – easy to detect
  ○ Parasite benefit - difficult to prove in some cases
  -completing life cycle in host is an obvious benefit e.g. sleeping sickness

Question 2

What is a mutualistic interaction?

Answer 2

• Both partners benefit from the interaction
  -This would be exemplified by organisms that are normally resident on the host.

Question 3

What is a commensal interaction?

Answer 3

• There is no harm perpetrated but no demonstrated benefit either
• This would be exemplified by organisms that are normally resident on the host

Question 4

What is normal microbiota?

Answer 4

• Microbes that are normally resident on a healthy individual
• Microbes participating in mutualistic and commensal interactions contribute to the normal microbiota of an animal, plant or human

Question 5

Sites of microbial residence on the human body

Answer 5

• Skin
• Respiratory tract
• Genitourinary tract
• Gastrointestinal tract

Question 6

Why is the skin a hostile environment for microbes?

Answer 6

• thick, keratinised surface layer
• low pH
• low H2O availability
• antimicrobial secretions from glands
• Distribution of glands is not uniform therefore resident microbiota not uniform (microenvironments)

Question 7

How to study skin microbiota- original methodology

Answer 7

• Sample skin with sterile swab
• Transfer to selective growth media
  ○ Different media support different organisms
  ○ No one medium that will support every organism

Question 8

Microbes of the Upper Respiratory Tract

Answer 8

· Microbes move from the skin to populate this region by colonising mucosal surfaces
· Nose
· Nasopharynx
· Oropharynx
· Resident bacterial populations are established by organisms capable of attaching to mucosal epithelial cells.
· Many species inhabiting this region are anaerobes (both facultative and obligate)

Question 9

What does facultative and obligate mean?

Answer 9

-“obligate” means that an organism can only survive by using one type of respiration (aerobic or anaerobic)

-“facultative” means that it prefers aerobic respiration but can survive on anaerobic respiration if needed.

Question 10

Microbes of the lower respiratory tract

Answer 10

□ Transient microbes may be detected but there is no permanent, resident population
□ Bronchi
□ Alveoli

□ This region is “sterile” due to the efficiency of the non-specific defence mechanisms
□ Alveolar macrophages
□ Mucocilliary escalator

Question 11

Microbes of the oral cavity

Answer 11

· A number of very different environments exist that are capable of supporting a microbial population
○ Teeth
○ Saliva
○ Buccal epithelium

· The saliva contains nutrients yet also contains inhibitory substances which regulate the population
○ Lysozyme
○ Lactoperoxidase

Question 12

How does the genitourinary tract protect itself?

Answer 12

• Vagina has a resident microbiota
• mucosal surface
• hormone regulated
• After puberty Lactobacillus spp. become the dominant population metabolising glycogen and releasing organic acids as a metabolic by-product
• Low pH makes the environment inhibitory to most other bacteria

Question 13

What is the function of the GI microbiota

Answer 13

· Intestinal microbiota contribute a large number of biochemical and metabolic functions to the system

• Vitamin synthesis thiamin, riboflavin, B12, K
• Gas production CO2, CH4, H2, H2S
• Nitrogen fixation Klebsiella pneumoniae
• Sterol metabolism
• Organic acid production acetic, propionic, butyric
• Stimulation of Immune System Development!
• Protection from GI infection
• Mechanism not known
• Blockage of pathogen adhesion sites?
• Direct antagonism of pathogens?
• Stimulation of immune function?
• Exploited in probiotic products
• “bioactive” yoghurts etc.
• Lactobacillus spp., Bifidobacterium spp.