U4 AOS 1 (O2) - Disease Challenges & Strategies

U4 - How Does Life Change and Respond to Challenges

disease

a condition in a living animal or plant body that impairs the normal functioning of an organ, part, structure or system

non-infectious

diseases that cannot spread from affected people to healthy people via the environment

infectious

diseases that can be transmitted between individuals and are caused by pathogens

pathogens

agents that cause diseases in their hosts

emerging disease

a disease caused by a newly identified or previously unknown agent

re-emerging disease

reappearance of a known disease after a significant decline in incidence

physical methods

methods of identifying pathogens based on size and shape

immunological methods

methods used to diagnose pathogens based on the presence of specific antigens or antibodies

ELISA

a technique known as enzyme linked immunosorbent assay, which can detect specific antigens or antibodies

direct ELISA

a form of ELISA where a primary antibody with an enzyme indicator directly binds to an antigen attached to a surface

indirect ELISA

a form of ELISA where a primary antibody attached to a secondary antibody with an enzyme indicator binds to an antigen attached to a surface

sandwich ELISA

a form of ELISA where an antigen attached to an antibody with an enzyme indicator binds to an antibody attached to a surface

Physical methods
Physical methods can assist in identifying viruses based on size and shape

• x-ray crystallography, which has determined the structure of many viruses

• electron microscopy, which has given us images that distinguish various kinds of virus.

Immunological methods
Immunological methods detect specific viral antigens or antibodies.

One of the main techniques used is the enzyme-linked immunosorbent assay (ELISA) technique which allows for the diagnosis of diseases (including viral diseases).
There are many different types of ELISA technique. T

• direct ELISA a form of ELISA where a primary antibody with an enzyme indicator directly binds to an antigen attached to a surface

• indirect ELISA a form of ELISA where a primary antibody attached to a secondary antibody with an enzyme indicator binds to an antigen attached to a surface

• sandwich ELISA a form of ELISA where an antigen attached to an antibody with an enzyme indicator binds to an antibody attached to a surface

Direct ELISA - expand

Direct ELISA is the simplest form of ELISA.

In direct ELISA, a viral antigen is placed on a surface. Matching primary antibodies bind to this antigen. These primary antigens have an enzyme indicator directly attached to them (as shown in figure 6.10a).

The steps of direct ELISA are as follows:

1. A plate (called microtiter plate) with wells is coated with an antigen (or toxins) specific to the disease being tested.

2. An antibody specific to a particular antigen is added to each well. This antibody also has an associated enzyme indicator.

3. During incubation, antibodies present in the sample bind to the antigen in the well.

4. The wells are then washed using a detergent solution to remove any unbound antibodies.

5. The substrate for the enzyme is added, leading to a colour change if an antigen–antibody complex formed. This indicates a positive test.

Indirect ELISA - expand

Indirect ELISA

Indirect ELISA is similar to direct ELISA. However, the primary antibody does not have an enzyme indicator.

Instead, the enzyme indicator is attached to a secondary antibody. This secondary antibody is attached to the

enzyme indicator. This can help amplify the signal, as multiply secondary antibodies can be used (as shown in

figure 6.10b).

Sandwich ELISA

Another type of ELISA, known as sandwich ELISA, involves antibodies bound to the surface (as shown in

figure 6.10c). This differs to direct and indirect ELISA, which use bound antigens. The steps in this process are

as follows:

1. A ‘capture’ antibody is used to identify the presence of a specific viral antigen through an antigen–antibody

reaction.

2. All unbound material is washed away with a detergent solution.

3. A second antibody with an enzyme indicator is then added. This binds to the antigen. As such, the antigen

is ‘sandwiched’ between two antibodies.

