PathogensPathogens PathogensPathogens A biological agent which can cause an infectious disease

What are pathogens?• Some pathogens can be classified as micro-organisms

(microscopic living things):

Bacteria Protists (worms, mites, lice, amoebae)

Fungi (yeasts & moulds)• Some pathogens can be classified as non-living biological

agents:

Viruses Prions

Most micro-organisms live harmlessly on our skin, in our water and soil and on surfaces all around us.

Some micro-organisms that are generally harmless to healthy people can sometimes cause illness in susceptible people.

About microorganisms

This photograph

taken by a special

electron microscope,

demonstrates the

size of bacteria

(yellow) in relation to

the tip of a pin

(mauve).

Viruses are even

smaller than bacteria.

Bacteria These are widely distributed in nature (including on/in our body)

Eg: Golden Staph, Salmonella, Meningococcus, E.coli etc

The majority of bacteria are not

pathogens, in fact some are

beneficial to humans

Microscopy Bacteria under the microscope are analysed by their shape and colour reaction (after a specialised stain is added to slide)

Round purple bacteria calledGram positive cocci

Large pink cellsWhite blood cells

Growth on agar plates

Bacteria grow as colonies on the surface of plates containing nutritional agar gel

Specimen placed here & spread out

Individual colonies used for analysis

Plates containing nutrient gel

Reproduction

Bacteria multiply very rapidly by simply ‘snapping’ into two.

Each bacterium doubles itself in 8-10 minutes, so one bacterium will produce millions of ‘offspring’ within hours

Antibiotics

Penicillin, the first antibiotic discovered, was

produced from a fungus called Penicillium

Sir Alexander Fleming who discovered Penicillin

Antibiotics can be used to cure diseases

caused by bacteria.

They are chemical substances which kill or

prevent the reproduction of bacteria.

Protists This is a very broad group of pathogens that can include worms, lice, mites, and protozoans such as amoebae

Protist caused infections are usually associated with poor hygiene or low immunity.

They may be very small (microscopic) or larger worms or insects

Eg: Giardia, Amoeba, Malaria, Hookworm, Threadworm, Headlice, Scabies (itch) mite

Giardia

Parasitic infectionsDiagnosed by directly detecting the

parasite or its eggs in the specimen with a microscope

Difficult to grow in laboratory

Eggs of Thread worm

Itch miteItch mite

Giardia in faecesGiardia in faeces

Malaria in Red Blood Cells

Normal RBC

RBC with malarial parasite

Life-cycles

Some protists have complex life-cycles with more than one host

A host is an organism that carries the pathogen during one or more stages of its life cycle

A primary host is the organism which carries the pathogen during its parasitic stage (is harmed by it)

An intermediate host is the organism which carries the pathogen during its larval phase

Giardia

Human: Primary Host

Pig: Intermediate Host

FungiWidely distributed in nature.

Two types: yeasts and moulds

Fungi don not often infect healthy people. They usually infect people with low immunity, people who have puncture their skin or people who have been using antibiotics

Eg: Candida (causes thrush), Aspergillus, dermatophytes etc

Aspergillus

Did you know…

That mushrooms are fungi?

Fungal infections

Diagnosed by directly detecting the fungus in

the specimen with a microscope

Fungi reproduce by producing spores which

are very light, airborne and are resistant to

damage

Like bacteria, fungi can be grown on plates with

agar gel

Candida seen under the microscope

Yeast cells

Candida grown on nutrient plates

Aspergillus seen under the microscope

Aspergillus (mould) grown on nutrient plates

Viruses Are not classified as living because they are only active once they invade living cells

Eg: Influenza, Hepatitis, HIV, Herpes, SARS, Measles, common cold, Rubella etc

Model of HIV virus enlarged millions of times

Herpes viruses seen by an electron microscope

Cell-hijackers

Viruses cause disease by invading cells

and using cell organelles to survive and

reproduce more copies of themselves

They disrupt normal cell functioning and

kill the cells they invade by causing them

to burst as they release the newly made

viruses

Viruses

Extremely small – cannot be seen by

ordinary microscope (only seen by

electron microscope)

Difficult to grow in laboratory because

they need to be grown within other cells

Antibiotics are not effective against

viruses

Prions Can be considered as infectious proteins. They are thought to be a form of protein that is folded in an unusual manner and able to replicate themselves and invade/kill cells

Only discovered recently and not much is known about them

Eg: Kuru, Cruetzfeld Jacob’s disease, Scra[ie. Bovine Spongiform Encephelopathy

Some good sites to learn about micro-organisms

• http://www.cdc.gov/

• http://www.who.int/health_topics/en/

• http://www.tmvc.com.au/info10.html

• http://www.dhs.vic.gov.au/phd/topics/diseases.htm

What is infection?Infection occurs when:• pathogens multiply in the body • Overcome the body's defences • pathogens cause general illness (eg. fever) or

a localised effect (eg. pus in a wound or urinary tract burning).

Infection can occur only if:• a pathogen can be transmitted from

person to person or from the environment to a person,

• person is prone to infection, and

• the pathogen can enter and then survive and multiply (e.g. through a wound).

Spread of Infection