4.1 - Microbial Infection

Question 1

Viruses

Answer 1

• not cells in their own right
• obligate parasites - must use a host
• contain DNA / RNA as genetic material
• replicate using host-cell nuclear synthetic machinery
• show host specificity, but infect most life forms including bacteria (bacteriophages)
• divide by budding out of host cell, or cytolysis
• various routes of infection - faecal-oral, airborne (influenza), insect vectors, blood borne (HIV, hepatitis)

Question 2

Examples of viruses

Answer 2

• HIV - retrovirus -> RNA genome requires reverse transcriptase which converts RNA->DNA to insert into host genomes, infects T cells, leads to AIDS, very low CD4 T cell count, increasing risk of infection
• smallpox
• polio
• HPV (–> cervical cancer)

Question 3

Bacteria

Answer 3

• prokaryotes - do not have internal membranes (except photosynthetic bacteria)
• haploid - single copy of each chromosome
• poorly defined cytoskeleton
• cell wall contains peptidoglycan –> leads to different shapes
• divide by binary fission
• can use flagella to move

Question 4

Examples of bacteria

Answer 4

• shigella - faecal-oral transmission
• Neisseria meningitidis - rapid progression, septic shock, severe inflammatory response - gram negative diplococci, nose and throat commensal, causes meningitis
• hospital acquired (nosocomial) infections - Clostridium difficile (spores) + MRSA
• Mycobacterium tuberculosis - intra-cellular rod, causes TB, respiratory disease but can be systemic, abscesses
• Helicobacter pylori - peptic ulcer and gastric cancer
• pathogenic E. coli
• leprosy

Question 5

Fungi (and example)

Answer 5

• single celled eukaryotes
• cause cutaneous, mucosal and/or systemic mycoses
• occur as yeasts, filaments or both
• yeasts bud or divide; filaments (hyphae) spread by extending
  EXAMPLE - CANDIDA ALBICANS
• combination yeast + filaments (depending on environment)
• natural yeast in our bodies - commensal of mouth and skin but opportunistic
• can cause thrush - cutaneous, mucosal spread, inflamed

Question 6

Protozoa

Answer 6

• unicellular eukaryotic organisms
• include intestinal, blood and tissue parasites
• replicate in the host by binary fission or by the formation of trophozoites inside a cell (asexual reproduction)
• complicated life cycle involving 2+ hosts
• infection is acquired by ingestion or through a vector e.g. insect or invertebrate vector

Question 7

Examples of protozoa

Answer 7

PLASMODIUM SPECIES

• malaria - infection acquired by a mosquito vector
• blood and tissue parasites
• formation of trophozoites inside a cell
• symptoms - fever, headache, anaemia, malaise, hepatosplenomegaly (large liver and spleen), jaundice

LEISHMANIA SPECIES

• leishmaniasis
• infection acquired via a sandfly vector
• blood and tissue parasites
• formation of trophozoites inside a cell

Question 8

Helminths

Answer 8

• multicellular eukaryotes, visible to the naked eye
• metazoa with eukaryotic cells
• life cycles outside the human host
• parasitic worms
• reproduce sexually with adult forms shedding eggs
• roundworms e.g. Ascaris
• flatworms (flukes) e.g. Schistosomiasis - Schistosoma species life cycle involves water snail then enters humans, symptoms depend on where egg deposits
• tapeworms