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Infection and Disease
Cellular and Non-cellular Pathogens
S
Disease
S A disease is any change that impairs the function of an
individual in some way.
S Diseases may be inherited or caused by many other factors.
S An infectious disease is one caused by a pathogen.
Pathogens
S A pathogen may be a:
S Multicellular organism, eg. fungus and worm
S Single-celled organism, eg. eukaryotes such as protozoan or
prokaryotes such as bacterium
S Non-cellular agent, eg. virus and prion
Transmission of Pathogens
S Some diseases can be transmitted from person to person because
the pathogenic organism or agent causing the disease can be
transmitted.
S Infective agents can be transmitted from one host to another by:
S Direct contact
S Droplets in air breathed or sneezed out by an infected person
S Sexual contact
S Contaminated food or water
S A carrying vector, eg. rats carrying fleas
S An injecting vector, eg. mosquitoes carrying malarial parasites
S Injection with infected needles and syringes
Bacteria
S Are prokaryotic organisms lacking a nuclear membrane.
S Have cell walls made of carbohydrates, lipids, amino acids and
sometimes other substances.
S Contain a single circular molecule of DNA.
S Can be classified by shape, eg:
S coccus (round)
S bacillus (rod shaped)
S spirochaete (spiral)
S This often relates to their naming – eg. Staphylococcus aureus are a
round-shaped bacteria
Bacteria
☢ Can be classified by their metabolic characteristics
☢ Reproduce exponentially and quickly (as little as 20
minutes) in favourable conditions.
S Can have special features, including
a flagellum, capsule or spore.
S Gram positive bacteria have a thick
peptidoglycan layer
S Gram negative bacteria have a thin
peptidoglycan layer and a
lipopolysaccharide wall, which makes
them more virulent
Bacteria
S Treatment of bacteria diseases:
S Chemotherapy (term used when a disease is treated with
chemicals).
S Antibiotics/antimicrobials
S Some drugs are:
S
narrow-spectrum and act against a limited variety of microorganisms;
S
broad-spectrum and act against many different kinds of pathogens.
S Treatment must be carefully administered as bacteria can
become antibiotic resistant. Eg. Golden Staph
Fungi, Yeast & Protozoa
S Fungi, yeasts and protozoa are all eukaryotic organisms that
can cause disease.
S Fungi
S Decompose dead organic matter
S Spread readily as spores
S Bacterial antibiotics do not act against fungi. Antifungal
ointments and oral preparations are used to treat fungal
infections.
Fungi, Yeast & Protozoa
S Yeasts are unicelluar fungi.
S Protozoa are single-celled organisms.
S Examples include Plasmodium, Giardia and
Amoeba
Arthropods and worms
S Arthropods
S Are generally parasitic but are not necessarily pathogenic
S Include head lice and body lice.
S Worms
S May be parasitic but are not necessarily pathogenic
S Include Echinococcus granulosis, a pathogenic hydatid tapeworm,
which normally lives in dog intestines but which can spend
part of its life cycle in human host tissues.
S Include parasitic nematode worms that live in plant tissue.
Viruses
S Viruses:
S Are extremely small
S Have a protein coat
S Have a core of either DNA or RNA
S Reproduce only after entering a host cell and using
its
components
S Are parasitic and pathogenic
S Can infect humans, other animals, plants and
bacteria
Viruses
S How do viruses affect a host cell?
S Viruses must enter a host cell to reproduce.
S They use components of the host cell as a source of energy
and material to make new viruses and ultimately destroy
the host cell.
S Different kinds of viruses may reproduce in slightly
different ways.
Types of viruses
Adenovirus
Retrovirus
S
DNA virus
S
RNA virus
S
Utilises host cell ribosomes to
replicate
S
S
Does not possess a viral envelope
Uses reverse transcriptase to
create DNA which is
incorporated into the host DNA
S
Has a viral envelope
Viruses
S Antiviral drugs
S Because viruses enter host cells, it is difficult to inhibit the
reproduction of a virus without interfering with the
metabolism of the host cell.
S Some antiviral drugs have been developed that specifically act
on enzymes important to the virus without damage to the host
cell.
Viroids and Prions
S Viroids:
S Contain very short pieces of naked RNA
S Are important as plant pathogens
S Are known to infect plants including potatoes, tomatoes and
grapevines.
S Completely dependent on the host cell for replication.
S
S
S
S
Prions:
Do not contain genetic material
Are an abnormal and infectious form of protein
Cause a range of brain diseases including spongiform
encephalopathy.
S No treatment is available for individuals infected with abnormal
prions.