MICROBES AND DISEASE

F 6

• PATHOGEN- a micro-organism which causes a disease when it infects.

• VIRULENCE-ability of a pathogen to cause disease

• INFECTIOUS DISEASE- which can be passed from one sufferer to another

• An infection does not always result in disease

Mode of Transmission

• Direct contact—person to person via the mucous membrane, which is thinner, softer and moister and thru blood• Horizontal kissing, sexual intercourse, sneezing less than 1 meter, touching• Vertical Placenta , breast milk

Indirect contact Air /dust—Fungi spores ---influenza and SARS viruses Food, Water, Fomites Vector borne Insects , Protozoans, poor hygiene, Cholera and Typhoid bacteria-sewage- contaminated water Salmonella bacteria (raw meat)Prion as disease agent(CJD)-Creutzfeldt-Jacob disease

Human pathogenic diseases and their means of transmission

• Mode of transmission Indirect contact – vector borne Example: bacterial dysentery

• * The bacterium Shigella causes a type bacterial dysentery .

• * Flies can spread Shigella when they carry infected faecal matter on their feet to drinking water or food.

• * Symptoms can vary from mild diarrhoea through to a more severe disease with watery or bloody diarrhoea, fever, stomach cramps and vomiting .

• Mode of transmission Indirect contact – vehicle borne Example: athlete’s foot

• * The fungus Trichophyton that causes athlete’s foot can be spread indirectly through towels, changing room floors etc.

• * The fungus thrives in the damp warm environment found between the toes.

• * The skin between the fourth and fifth toe is usually affected first. A flaky itchy red rash develops. The skin becomes cracked and sore and small blisters may appear.

• * If the infection is left untreated it can spread to other parts of the body.

• Fomite - a non-living object that can carry disease-causing organisms.

• Mode of transmission Indirect contact – vehicle borne Example: tuberculosis (TB)

• * Air/dust

• * The bacterium Mycobacterium tuberculosis causes tuberculosis (TB).

• * TB is spread from person to person through the air.

• * When a person with active TB coughs or sneezes, droplets loaded with the infectious organism are propelled into the air.

• * The moisture evaporates from these particles to leave droplet nuclei that can remain airborne for days and spread long distances.

• * The Mycobacterium has a waxy coat, which protects it from drying out allowing it to survive for many months in the air and dust.

• Mode of transmission Direct contact – person to person Example: German measles

• * Rubella virus causes German measles. • * When infection occurs during pregnancy the virus

crosses the placenta in the blood leading to infection of the fetus .

• * The virus can affect all the organs of the developing fetus .

• * T he risk to the baby is highest in the first 3 months – up to 85% of babies are affected if infected during this period.

• Eight week old fetus attached to its placenta by the umbilical cord. Vertical across the placenta or via breast milk

• Mode of transmission Direct contact – person to person Example: German measles

• * Congenital rubella syndrome is the name given to a group of defects that occur in a child when infected as a fetus.

• * Defects are

• # deafness (most common)

• # eye problems such as cataracts

• # heart disease

• # impaired mental development

• # bone deformities

• # liver damage

• * The number of cases has dropped significantly due to the introduction of rubella vaccine which is offered to all children as part of the MMR jab.

• Mode of transmission Direct contact – person to person Example: SARS

• * Horizontal – sneezing closer than 1 metre

• * SARS - associated coronavirus causes severe acute respiratory syndrome (SARS).

• * SARS is transmitted when an infected person coughs or sneezes infectious droplets onto a nearby person .

• * T he droplets land on another person ’ s face or hands, and become introduced to the nose or mouth.

• Jets of droplets erupt from a man’s nose as he sneezes.

• Mode of transmission Direct contact – person to person Example: SARS

• * The main symptoms of SARS are a high fever >38.0°C, dry cough and breathing difficulties.

• * Other symptoms may include headaches, loss of appetite and body aches.

• * About 10 -20% of patients have diarrhoea.

• * Most patients develop pneumonia.

• Mode of transmission Direct contact – person to person Example: cold sore

• * The virus particles invade the cells of the skin around the mouth and enter the nerve tissue where they lie dormant until something triggers their reactivation.

