Microbiology- Causes of Infection

Question 1

Define the agents that can cause infections?

Answer 1

• Viruses
• Bacteria
• Fungi
• Parasites
• Prions

Question 2

Name a few facts about viruses?

Answer 2

Smaller than cellular organisms
Metabolically inert
Simple structure
Need living host cells to replicate
>1/100th size of bacteria
too small to see with light microscope
Different shapes
Infect animals, plants and bacteria (bacteriophages)

Question 3

What are the three properties on viral replication?

Answer 3

Viruses contain genetic material but no organelles
They rely on their host’s organelles / systems to reproduce
They use surface protein(s) to bind to a cell, insert their genetic material into it

Question 4

Why do viruses infect hosts for different lengths of time?

Answer 4

Some can stay dormant in host – with symptoms re-appearing months or years later

Question 5

What are the three examples viruses can lay dormant and appear later on in life?

Answer 5

Chickenpox virus- can lay dormant for decades, emerging to cause shingles
Hepatitis C virus- causes chronic liver infection over years
Rhinovirus - infects hosts for days, causing a cold

Question 6

Give facts on bacteria?

Answer 6

Unicellular organisms
Cell membrane
Cell wall
No nucleus
Genetic material is DNA but not bounded by a membrane
Reproduce asexually
Some move using flagella and attach via fimbriae

Question 7

What colour does gram +ve bacteria stain?

Answer 7

G+ve stain purple

Question 8

What colour does gram -ve bacteria stain?

Answer 8

G –ve stain pink

Question 9

Give 4 examples of bacterial diseases?

Answer 9

Meningococcal sepsis
Bacterial endocarditis
Cellulitis
Streptococcal throat infection

Question 10

Define fungi?

Answer 10

Kingdom of their own
Eukaryotes
Cell membrane, cell wall, nucleus and cytoplasmic structures
Reproduce sexually and asexually

Question 11

What is dimorphic fungi?

Answer 11

Diamorphic fungi (can switch between types)

Question 12

What is the difference between yeast and mould?

Answer 12

Yeast is the infection and mould is the reproducing stage.

Question 13

Give examples of yeast infections?

Answer 13

Mild infections
thrush, athletes foot, ringworm
Severe infections–In the Immunocompromised
Cryptococcal meningitis in HIV patients•Invasive candida in ICU–Or Immune competent

Question 14

What are the different types of parasites?

Answer 14

Ectoparasites–live outside body
Fleas, Ticks
Endoparasites–Iive inside body
Worms
Epiparasites–a parasite which lives on another parasite
Malaria (mosquito)

Question 15

What are the two general classes of human parasites?

Answer 15

Unicellular organisms-
Protozoa
Worms- Helminths

Question 16

What is giardia?

Answer 16

Cause bloody diarrhoea
Caught from drinking infected water
May be seen in stool under a light microsope
Cyst form aids survival + spread

Question 17

What are the different types of helminth worms?

Answer 17

Cestodes (tapeworms)
Segmented, flat
Trematodes (flukes)
Unsegmented, flat
Nematodes (round worms)
Cylindrical, have digestive tract with lips, teeth and anus

Question 18

What does cestodes cause?

Answer 18

Tapeworms–Fish, pork, beef tapeworms
Cause variety of disease:
–Malabsorption
–Malnutrition in chronic disease
–Cysts in muscle or brain

Question 19

What does nematodes cause?

Answer 19

Round worms
Biggest of helminth family
–Huge burden of disease worldwide
–Diarhoea / malabsorption
Problems often caused because worms are not “usual” human parasites–E.g. Elephantiasis

Question 20

Define prions?

Answer 20

Smallest infective agents known
Proteinaceous Infectious particles
Lack nucleic acid - not a ‘living organism’
Proteins fold abnormally and accumulate, mainly in neural tissue
They are very difficult to destroy
Concerns over cleaning surgical instruments

Question 21

What are the different types of prion diseases?

Answer 21

CJD- fatal, degenerative neurological disease
–Affects 1 in a million people each year
–Transmitted through contaminated human growth hormone, surgical instruments and corneal grafts
•Variant CJD- typically occurs in young adults
•BSE- occurs in cattle
•Scrapie- occurs in sheep
•Kuru- similar to vCJD, –occurred in Papua New Guinea in 1950s–thought to be spread by cannibalism

Question 22

What are the growth requirements for microorganisms to reproduce?

Answer 22

Nutrient provide the necessary elements such as carbon, hydrogen, oxygen and nitrogen (CHON)
Microbes are very successful in obtaining nutrients from variety of sources. This helps them inhabit all possible environments
Growth and reproduction require synthesising new molecules.
Energy is needed to build those molecules.

Question 23

Define the differences between anabolic and catabolic metabolism?

Answer 23

Catabolic pathways
Break down molecules to produce energy. (lerger substrates, smaller products)
Anabolic pathways
§Use energy to combine small molecules into macromolecules (energy+ source of elements espcially carbon)

Question 24

What are the three entry of molecules into a cell?

Answer 24

Passive Diffusion: No energy needed. Driven by concentration gradient.
Active Transport: requires energy. Requires recepters and work even against concentration gradient.
Group Translocation: less energy, involved phosphorylation of the molecule.

Question 25

What affect does temperature have on growth?

Answer 25

Temperature affect protein structure and fluidity of the cytoplasm and the cytoplasmic membrane.
Minimum growth temperature: lowest temperature that a given organism can carry out its metabolic activities
Maximum growth temperature
Optimum growth temperature: temperature at which metabolic activity is at its peak and thus growth rate is the highest.

Question 26

define water activity Aw?

Answer 26

Water activity (Aw): the vapour pressure of a sample divided by that of pure water at the same temperature.

Question 27

How does the pH affect growth?

Answer 27

PH effect: High or low PH inhibit microbial growth except for few!
Acidic conditions are body defense mechanism e.g. stomach and vagina (via fermentation by normal flora)
Acidophiles can tolerate acidic conditions. Some are obligate acidophiles
Alkalinophile lives in alkaline media like soil and alkaline water (e.g. Vibrio cholerae).
In both, interacellular PH is near normal

Question 28

Define the different O2 requirements ?

Answer 28

Obligate aerobes: requires O2 for energy production (respiration)
Anaerobes : do not use O2 for energy production
Facultative Anaerobes: can survive with or without O2 Microaerophiles: can tolerate oxygen levels from 2-10% (limited ability to detoxify hydrogen peroxide and superoxide radicals)
Aerotolerant: Anaerobes but have some enzymes that detoxify oxygen’s poisonous forms.
Classification of Organisms Based on O2 Requirements