disease of plants and animals

Pathogens and communicable diseases

Microorganisms that cause diseases are called pathogens.

All diseases caused by pathogens are communicable diseases.

The transfer of pathogens from an infected host to an uninfected host is disease transmission.

Pathogens from four groups of microorganisms:

bacteria

viruses

fungi

protoctists

Bacteria

Prokaryotic organisms have a huge diversity of different types.

Cause human diseases such as tuberculosis and bacterial form of meningitis.

Not all pathogenic bacteria infect cells; some remain in body spaces.

The meninges are the tissues that surround the brain and spinal cord; they prevent the entry of most bacteria, but N. meningitids is one of the few that can pass through this barrier and causes the disease known as bacterial meningitis.

Bacterial pathogens of plants tend to kill their hosts then feed on the dead and decaying tissues.

Bacterial diseases can cause ring rot in potatoes by infecting vascualar tissue- xylem and phloem-and block it so less water reaches the leaves, and they wilt.

No nucleus or membrane bound organelles

Reproduce by binary fission

Produce toxins that damage cells and tissues.

Some directly invade and destroy cells.

Viruses

Viruses infect cells by hijacking the host cell’s metabolism to make more of themselves.

Three types of influenza viruses are A, B, and C.

The most common form of influenza is caused by virus type A, and it has a capsid that surrounds 8 single-stranded RNA molecules that, together, encode 11 genes- this virus infects the cells lining the airways of the gas exchange system.

As virus particles leave their host cells, they are enveloped in a phospholipid bilayer derived from the cell surface membrane- this membrane contains two types of glycoprotein, which are involved in infecting new host cells and are coded by the genes in the viral RNA.

HIV infects certain cell types, including brain cells and some in the immune system; it is an enveloped virus with RNA as its genetic material.

The protein capsid is surrounded by a matrix of viral protein and then by a phospholipid bilayer formed from the cell surface membrane of the host cell from which it emerged.

Inserted into this membrane are molecules of glycoprotein that fit into molecules of a cell surface protein of the cells that the virus can infect.

structure of a virus

HIV is a retrovirus; its RNA is used as a template to make DNA, which is the reverse of what normally happens in cells.

The enzyme reverse transcriptase uses the viral RNA as a template to make single-stranded DNA, which is replicated by DNA polymerase so the DNA becomes double-stranded.

The DNA enters the nucleus where the viral enzyme integrase attaches it to host DNA- this incorporated viral DNA is a provirus and may remain inactive for several years.

When activated, the DNA provirus is used as a template for host RNA polymerase to make RNA as the genetic material for new viruses and mRNA to make viral proteins.

Viral protease cuts the proteins produced on the host cell’s ribosomes into short sections that are assembled around RNA to make the capsid and matrix of new viruses.

These viruses travel to the cell surface membrane and leave surrounded by the host cell membrane, with HIV glycoproteins incorporated.

Protoctista

The parasite that causes malaria is classified with other unicellular eukaryotes in the kingdom Protoctista.

Fungi

Fungi are eukaryotic organisms that are composed of filaments known as hyphae that form an extensive network through soil; in the case of parasitic fungi, over the surface or within the body of their hosts.

Athlete’s foot and ringworm of cattle are two diseases caused by fungi that grow over the surface of the skin.

Black Sigatoka is caused by a fungus that causes black streaks in banana leaves; the fungus spreads through the leaf tissue, reducing plants’ ability to photosynthesise.

As the disease spreads, the whole leaf dies, reducing the production of carbohydrates that are destined to be transported to the fruits.

Direct transmission of pathogens

Involves contact between two individuals.

In a field of tobacco plants, leaves infected with TMV may touch the leaves of an uninfected plant, so virus particles are transmitted.

Black Sigatoka grow small tubes that penetrate their host through their cuticle or through their stomata.

Pathogens can be breathed in by uninfected people.

HIV is not a very robust parasite and is only transmitted by direct contact involving some body fluids; this can happen through:

Unprotected sex, infected needle, blood transfusion, breast milk.

