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 oxygen GroupCharacteristicExamples and disease they cause AerobesGrow 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 botulinum Causes: 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 host 2. reproduction within the host 3. 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  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 tests 1.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