• * Common reasons for the virus becoming reactivated are tiredness, illness, stress and sunlight.

Bacterial infection

• Intracellular---Chlamydia• Extracellular---Streptococcus

Extra cellular bacteria

• Damage tissues from outside cells.• Host cells produce antibodies – OPSONINS• Produce acute disease• Last for short time• Streptococcus--------Gram +ve bacteria• S.agalactiae- mother to infant during child birth• S.pyogenes cause-strep throat, rheumatic fever and

scarlet fever

An Opsonin

• is any molecule that acts as a binding enhancer for the process of phagocytosis

• Action of opsonins a phagocytic cell recognises the opsonin on the

surface of an antigen

• Capsule of Streptococcus pneumonia produces proteins that inhibits phagocytosis by human immune system

Intra cellular bacterial infection

• Are able to grow and reproduce only within the cells of other organisms

• Eg., Chlamydia, Rickettsia• Chlamydia trachomatis- blindness

The Rickettsia survival depends on entry, growth, and replication within the cytoplasm of Eukaryotic host cells.

Rickettsia species are carried as Parasites by many ticks, fleas and lice.

• Typhus• African tick bite fever

Chlamydia spc.

• are the most common bacterial sexually transmitted infections in humans

• Leading cause of infectious blindness

Bacterial toxin

• Exotoxin• Endotoxin----- • Lipopolyscaccharides in the wall of the Gram –ve

bacteria For eg., Salmonella• Released when the bacteria die or cell wall is

damaged• Causes fever and aches• Only gram+ve – Listeria monocytogenes

Exotoxin

• A protein secreted by the bacterium.• may produce even in the absence of

bacterium.• majority of them can be destroyed by heating• Both gram-ve and +ve produce exotoxin• Can be destroyed by antibodies, but some are

so toxic that they may be fatal before immune system can respond.

• Vibrio cholera

Three main types of exotoxin

• Acts on connective tissue-spread the bacteria• Acts as enzymes-enter the cell and effect their

metabolism• Membrane damaging toxins-puncture the CM-

killing cells.

Clostridium tetani-gram +ve

• Tetanus• Exotoxin- neurotoxin-tetanospasmin• Infection thru wound contamination• Muscles spasmsin the jaw-lockjaw—difficulty

in swallowing, and muscle stiffness and spasms.

• Sustained contraction of skeletal muscle

Exotoxin

• Clostridium botulinum –inhibitor of neuromuscular junction

• Small doses of toxin is used in commercial preparation of –botox

Methods of controlling microbial growth

• Antiseptics—to prevent sepsis• Carbolic acid (phenol) the first antiseptic • Today-chlorohexidine –mouthwash-• and alcohols---ethanol and isopropanol

Disinfection• Destruction of infective organisms• Hypochlorite ---purify water in swimming

pools• Disinfect feeding bottles• Toilets—bleaching agents-pH 5• Active ingredient is HOCl-hypochlorous acid• Kills bacteria by releasing oxygen, and act as

oxidising agent

• Effective when used in warm solution• Bacteriocidal- to kill the bacteria• Bacteriostatic –to prevent multiply

The action of bacteriocidal and bacteriostatic agents.

Pastuerisation

• Sterilise food• Milk• Boiling upto72 celsius for 15 seconds

Radiation

• Gamma radiation • used to sterilise culture media• Medical supplies• Drugs• Bacterial and fungal cells-need low dose• Spores and viruses need higher dose • Spices and condiments

In lab

• 1.Culture media---all nutrient• Agar-complex polysaccharide ---extracted from

seaweed and can be warmed to 370 C.• Agar is mixed with liquid medium to prepare a

solid medium.• 2. Inoculation-introduction of bacteria• 3. Incubator• Liquid media/ broth-incubated in a waterbath.

Condition for growth• Temperature- bacterial growth increases with slight

increase in temperature• Wide range of temperatures. • Most grow at neutral pH –use buffers• Obligate aerobes require molecular oxygen so on

solid agar they grow on surface and if growing in liquid medium it must be well aerated.