4. The substrate for the enzyme indicator is added. If colour appears, the specific viral antigen is present.

Often, rather than having the enzyme indicator on the antibody that binds to the antigen, an additional antibody

is added with this indicator. This binds to the antibody (in a similar way to indirect ELISA).

molecular techniques

methods using DNA or RNA to identify a pathogen

probe

single-stranded segment of DNA (or RNA) with a radioactive label that locates and binds to a target sequence (molecular techniques)

phenotypic methods

techniques to differentiate bacteria on the basis of microscopy, different media and biochemical tests

Phenotypic methods

Phenotypic methods use techniques that involve identifying particular traits or features in bacteria. They include:

• use of microscopy to differentiate bacteria on the basis of differences in cell shape, size and response to Gram stain, and physical features such as the presence or absence of a capsule.
→ Gram-staining (gram pos - dark blue, gram neg - red):

• use of a range of biochemical tests eliciting different bacterial responses

• use of different media to differentiate bacteria on the basis of variation in growth patterns. This may be done by observing growth on non-selective, selective and differential media.

• Non-selective media or agar can be used to detect and count the number of bacteria in the sample.

• Selective media contain compounds, such as antibiotics or growth nutrients, that selectively inhibit or enhance the growth of specific bacteria. For example, selective media can be used to distinguish aerobic bacteria from facultative anaerobes (which can survive with or without oxygen) and obligate anaerobes (which can only survive in anaerobic conditions).

• Differential media contain a substrate that, under the action of an enzyme, produces a coloured or fluorescent product. This can identify bacteria according to various chemical reactions that are carried out during growth

peptidoglycan

a polymer consisting of sugars and amino acids that forms a major part of the cell wall of Gram-positive bacteria

Genotypic and molecular methods

methods of identifying bacteria by examining its genetic material

serotypes

variants within a species of bacterium or virus that are distinguished by their surface antigens

Genotypic and molecular methods - Expand

Key Advantages:

• Higher speed and accuracy compared to traditional culturing.

• Allows for precise identification of specific strains/serotypes.

Three Main Techniques:

• Gene Probes: Short, radioactively/fluorescently labelled nucleic acid sequences that bind to specific complementary genes unique to a bacterium.

• Sequence Analysis: Determining the exact order of nucleotides in the bacterial DNA to find matches in a database.

• Plasmid Fingerprinting: Using DNA profiling to identify unique genetic patterns in plasmids, allowing for the identification of specific bacterial strains.

Serotyping via Antibodies:

• Uses solutions of specific antibodies to test for the presence of matching surface antigens. A positive result (clumping/binding) confirms the specific serotype.

Immumological methods (bacteria) - Expand

Immunological methods use techniques including monoclonal antibodies, ELISA and immunofluorescence to identify bacteria.

These immunolobical methods are used in the following ways:fy bacteria. These immunolobical methods are used in the following ways:

• Monoclonal antibodies are antibodies that are designed to have a specific antigen-binding site. These can be formed to target and bind to particular bacterial antigens.

• ELISA was introduced in the subsection on identifying viruses, and works in a similar way for bacteria. ELISA for bacteria can be used not only to detect antigens or antibodies, but also to detect toxins specific to a certain bacterium. Direct, indirect and sandwich ELISA may all be usedfor this. Another type of ELISA, known as reverse ELISA, may also be used to identify particular strains ofbacteria. This technique does not use traditional wells, but leaves the antigens suspended in fluid.

• Immunofluorescence, in a similar way, uses an antibody with a fluorescent marker to bind to and detectspecific antigens or antibodies in serum.

Describe how ELISA can be used to detect subsances (4 marks)

A plate with wells is coated with an antigen

(bacterial toxin) specific to the disease being

tested (1 mark).

• An antibody specific to a particular antigen

is added to each well. During incubation,

antibodies present in the sample bind to the

antigen in the well (1 mark).

• Next, enzyme- linked secondary antibodies

are added to the well. If the antigen is

present, then the secondary antibodies bind

to the antigen (1 mark).

• A substrate is added which reacts with

enzymes to produce a colour change,

indicating a positive test (1 mark).

