1 2 think about… 24.1causes of infectious diseases 24.2ways of transmission and control measures...

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Think about…

24.1 Causes of infectious diseases

24.2 Ways of transmission and control measures of infectious diseases

24.3 Treatment of infectious diseases

Recall ‘Think about…’

Summary concept map

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Severe Acute Respiratory Syndrome (SARS) (嚴重急性呼吸系統綜合症 ) 　 first appeared in Guangdong in late 2002.

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In less than a year, this infectious disease spread to 30 countries, taking hundreds of lives in over 8400 cases.

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The outbreak of SARS not only affected health, but also caused public fear and serious economic losses.

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Luckily, the causative agent of SARS was quickly identified with the joint efforts of scientists.

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A range of measures were also designed and taken to control the spread of the disease.

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What causes infectious diseases1

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How do infectious diseases spread2

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How can infectious diseasesbe treated3

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What are infectious diseases?

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Infectious diseases can be transmitted from one person

to another.


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Examples are SARS and influenza.


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Non-infectious diseases cannot be transmitted from

one person to another.


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An example is cancer.


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Then what are the causative agents of infectious diseases?


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• caused by pathogens (病原體 ):

viruses (病毒 )

bacteria


protists fungi

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• pathogens enter the body, then multiply and cause harm to us


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Viruses• non-cellular particles that consist of a

protein coat enclosing the nucleic acid


surface proteins

protein coatnucleic acid

• take over the machinery of a cell (host) to produce new viruses

3D model

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• cause diseases by:

- destroying cell membranes of host cells

- stopping the synthesis of nucleic acids or proteins in the host

Viruses

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e.g.

influenza virus coronaviruses (冠狀病毒 )

Viruses

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e.g.

dengue viruses measles viruses

Viruses

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Bacteria• may live inside or outside host cells


• damage or affect functioning of tissues by toxins (毒素 ) or enzymes

e.g. Vibrio cholerae

- causes cholera (霍亂 )

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Bacteria• may live inside or outside host cells


• damage or affect functioning of tissues by toxins (毒素 ) or enzymes

e.g. Mycobacterium tuberculosis

- causes tuberculosis (結核病 )

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Protists• some are parasites that cause diseases


e.g. Plasmodium (瘧原蟲 )- causes malaria (瘧疾 )- reproduces in liver and then in red blood cells

- infected red blood cells finally burst, releasing new parasites

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Protists24.1 Causes of infectious diseases

- toxic substances cause:1 shivering (顫抖 )2 fever3 sweating

Animation

• some are parasites that cause diseases

e.g. Plasmodium (瘧原蟲 )

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Fungi• some are harmful


e.g. fungi that cause athlete’s foot

- live on skin surface

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Fungi24.1 Causes of infectious diseases

e.g. fungi that invade the lungs

• some are harmful

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Other causative agents• parasitic worms


e.g. liver flukes (肝吸蟲 )

- live in the bile duct

- lead to inflammation

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Other causative agents• not all infectious diseases have 　　　　　　　　　　　　　　 a known cause


e.g. Creutzfeldt-Jakob disease 　　　　　　　　　　　　　　 (克雅二氏症 )- a brain degeneration disease

- may be caused by prions 　　　　　　　　　　　　　　 (病原性蛋白顆粒 )

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‘infection’ = ‘disease’


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An infection results when a pathogen enters and begins to grow within the host. A disease results only if the growth of a pathogen causes damage to the host.

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1 diseases can be

transmitted from one person to another whereas diseases cannot.

Infectious


non-infectious

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the host.

. Their activities

2 Infectious diseases are caused by the invasion of a host bypathogens


harm

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that can cause diseases.

3aPathogens are and viruses


organisms

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3b

Virus


tuberculosis

Pathogen Diseases it causes

Influenza, SARS, dengue fever, measles

Bacterium ,Cholera

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3b


Pathogen Diseases it causes

Protist

Fungus Athlete’s foot

Malaria

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To control the spread of an infectious

disease, we should know how it is transmitted.

