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Biology 4
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Microbes can be harmless
(pathogen) or helpful
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There are different types of pathogens
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They are much
smaller than our body cells
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They invade the body where they multiply and spread throughout the body
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Viruses contain genetic material but are not living
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So they are
infectious diseases
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There are many methods of transmission:


Water may contain cholera bacteria

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Food may contain food-poisoning bacteria such as Salmonella

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Airborne: Influenza, cold viruses and TB bacteria are spread in
air e
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from coughs and sneezes

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Contact: Athlete's foot fungus is usually transferred to skin by moist surfaces such as
swimming pools

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Body fluids: The HIV virus is transferred in body fluids, for example by drug addicts
sharing needles

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Vector-borne means that the microbes are carried by an animal, often an insect
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g
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These are much smaller than any plant or animal cell and do not have a nucleus
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They come in many shapes and sizes, but even the
largest are only 10 micrometres long - that's 10 millionths of a metre
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Bacteria are living cells and, in favourable
conditions, can multiply rapidly
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Salmonella food poisoning is spread by bacteria eaten in food, or on food prepared in
unhygienic conditions
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Fever, abdominal cramps, vomiting and diarrhoea are caused by the bacteria and the toxins
they secrete
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They are among the smallest organisms
known and consist of a fragment of genetic material inside a protective protein coat
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A
virus can get inside a cell and, once there, take over and make hundreds of thousands of
copies of itself
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Measles is a viral disease showing symptoms of fever
and a red skin rash
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For this reason
most young children are vaccinated against measles
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HIV initially causes a flu-like illness
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Late stage HIV infection, or AIDS, occurs when the
body's immune system becomes so badly damaged it can no longer deal with other
infections or cancers
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Tobacco mosaic virus (TMV) is a widespread plant pathogen
affecting many species of plants including tomatoes
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It can be spread by touching other plants or from the clothes or hands of
farm workers
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FUNGAL DISEASES

Fungi are a type of cell – similar to normal plant and animal cells with a nucleus Eukaryotes
Rose black spot is a fungal disease where purple
or black spots develop on leaves, which often
turn yellow and drop early
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It is
spread in the environment by water or wind
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PROTIST DISEASES
Protists are a type of single celled organism
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Malaria causes repeated high fever and can lead to death
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Mosquitos can bite and suck the blood
from someone with malaria and then pass the pathogen from their saliva to the next person
they bite
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Travellers to countries where malaria is endemic (regularly found) can take antimalarial
drugs
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g
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The walls are a physical barrier – they stop the
enemy getting in
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The outer layer of the skin consists of dead cells
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When we get a cut, the blood clots and the wound quickly seals over to
restore the protective barrier of the skin
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Tears in your eyes contain an antiseptic chemical
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When you blink this helps to spread the liquid over the
whole surface of your eyes
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If we are constantly
letting air into our bodies, then there is a danger here that there might
be pathogens in the air which can enter our body and harm our delicate
lungs
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In the nose, nasal hair and mucus traps dust and larger microorganisms and stops them
entering
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This divides
into two bronchi which go down to the lungs
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Other cells produce a thin layer of sticky mucus
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Any dirt or microorganisms get ‘caught’ on the sticky
sides
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When it reaches your throat, you swallow it
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When we swallow the mucus and microbes from
the air tract, the stomach acid kills any pathogens which might be there
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SECOND LINE OF DEFENCE
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If pathogens get through the first barrier: the physical barrier of the skin or nose or past the
chemical barrier of the eyes then the body gas a second line of defence
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When a
microorganism invades the body white blood cells move towards them
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This way the microbes are changed into harmless chemicals
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The type which move out from the blood vessels
and digest microbes are called phagocytes
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THIRD LINE OF DEFENCE
The immune system is the body’s system to resist infection
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The third line of defence also involves white cells – but these are different white cells called
lymphocytes
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They treat different pathogens in different
ways
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Another of these white
cells might only react to a different sort of invading microbe
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The non-specific immune system
I eat anything

I eat anything

I eat anything

I only eat oranges

I only eat bananas

The specific immune system
I only eat apples

The third line of defence is activated when a pathogen gets through the first two
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There are millions of different
forms of antibody
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The white cells send out the antibodies
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Once the correct one has been
recognized, white cells produce more of the correct sort of antibody
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antigen

bacterium
an antibody has
locked onto the
correct antigen
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antibodies

The different types of antibodies attack microbes in different ways
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This makes it easier for the
phagocytes to digest them
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These antibodies are called antitoxins
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These white
cells are called memory lymphocytes
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(Unfortunately there are hundreds of different types of pathogen for the common cold
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The important thing is that this inactive form must carry the same antigens
as ‘real’ live pathogens
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If the same
pathogen re-enters the body, the white blood cells respond quickly to produce the correct
antibodies, preventing infection
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This
contains forms of measles, mumps and rubella pathogens which do not affect people
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This will
’refresh’ the memory lymphocytes
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This is called herd
immunity
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In 2015 there were 1 200 cases of measles in the UK and 0 people died
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As a result of the hoax, many parents refused to immunise their
babies – and for a few years there was in increase in cases of measles
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They have no effect
on viruses
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Penicillin
was the first antibiotic to be discovered in 1928
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Antibiotics destroy bacteria in different ways: Some weaken the cell wall of bacteria – so
that they burst; some stop bacteria from reproducing
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Up until the mid 19th century
it was the commonest cause of early death in Great Britain
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The bacterium usually infects the lungs
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It is a chronic (long-term) illness
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They will begin
to cough a lot and eventually cough up blood
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They will get weaker and weaker until they die
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When an infected person coughs or sneezes,
tiny droplets spread in the air
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Other
people may breathe them in
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In 1913 there were 36 000 deaths from TB in the UK
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In 2013 there were 280 deaths
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You have to go on taking them for a
long time – usually several months
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Then the disease can come back again
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Unfortunately, there are now many
strains of TB which are resistant to many of the different antibiotics that are used against it
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Most
of them will be genetically identical, but one or two might be a different type which are harder
to kill than the rest (a mutation – a new type of gene that develops by chance)
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Without the normal ones present, the resistant ones can easily multiply at a great rate
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normal bacteria
destroyed by
antibiotics
normal TB
bacteria invades
body and

resistant mutation
occurs

person takes antibiotics

To deal with this problem, patients are often prescribed at least two antibiotics which they
take at the same time
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If someone goes to the doctor with a bad cold, the doctor cannot cure it
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Also, because pathogenic viruses live inside body cells, it is difficult to kill the virus without
damaging the cell
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These will treat
the symptoms of a disease – but not destroy the pathogen
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• The heart drug digitalis originates from foxgloves
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• Penicillin was discovered by Alexander Fleming from the Penicillium mould
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However, the
starting point may still be a chemical extracted from a plant
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1
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These first tests are mainly finding out if the drug works – that it
does what it is supposed to do - testing the efficacy of the drug
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Very low doses of the drug are given to healthy volunteers or some patients
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3
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Finally, it is given to a large number of patients to finalise the safe dose and efficacy

Usually when these tests are conducted they divide the individuals into two groups
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Some people
psychologically feel better just because they have been given a tablet
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Some of these trials are called double blind
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So that they will not be biased when they assess the results
New drugs for TB
Because so many people in the world suffer from TB, and because the bacterium is becoming
resistant to the antibiotics that we use, we need to find new drugs if we are to win the fight
to bring TB under control
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At the time of writing these notes (2018) a new drug (PA-824) has passed stage 1
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