09 Infectious Disease

7/27/2019 09 Infectious Disease

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- GCSE Bitesize - Infectious disease

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Science

Infectious disease

nfectious diseases are caused by the transmission of

pathogens, which are microorganisms such as bacteria and

viruses. Pathogens can be transmitted by direct contact -horizontal and vertical t ransmission - and by indirect

contact - vehicle-borne transmission and vector-borne

transmission.

Pathogens

Pathogens are microorganisms that cause infectious disease.

Pathogens are mostly bacteria but some are viruses, fungi and

protoctists.

Bacteria

Bacteria come in many shapes and

sizes, but even the largest are only

10 micrometres long (10 millionths

of a metre).

Bacteria are living cells and, infavourable conditions, can multiply

rapidly. Once inside the body they

release poisons or toxins that make

us feel ill.

Viruses

Viruses are many times smaller than

bacteria. They are among the

smallest organisms known and

consist of a fragment of genetic

material inside a protective protein

coat.

Viruses can only reproduce inside

Salmonella bacterium cell





host cells, and they damage the

cell when they do this. A virus can

get inside a cell and, once there,

take over and make hundreds of

thousands of copies of itself.

Eventually the virus copies fill the whole host cell and burst it

open. The viruses are then passed out of the body in the

bloodstream, the airways, or by other routes.

Transmission of pathogens

Microorganisms can be transmitted in two main ways: direct

contact and indirect contact.

Direct contact

Direct contact means that the disease-causing microbe is

passed from one person to another when their bodies touch in

some way.

Vertical transmission happens when microorganisms pass

from a mother to her unborn baby through the placenta. German

measles and HIV can be passed on this way.

Horizontal transmission happens when microorganisms passfrom one person to another by touching, kissing or sexual

ntercourse.

Examples of hor izontal transmission

Type of

contact

Bacterial disease Viral disease

A hepatitis C virus showing

DNA enclosed in a protein

coat.





touching bacterial

gastroenteritis

chickenpox

kissing bacterial

meningitis

glandular fever, cold

sores

sexual

intercourse

gonorrhoea,

syphilis

HIV, hepatitis B

Indirect contact

ndirect contact happens when microorganisms are carried to a

person in some way, instead of by actual body to body contact.

Vehicle-borne transmission involves an object carrying the

disease-causing microorganism.

Examples of vehicle-borne transmission

Vehicle Bacterial disease Viral

disease

droplets in the

air

tuberculosis (TB) colds, flu

water cholera polio

sharp objects tetanus HIV

food Salmonella food

poisoning

hepatitis A

Vector -borne transmission involves an animal such as an

nsect. For example, malaria is transmitted by mosquitoes,

dysentery by houseflies and plague by fleas.

Body defences

Most pathogens have to get inside our body to spread infection.

Once they are inside, the body provides ideal living conditions,

ncluding plenty of food, water and warmth. Standing in their way

s our body's immune system - the body's co-ordinated

response to the invading pathogens.





The first line of defence is the body's natural barriers. These

nclude:

skin

nasal hairs, mucus and ciliacilia: Tiny hairs, which line

the respiratory tract. They beat continuously to move

mucus and dirt up the bronchi and trachea.tears

stomach acid - it destroys the protein structure of the

bacteria’s enzymes. This results in the bacteria being

unable to carry out its bodily processes.

The skin

The skin covers the whole body,

protacting against physical damage,microbe infection and dehydration.

Its dry, dead outer cells are difficult

for microbes to penetrate, and the

sebaceous glands produce oils that

help kill microbes.

Nasal hairs, mucus and cilia

The respiratory system (or gaseous exchange tract) is protected

n several ways. Nasal hairs keep out dust and larger

microorganisms. Sticky mucus traps dust and microbes, which

are then carried away by cilia (tiny hairs on the cells that line the

gaseous exchange tract).

Tears

Tears, saliva and mucus contain an enzyme called lysozyme.This destroys microorganisms.

Stomach acid

The stomach contains hydrochloric acid , which destroys

microorganisms.

Plant defences

Cross-section of skin





Plants produce antibacterial chemicals to help so that they can

defend themselves against microorganisms. Some of these

chemicals are now used by humans.