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Human reservoirs and transmission of infectious agentsReservoir Transmission route Infectious Agent

Blood Blood, needle stick, other contaminated equipment

Hepatitis肝炎 B and C; HIV/AIDS, S. aureus金黃色葡萄球菌 ,

Tissue Drainage from a wound or incision

S. aureus, E. coli大腸桿菌.

Respiratory tract Droplets from sneezing or coughing

Influenza viruses, Strep spp. 鏈球菌 , S. aureus

Gastrointestinal tract

Vomitus, feces, bile, saliva

Hepatitis A, Shigella spp痢疾 , Salmonella spp沙門氏菌

Urinary tract Urine E. coli, enterococci腸球菌Reproductive tract and genitalia

Urine and semen N. gonorrhoeae淋病 , T. pallidum梅毒螺旋菌 , Herpes simplex virus type 2疱疹 , Hepatitis B

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By droplets• expelled from the respiratory system

when talking, coughing or sneezing


droplets (飛沫 )

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By droplets24.2 Ways of transmission and control measures of infectious diseases

Can travel 1 m!

• expelled from the respiratory system when talking, coughing or sneezing

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• when droplets containing pathogens deposit on mucous membranes


transmission of diseasese.g. influenza, common cold

and SARS

By droplets

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• to reduce droplet transmission:


1 Cover the mouth with tissue paper during coughing or sneezing

By droplets

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3 Avoid going to crowded places

• to reduce droplet transmission:2 If there is any respiratory symptom,

wear a surgical mask

By droplets

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• dust particles, droplet nuclei (飛沫核 ) and fungal spores can suspend and travel in air


• inhalation of these substances carrying pathogens can cause diseases

e.g. measles and tuberculosis

By air

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• to reduce the spread of airborne (空氣傳播的 ) diseases:

Maintain good ventilation


By air

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By water• water is contaminated with pathogens

when faeces from an infected person go into the water


• drinking contaminated water can cause diseases

e.g. cholera, gastroenteritis (胃腸炎 )

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• to control waterborne diseases:


1 Supply of clean drinking water

2 Proper disposal of faeces

By water

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By food• food may be contaminated when


- food is prepared by unwashed hands

- crops are fertilized with faeces from infected persons

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By food• food may be contaminated when


- food contains meat, milk and eggs from animals carrying pathogens

raw eggs may contain pathogens

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By food24.2 Ways of transmission and control measures of infectious diseases

• eating contaminated food may cause diseases

e.g. cholera and food poisoning

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By food• to reduce the transmission of foodborne

diseases:


1 Wash your hands properly before handling food

Proper handling of food

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2 Cook food thoroughly, especially meat, eggs and seafood

• to reduce the transmission of foodborne diseases:


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3 Keep meat, milk and egg products refrigerated below 4oC

• to reduce the transmission of foodborne diseases:


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By body fluids• blood from one person may enter the

bloodstream of another person through


- wounds

- sharing of injection needles

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By body fluids• blood from one person may enter the

bloodstream of another person through


- blood transfusion

- child birth

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By body fluids• semen and vaginal fluid are exchanged

during sexual intercourse


• body fluids may contain pathogens that cause:hepatitis B (乙型肝炎 ) and Acquired Immune Deficiency Syndrome (AIDS) (後天免疫力缺乏症／愛滋病 )

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By body fluids• to reduce the spread of these

diseases:


1 Wear gloves when handling wounds

2 Cover any wound with a dressing

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By body fluids• to reduce the spread of these

diseases:


3 Screen the blood used in blood transfusion

4 Never share injection needles

5 Stay with one sex partner

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By body fluids• to reduce the spread of these diseases:


6 Always use a condom correctly

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By body fluids• to reduce the spread of these diseases:


7 Obtain protection by receiving vaccinations (接種 )

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By vectors (媒介 )

• organisms that carry pathogens to a new host


- flies

• common vectors:

flies may carry pathogens on their body and contaminate our food

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By vectors (媒介 )24.2 Ways of transmission and control measures of infectious diseases

- cockroaches

• common vectors:


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By vectors (媒介 )24.2 Ways of transmission and control measures of infectious diseases

- mosquitoesAedes albopictus (白紋伊蚊 ) transmits dengue fever

• common vectors:


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• to reduce the spread of vector-borne diseases:


1 Remove the breeding place of the vectors

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2 Killing the vectors, e.g. by pesticides

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3 Prevent contact with the vectorswear long-sleeved clothes and trousers to avoid mosquito bites

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By direct contact• physical contact between an infected

person and a non-infected person


- athlete's foot- genital herpes (生殖器疱疹 )

e.g. touching the wounds, kissing, sexual contact

• can transmit:

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By direct contact• to reduce the transmission of these

diseases:


1 Minimize physical contact with infected people

2 Maintain good personal hygiene

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To prevent and control infection

The Chain of Infection

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Allows us to interrupt the links in the chain of infection

And stop the spreading of diseases

Epidemiology

The study of the components and the movement of pathogens in the chain of infection

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Breaking the Chain of Infection Rapid identification Rapid identification

Env. Sanitation

Disinfection

Employee health

Env. Sanitation

Disinfection

Employee health

Handwashing

Control of excretions and secretions

Trash disposal

Handwashing


Trash disposal

Isolation, Food handling

Air flow control, Handwashing



Aseptic technique

Wound care

Personal hygiene

Aseptic technique

Wound care

Personal hygiene

Treatment of disease

Recognition of high-risk patients



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Breaking the Chain of Infection Rapid identification Rapid identification

Env. Sanitation

Disinfection

Employee health

Env. Sanitation

Disinfection

Employee health

Handwashing


Trash disposal

Handwashing


Trash disposalIsolation, Food handling




Aseptic technique

Wound care

Personal hygiene

Aseptic technique

Wound care

Personal hygiene





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• Wear a if we have any respiratory symptoms

• Cover the during coughing or sneezing

Ways of transmission and how to reduce the spread of the diseases

mouth


1 By droplets

surgical mask

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, common cold and

• Avoid going to placesExamples of diseases:

Influenza


1 By droplets

SARS


crowded

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• Provide good

and

ventilation


2 By air

Measles

Examples of diseases:

tuberculosis


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and

• Supply of clean drinking waterfaeces


3 By water

Cholera


gastroenteritis

• Proper disposal of


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Cholera and

• Proper handling of food


4 By food

food poisoning



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• Wear when handling wounds

gloves


5 By body fluids (e.g. blood)

blood transfusion

• Cover any wound with a dressing • Screen the blood used in


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• Use a correctly during sexual intercourse

• Never share injection needles



condom

• Stay with sex partner


one

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and

• Receive vaccinations



Hepatitis B


AIDS


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• Remove the of the vectors

breeding place


6 By vectors (e.g. mosquitoes)

• Kill the vectors • Prevent contact with the vectors


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6 By vectors (e.g. mosquitoes)

and


Dengue fever malaria


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• Minimize with infected people

physical contact


7 By direct contact

• Maintain good personalhygiene


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andAthlete’s foot


7 By direct contact

genital

herpes



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• usually involves the use of drugs to:

- kill pathogens- inhibit the growth of pathogens

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Antibiotics (抗生素 )• chemicals produced synthetically or

naturally from microorganismsmicroorganisms


• can kill or inhibit the growth of other microorganisms

• widely used in the treatment of infections and diseases caused by bacteriabacteria

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a inhibit the formation of bacterial cell walls

b damage the cell membranes of bacteria

Action of antibiotics

c inhibit the synthesis of nucleic acids

d inhibit protein synthesis

Antibiotics (抗生素 )

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paper discs soaked with different antibioticsbacterial lawn (a continuous layer of bacteria)

Action of antibioticsVideo


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no bacterial growth around disc

Action of antibiotics

• some act on many types of bacteria• others act on one or only a few types


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• not effective against viruses. WHY?

Action of antibioticsAntibiotics (抗生素 )

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• Antibiotics are NOTNOT effective against viruses, WHY?

a inhibit the formation of bacterial cell walls ?

b damage the cell membranes of bacteria ?

c inhibit the synthesis of nucleic acids ?

d inhibit protein synthesis ?

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• Antibiotics are not effective against viruses. WHY?

a inhibit the formation of bacterial cell walls ? Virus has no cell wall

b damage the cell membranes of bacteria ? Virus has no cell membrane

c inhibit the synthesis of nucleic acids ? inhibit protein synthesis ?

Virus has no metabolism of its own, and has different mechanism of nucleic acid and protein synthesis as in bacteria.

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Indiscriminate use• adding antibiotics to

animal feed to kill pathogenic bacteria


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• taking antibiotics unnecessarily when we have viral infections

Indiscriminate use

• not finishing the course of antibiotics when we start to feel better


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Consequences• speeds up the development of antibiotic

resistance (抗生素抗性 )

1 The population of bacteria is not resistant to the antibiotic.

Animation


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• speeds up the development of antibiotic resistance (抗生素抗性 )

2 A bacterium develops antibiotic resistance as a result of a change in its genetic material.

Consequences


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3 In an environment of heavy antibiotic use, only the antibiotic resistant bacterium is selected to survive.