Transmission

passing a pathogen on to another individual

Quarantine

the act of isolating infected individuals to prevent the spread of a disease

Reservoir

• the habitat in which a pathogen lives, grows and multiplies

• do not experience symptoms of the disease.

• can include: humans, animals; zoonotic diseases are infectious diseases that are transmitted from animals to humans. Some examples of zoonotic diseases include plague (rodents), anthrax (sheep) and rabies (dogs and other mammals), the enviroment; plants, soil and water are alsoreservoirs for some infectious agents

Zoonotic disease

diseases that have been transferred from other animals

Modes of transmission

Infectious diseases may spread from infected people to healthy people by various means:

• by direct transmission, such as by person-to-person contact, through kissing or sexual contact

(e.g. chickenpox, chlamydia and conjunctivitis)

• by indirect transmission, such as:

• by airborne droplets or particles, such as an uncovered sneeze or cough (e.g. influenza, rotavirus and

coronavirus)

• by contact with contaminated objects, such as bedding, cups or medical instruments (e.g. glandular fever

or tetanus)

• by ingestion of contaminated food or water (e.g. salmonella, cholera and gastroenteritis)

• by biological vehicles, such as contaminated blood, sputum, or faeces (e.g. HIV and hepatitis B and C)

• by vectors that carry pathogenic agents and spread them to people through bites from infected ticks, mites, fleas or mosquitoes, or through contaminated particles that they leave on material, such as fly droppings on food. (e.g. bubonic plague, psittacosis, anthrax, West Nile virus disease and rabies). Many of these vectors transmit pathogens from their natural host (e.g. birds, bats or rodents) to people. Thesediseases are referred to as zoonotic diseases or zoonoses.

direct transmission

mechanism of transmission of pathogenic agents that involves direct person-to-person contact, such as by kissing or sexual contact

indirect transmission

mechanism of transmission of pathogenic agents that does not involve direct person-to￾person contact, such as by airborne droplets or by ingestion of contaminated food

vector

organisms that carry pathogenic agents and spread them to other organisms

asymptomatic carrier

person with an infectious disease showing no symptoms but able to infect others

R0 value

the basic reproduction number that identifies the expected number of individuals a person with a certain disease will infect

Methods of disease control

Prevention. The transmission of disease can be prevented by changing behaviours. Examples include

practising personal hygiene such as washing hands, using condoms to prevent the spread of sexually

transmitted diseases and using insect repellent to prevent the spread of a particular disease by vectors. The

access to improved sanitation and clean drinking water is also a vital measure to prevent against diarrhoeal

and parasitic diseases, such as cholera.

• Vaccination. Vaccination is a way of providing long-term protection against infectious diseases. Vaccines help in

preventing and, in some cases, eradicating diseases. For example, child immunisation schedules have resulted

in the dramatic decline of diseases such as measles and polio.

• Medication. Antibiotics for bacterial infections are just one of many medications now used to manage

infectious diseases.

• Surveillance. The global monitoring of disease outbreaks is another tool used to control the spread of several

diseases.

• Modification of the environment. The environment can be made less suitable for the microbes to grow and be

transmitted. Examples may include vector control like in case of malaria.

• Infection control standards. These help in preventing the spread of infectious diseases, and include

antibiotics

antibiotics a class of antimicrobial drug used in the treatment and prevention of bacterial infections that act either by killing pathogenic bacteria or by inhibiting their growth

sensitivity test

test to establish the most effective drug to use for treatment against a particular bacterial infection

Other chemical agents to control pathogens

Other ways of controlling the spread of pathogens are:

• sterilisation. This is the removal or killing of the microbes from surfaces. Sterilisation is done by heat

through such methods as autoclaving. This is a very effective method and relies on pressurised steam at a

high temperature.

• chemical agents. Antiseptics and disinfectants are also used to control the

spread of pathogens. The inhibition or killing of pathogenic organisms

on non-living surfaces, such as taps and door handles, is called disinfection.

Antiseptics are used for inhibiting the growth of pathogens on living surfaces