Pathogens: Bacteria

Pathogen = a disease-causing agent

“Disease causing agent”

A foreign body that causes infectious disease.

May be non-cellular (prions and viruses), single-celled (bacteria) or multi-cellular (fungi, worms etc)

Bacteria

Typically single celled Prokaryotic Cell wall One large, circular chromosome

Sphere-shaped Staphylococcus bacteria. Causes sore throat (among other things)

Rod-shaped Bacillus cereus bacteria. Causes food poisoning.

Virulence The ability of the bacterium to cause

disease Higher virulence = more likely to

cause disease Factors that increase virulence:

Capsule Production of spores Resistance to traditional drugs No teichoic acid in the cell wall (Gram

negative)

The genius of bacteria

Bacteria exists in all ecosystems on the planet

Many bacterial species can withstand extreme environments (extremophiles)

Some bacterial species even live in lava pools! (Thermophiles)

Aerobic/anaerobic bacteria Some bacteria live in the absence of

oxygenGroup Characteristic Examples and disease they

cause

Aerobes Grow in presence of oxygen

Pseudomonas aeruginosa (external ear infection)

Facultative anaerobes

Grow whether there is oxygen present or not

Streptococcus pyrogenes (tonsilitis)

Obligate anaerobes

Grow only in the absence of oxygen

Clostridium botulinum (botulism – food poisoning)Clostridium tetani (tetanus)Clostridium perfringens (gangrene)

GANGRENE

Clostridium botulinumCauses:

Muscle weakness

Disrupts nervous system

Nausea

Blackouts

Dry mouth

Eventually, paralysis… then death

Source of energy for bacteria

Either photosynthetic, or chemosynthetic

Some are also heterotrophs

Most human pathogens are chemosynthetic heterotrophs, which oxidise organic material for energy

Identifying bacteria in the lab

Plain nutrient agar: provides energy source for bacteria. Can determine bacteria from other infective agents

Blood agar is required to grow Staphylococcus colonies.

The ability of an organism to grow on a plate depends on its nutritional requirements and the ingredients of the agar

Endemic bacteria Bacteria live on and around us and

don’t cause infection Why? Because they receive the

nutrients they require, so they don’t bother to attack

HOWEVER! If you fall ill, or your immune system is otherwise compromised, your endemic bacteria may turn on you!

How do bacteria cause disease?

Require certain conditions:1. Entry to a host2. reproduction within the host3. Acting adversely (negatively)

on host tissue

Common modes of transmission

Food poisoning Coughs or sneezes Contamination of drinking water Sharing of body fluids Transport from one host to another,

via a VECTOR (another organism)

Food Poisoning Food poisoning is most common in

poorly-prepared chicken/meat However, Bacillus cereus, which

also causes food poisoning, reproduces on cooled rice and pasta, and infects if it is not reheated sufficiently

High temperatures kill most food-borne bacteria. Low temperatures simply stop them reproducing.

Cross-contamination of raw meat products onto fresh fruit and vegetables also causes infection

Typhoid Mary

Chef around 1900. Carried Typhoid, but was not infected

by it Caused the infection of many

customers, due to her contamination of the food she cooked

Was quarantined by authorities, but still continued to cook for others

Water contamination Our water sources are constantly tested for

dangerous levels of bacteria Legionnaires disease (like pneumonia) is

frequently spread via droplets from large air conditioning systems (eg. Shopping centres)

Natural disasters often cause the leakage of sewage into water systems – disease like cholera may break out if people drink the contaminated water

Bacterial reproduction

Reproduce about every 20 minutes, when in optimal conditions

When limiting resource is used up (eg. Nutrient), the bacterial colony can no longer grow, and eventually begin to die

Bacterial reproduction

Bacterial effect on tissues

Death of tissue due to lack of blood supply: gangrene

Enzyme, collegenase, is produced. Breaks down collagen in blood vessels Also produces a gas, that appears that the

surrounding tissues are swollen Results in necrosis (death of the tissue) Fingers, toes, even arms and legs may be lost

Exotoxins Exotoxins are chemicals produced by the

bacterium and released into the surrounds. Remain in the food, even if the bacteria

itself has been removed Have varied results:

Inhibition of protein synthesis Damage cell membranes or disrupt transport of

material across cell membranes Interfere with normal nervous function

Endotoxins

Endotoxins are released when the infecting cell is lysed.

Cause fever and diarrhoea

Treatment of bacterial diseases

Chemotherapy: treated with chemicals

Antibiotics: kill or inhibit bacterial processes

Broad-spectrum antibiotics

Target many different types of pathogens

Many bacterium have developed RESISTANCE – they are no longer susceptible to the effects of the drug

Eg. Penicillin (harvested from a fungus, Penicillium)

Narrow-spectrum antibiotics

Act on only one or a few types of pathogen

More targeted usually means more effective

Antibiotic sensitivity tests1. Plate is “loaded” with bacteria

2. Each lettered disc is inoculated with a particular antibiotic

3. Discs are laid on the plate over the bacteria smear

4. Plates are incubated so bacteria grows for a couple of days

5. Areas of no growth of bacteria show that the corresponding antibiotic was effective. Diameter and completeness of no-growth zone tells us HOW effective it is against this particular bacteria

Most effective: large, complete no-growth zone

Not effective at all