Consequences


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4 Without competition from other bacteria, the antibiotic resistant bacterium reproduces rapidly.

Consequences


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4 The whole population is now resistant to the antibiotic.

Consequences


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• outbreak of diseases may lead to heavy loss of lives

Consequences• previously treatable diseases may

become untreatable

• development of new antibiotics uses many resources


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Consequences• antibiotics also kill the beneficial

bacteria may encourage the growth

of pathogens


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• to alleviate the consequences:a use antibiotics only when they are truly needed reduce the exposure of bacteria

to antibiotics


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b complete the course of antibiotics

to kill all pathogenic bacteria

Antibiotics (抗生素 )• to alleviate the

consequences:

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c implement effective measures to control the spread of pathogenic bacteriae.g. maintain good hygiene


consequences:

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d understand how bacteria cause diseases to develop new drugs or

alternative treatment


consequences:

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Sulpha drugs (磺胺類藥物 )• a group of chemicals derived from a

sulphur-containing red dye


• used in the treatment of bacterial infections

• competitive inhibitors

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Sulpha drugs (磺胺類藥物 )• without sulpha drug:


normal substrate

enzyme

essential metabolic product produced

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Sulpha drugs (磺胺類藥物 )• with sulpha drug:


sulpha drug

essential metabolic product cannot be produced

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Cocktail therapy24.3 Treatment of infectious diseases

• many metabolic functions of viruses depend upon the cellular machinery of the host drugs that work against viruses may

also affect host cells

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Cocktail therapy• AIDS is caused by the human

immunodeficiency virus (HIV) (人類免疫力缺乏病毒 )


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• HIV attacks the immune system (免疫系統 ) that defends against diseases• HIV develops drug resistance rapidly if one drug is given at a time

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• David Ho proposed the cocktail therapy (雞尾酒療法 ) in 1996

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• involves a combination of drugs that target different points in the reproductive process of HIV HIV is less likely to develop

resistance to all the drugs at the same time death rate of AIDS decreases

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• but cost is high

• AIDS is still incurable

Antimicrobial Medications

• Discovery of antibiotics

– Alexander Fleming

• Discovered penicillinpenicillin while working with Staphylococcus

• Noticed there were no Staph colonies growing near a mold contaminant

– The colonies appeared to be melting

• Identified mold as Penicillium and was producing a bactericidal substance bactericidal substance that was effective against a wide range of microbes

• Fleming unable to purify compound

History and Development ofAntimicrobial Drugs

• Discovery of antibiotics

– Chain and Florey successfully purified penicillinpurified penicillin

– In 1941 tested on human subject with life threaten Staphylococcus aureus infection

• Treatment effective

Mass production of penicillin during WWIIWWII

– Waksman isolated streptomycinstreptomycin from soil bacterium Streptomyces griseus

History　and　Development　ofAntimicrobial　Drugs

Features of Antimicrobial Drugs

• Most modern antibiotics come from organisms living in the soil

– Includes bacterial species Streptomyces and Bacillus as well as fungi Penicillium and Cephalosporium

– In some cases drugs are chemicallychemically alteredaltered to impart new characteristics

• Termed semi-syntheticsemi-synthetic

• Selective toxicity

– Antibiotics cause greater harm to cause greater harm to microorganisms than to human hostmicroorganisms than to human host• Generally by interfering with

biological structures or biochemical processes common to bacteria but not to humans

Features　of　Antimicrobial　DrugsFeatures　of　Antimicrobial　Drugs

• Antimicrobial action– Drugs may kill or inhibit bacterial growth

• Inhibit = bacteriostaticbacteriostatic• Kill = bacteriocidalbacteriocidal


Bacteriostatic drugs rely on host immunity to eliminate pathogen

Bacteriocidal drugs are useful in situations when host defenses cannot be relied upon to control pathogen

• Spectrum of activity

– Antimicrobials vary with respect to range of organisms controlled

• Narrow spectrum

– Work on narrow range of organisms

» Gram positive only OR Gram negative only

• Broad spectrum

– Work on broad range of organisms

» Gram positive AND Gram negative Disadvantage?


disruption of ‘normal flora’

• Adverse effects

– Allergic reactions

– Toxic effects

– Suppression of normal flora

– Antimicrobial resistance


Mechanisms of Action of Antibacterial Drugs

Mechanisms of action?