Antibacterial plants

Plant How it works

Mint is not just used in toothpaste

because of its fresh taste: it has

antibacterial properties too.

Witch hazel (Hamamelis) can be

used to prevent an infection getting

in through the skin.

Antiseptics and antibiotics

Antib iot ics and antifungals

Antibiotics are substances that kill bacteria [bacteria: Single-

celled microorganisms, some of which are pathogenic in

humans, animals and plants. Singular is bacterium. ] or stop

their growth. They do not work against viruses: it is difficult to

develop drugs that kill virusesvirus: ultramicroscopic non-

Sprig of mint (Mentha

spicata) on a wooden

surface.

Witch hazel

(Hamamelis mollis)





cellular organism that replicates itself inside the cells of living

hosts without also damaging the body’s tissues.

Antifungal agents kill fungi. An example of an antifungal is

nyastatin which treats the fungus candida albicans.

How some common antibiotics work

Ant ibiot ic How it works

penicillin breaks down cell walls

erythromycin stops protein synthesis

neomycin stops protein synthesis

vancomycin stops protein synthesis

ciprofloxacin stops DNA replication

Penicillin

The first antibiotic - penicillin - was

discovered in 1928 by Alexander

Fleming. He noticed that some

bacteria he had left in a petri dish

had been killed by naturally

occurring penicillium mould.

Since the discovery of penicillin,

many other antibiotics have been

discovered or developed. Most

antibiotics used in medicine have been altered chemically to

make them more effective and safer for humans.

Cleanliness and antiseptics

Chemicals that kill microorganisms outside the body are known

as antiseptics . Antiseptics can be used to clean an open

wound as well as surfaces on objects such as toilets.

One simple way to reduce the risk of infection is to maintain

personal hygiene and to keep hospitals clean. In the 19th

century, Ignaz Semmelweis realised the importance of

A bacterium damaged and

distorted by penicillin





WatchYou may wish to view

this BBC News item

(2007) about how

drug-resistant strains

of TB are putting

European Union

cleanliness in hospitals. However, although his ideas were

successful, they were ignored at the time because people did

not know that diseases were caused by pathogens that could be

killed.

Now try a Test Bite.

Antibiotic resistance - Higher t ier

Over time, bacteria [bacteria: Single-celled microorganisms,

some of which are pathogenic in humans, animals and plants.

Singular is bacterium. ] can become resistant to certain

antibiotics [antibiotics : Substances that kill bacteria. ]. This is

an example of natural selection . In a large population of

bacteria, there may be some that are not affected by the

antibiotic. These survive and reproduce, creating more bacteria

that are not affected by the antibiotic. The numbers of antibiotic-

resistant bacteria have increased, partly due to the misuse of

antibiotics.

MRSA

MRSA is methicillin-resistant Staphylococcus aureus. It is very

dangerous because it is resistant to most antibiotics. To slowdown or stop the development of other strains of resistant

bacteria, we should:

always avoid the unnecessary use of antibiotics

always complete the full course

Tuberculosis

Tuberculosis, or TB for short, is a

disease caused by a bacterium called

Mycobacterium tuberculosis. Most

people who are infected do not show

any symptoms. But about 10 per cent

go on to develop serious symptoms

including shortness of breath,

coughing, fever, and it can even

http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel/problems_in_environment/infectiousdisease/quiz/q37712212/





states at risk of a

deadly outbreak.

Watch

cause death.

Infected people without symptoms

are usually given a course of one

antibiotic [antibiotics : Substances

that kill bacteria. ]. Those who show symptoms need a course of

several antibiotics at once. This is to reduce the chance of strains of antibiotic-resistant bacteria emerging.

Development of resistance

The main steps in the development of resistance are:

1. Random changes or mutations occur in the genes of

individual bacterial cells.

2. Some mutations protect the bacterial cell from the effects

of the antibiotic.

3. Bacteria without the mutation die or cannot reproduce

with the antibiotic present.

4. The resistant bacteria are able to reproduce with less

competition from normal bacterial strains.

Now try a Test Bite - higher tier.

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http://news.bbc.co.uk/player/nol/newsid_6030000/newsid_6036000/6036085.stm


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09 Infectious Disease