• 4. Culture-bacteria grows in a medium one bacterial strain or many strains growing together. Colony- visible growth of bacteria on an agar plate Streak Plate is a method of inoculating an agar plate

with bacteria so that they are gradually diluted.

Lawn- is a layer of bacteria growing on the surface of agar plate. To test the effectiveness of antibiotics or antiseptics.

Viruses

• Obligate parasites• Grow in specific living host cells.• Tissue culture techiques

Growth of bacterial cells

• Counting number of cell• Hemeocytometer- individual cells under

microscope-used for large cells such as yeast

Turbidometry-less accurate• Liquid culture in a cuvette in colorimeter or

spectrophotometer and the absorbance of light is measured.

Mechanisms of action of antibiotics

• Originally defined as susbstance produced by microorganism that kill or inhibit growth of microorganisms

• Sir Alexander Fleming in 1928 • first antibiotic discovered Penicillin from a

fungus Penicillium notatum

Antibiotics

• Treat infectious bacterial diseases• Microbicidal –kill the bacteria• Microbistatic- slow down the growth of bacteria• Generally gram+ve bacteria are more sensitive to

antibiotics than gram-ve bacteria.

• Chloramphenicol and Tetracyclines- are broad spectrum of antibiotics and acts on wide range of bacterias

Action of antibiotics

• Effectiveness of antibiotics---resistance• Natural resistance– mutation,conjugation

Epidemiology• Study of spread of disease• To find the source of an infection• To establish mechanism of transmission • Pathogens come from infected person or from other

reservoir of infection. Three main reservoirs of human infection-

a. Person who have had an infection but do not show symptom. Carriers.

b. Some animals contain pathogens, which infect both animals and human called zoonosis.

c. Many pathogens are found in soil and water

Some pathogens

• English doctor in 1799• Edward Jenner • Vaccine to prevent a human disease. Small

pox.

W H O

Epidemic-occurs when a large number of people in several communities suffer from the infection

Pandemic- occurs when a large number of people in several countries all suffer from the same infection

Pandemic-emergence of a disease or new strain to the human population, infects human, causing serious illness and spreads among human.

Influenza virus

• A, B, C, D.• New strains of A commonly emerge• C and D are stable • Epidemics occur on regular seasonal basis• Pandemics- 1918, Spanish flu(A2 strain) killed more

than 20 million people.• -attack the epithelial lining of bronchioles• infection ----Droplet

2004- avian influenza(H5N1) was detected in birds in S .E. Asia

1300 –Bubonic plague ---Yersina pestis spread by fleasHIV infection

Spongiform encephalopathy (SE)• Affects the brain of Humans and animals• tiny hole appears in cortex of the brain , look like a

sponge.• BRAIN FUNCTION IS IMPAIRED • Memory loss• Personality changes• Problem in movement• Creutzfeldt Jacob disease (CJD) most common form

of SE. • Rare• Ages 60-65.

Prions (proteinaceous particle)• Misfolded, single type of proteins—lack nucleic acid,• Carry disease , cause deterioration of the brain

leading to death• They can be inherited, occurs spontaneously or

spread via infection• Transmission occurs when healthy animals eat

infected tissues• Britain in 1980s cows were infected by eating the

processed remains of sheep nervous tissue.• Bovine form of spongiform encephalopathy (BSE-

mad cow disease)

• A new variant Creutzfeldt Jacob (nvCJD)• Occurs early in their life -20s• Believed that nvCJD is transmitted by eating

nervous tissue of cows infected with BSE.

Prions

• Lack nucleic acid• Single type of protein• To infect and propagate by refolding into

abnormal shape• Resistant to denaturation, radiation and heat

Prion hypothesis 1960s

•

• Infectious agent of SE was resistance to UV radiation ( break the nucleic acid)

• But responded to chemicals and treatments that disrupt proteins

• Stanley Prusiner 1982 purified and confirmed the presence of a protein.

• news.bbc.co.uk/2/hi/health/medical_notes/355601.stm