Mechanisms of Action of AntibacterialAntibacterial Drugs

• Mechanism of action include:– Inhibition of cell wall

synthesis– Inhibition of protein

synthesis– Inhibition of nucleic acid

synthesis– Inhibition of metabolic

pathways– Interference with cell

membrane integrity– Interference with essential

metabolic processes

Resistance to Antimicrobial Drugs

• Mechanisms of resistance

– Drug-inactivating enzymes

• Some organisms produce enzymesenzymes that chemically modify drug

– Penicillinase breaks β-lactam ring of penicillin antibiotics

– Alteration of target molecule

• Minor structural changes in antibiotic target can prevent binding of antimicrobial agents to target molecules in pathogens

• Acquisition of resistance

– Can be due to spontaneous mutation– Or acquisition of new genes

Resistance　to　Antimicrobial　DrugsResistance　to　Antimicrobial　Drugs

• Staphylococcus aureus

– Common cause of hospital-acquired infections

– Becoming increasingly resistant

• In past 50 years most strains acquired resistance to penicillin

–Due to acquisition of penicillinase genes

• Until recently most infections could be treated with methicillin (penicillinase resistant penicillin)

–Many strains have become resistant

»MRSA methicillin resistant Staphylococcus aureus


Methicillin-resistant Staphylococcus aureus (MRSA) is a bacterium responsible for several difficult-to-treat infections in humans. It is also called multidrug-resistant Staphylococcus aureus

• Streptococcus pneumoniae 肺炎鏈球菌– Has remained sensitive to penicillin

• Some strains have now gained resistance

– Resistance due to modification in genes coding for penicillin-binding proteins

» Changes due to acquisition of chromosomal DNA from other strains of Streptococcus


• Slow emergence and spread of resistance– Responsibilities of physicians and healthcare workers

• Prudence / care in prescribing antibiotics• Educate patients on proper use of antibiotics

– Responsibilities of patients• Follow instructions carefully• Complete prescribed course of treatment

–Misuse leads to resistance


Mechanisms of Action of Antiviral Drugs (not considered

antibiotics)

Mechanisms of Action of Antiviral Drugs

• Available antiviral drugs effective specific type of virus

– None eliminate latent virus

• Targets include

– Viral uncoating

– Nucleoside analogs

– Non-nucleoside polymerase inhibitors

– Non-nucleoside reverse transcriptase inhibitors

– Protease inhibitors

– Neuraminidase inhibitors

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1 are chemicals produced by bacteria and fungi. The can kill or inhibit the growth of .

Antibiotics

bacteria


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2 How antibiotics kill or inhibit the growth of bacteria:

a They may inhibit the formation of bacterial .

cell walls


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b They may damage the cell


of bacteria.

membranes


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in bacteria.


c They may inhibit the synthesis of nucleic acids


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in bacteria.


d They may inhibitprotein


synthesis

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in large populations of bacteria is speeded up.

3 The consequences of indiscriminate use of antibiotics:

a The development of resistance


antibiotic

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b Previously treatable diseases may become .untreatable


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c Outbreak of diseases may lead to heavy .loss of lives


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d Development of new antibiotics will use many of the

in the society.resources


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e Antibiotics will kill the beneficial bacteria as well and this may encourage the growth of

.pathogens


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.

. They can inhibit the growth of

4 Sulpha drugs are a group of chemicals derived from a sulphur-containing red dye


bacteria

145

that target different points in the reproductive process of HIV. The use of multiple drugs makes HIV difficult to develop to all these drugs at the same time.

5 The ‘cocktail therapy’ involves a combination of drugs


resistance

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What causes infectious diseases?1Infectious diseases are caused by pathogens.

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How do infectious diseases spread?2Infectious diseases can spread through droplets, air, water, food, body fluids, vectors and direct contact.

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How can infectious diseases be3Infectious diseases may be treated by the use of drugs to kill or to inhibit the growth of the pathogens.

treated?

149

Infectious diseases

caused by

pathogensinclude

viruses bacteria protists fungi

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bacteriakilled or inhibited by

antibioticsindiscriminate use speeds up development of

antibiotic resistance

sulpha drugs

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commonly transmitted by

Infectious diseases

droplets

air

water body fluids

food vectors

direct contact

1 2 think about… 24.1causes of infectious diseases 24.2ways of transmission and control measures...

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