After transmission of HIV

After HIV transmission, there is a short incubation period of several weeks, then there are mild flu symptoms; the infection is symptomless for a fairly long time until a variety of opportunistic diseases appear, such as thrush, tuberculosis, pneumonia and these diseases develop due to the decrease in lymphocytes, destroyed by the HIV infection.

The collection of opportunistic diseases due to HIV is known as AIDS.

A more complex method of direct transmission is the formation of spores produced specifically for transmission via some medium, air or water.

For example, P. infesters produces hyphae that grow out through stomata; these hyphae swell to produce sporangia, which are pear-shaped structures. Sporangia may be blown by the wind to land on uninfected leaves; there they produce specialised hyphae that enter the plant to begin a new infection.

Indirect transmission of pathogens

Malaria is spread by a female Anopheles mosquito.

If a female mosquito takes a blood meal from someone infected with Plasmodium, she will take in many reproductive forms of the parasite- thrdr reproduce inside her gut and move to her salivary glands, ready to infect another human when she takes another blood meal.

Transmission is complete when she takes a blood meal from an infected person, injecting her saliva to stop the blood clotting; the mosquito is the vector of the disease.

An advantage of insects for pathogens is that, in suitable conditions, insects reproduce in very large numbers, increasing the chance of transmission.

Factors that influence transmission of communicable diseases

The presence of infected individuals affects the transmission of all diseases- if the pathogen is not present in a population, then no potential hosts can be infected.

Diseases that are always present in a population are known as endemic diseases; malaria is endemic in many tropical countries but not elsewhere; TB is endemic across the world.

Many pathogens have several different stages in their life cycle, and only some of these stages are infective. In malaria, the only infective stage is found in the salivary glands of anopheles; Plasmodium has a complex life cycle.

Animals and plants are resistant to some diseases- this means that they have inherited genes that code for some mechanism that prevents infection by specific pathogens or prevents their spread within the body.

Resistance is different to immunity because immunity means they develop symptoms the first time they are infected by a specific pathogen but are very unlikely to develop symptoms when infected by this pathogen on any subsequent occasion.

The higher the proportion of resistant or immune individuals in a population, the lower the chances of transmission occurring.

Factors affecting direct transmission

Diseases spread by droplet infection are dependent on potential hosts being close in proximity.

Influenza spreads to infect more people if people live in high densities- higher infection rates in schools than in the general population.

Crop plants are usually grown in monocultures at high densities, which means infected TMV leaves can easily touch uninfected leaves.

Factors affecting indirect transmission

Transmission by vectors such as mosquitoes and aphids is influenced heavily by factors that affect their biology; the population of these insects is influenced by climate and by weather.

Anopheles mosquitoes need small areas of water in which to breed- they breed more frequently in wet conditions than in dry- transmission of malaria is much higher during wet, rainy seasons than during dry seasons.

The development of the malaria parasite within mosquitoes is temperature dependent.

If the temperature decreases below 20 degrees, the parasite cannot complete its stages within mosquitoes.

Factors affecting transmission of human diseases

Waterborne diseases such as cholera, typhoid and polio are spread when human faecal waste contaminates drinking water- which can happen in areas of poor housing where there is little or no sanitation or sewage treatment.

Poor housing may also be unhygienic, so transmission of other diseases is possible, especially if people aren’t able to cook their food thoroughly.

Diseases transmit rapidly when brought into new populations with little to no natural resistance or no immunity.

Increased movement of people around the globe has increased the chances of disease transmission.

Plant defences against pathogens

Plants have mechanisms to protect them against infection.

These mechanisms are divided into passive and active.

Passive defence mechanisms

Present all the time.

Some are physical, and some are chemical that deter the growth of pathogens.

Examples of passive defences are:

physical:

waxy cuticle over leaf epidermis, bark, cellulose cell walls, Casparian strip in the endodermis of the root, stomata that close to stop pathogens from entering.

chemical:

secreting compounds to support growth of microorganisms that compete with pathogens, secretion of compounds toxic to pathogens, secretion of inhibitors of enzymes, receptor molecules on the cell surface membrane that detect pathogens and activate plant defences.

Physical barriers such as waxy cuticles reduce the chances of plant pathogens gaining entry or spreading very far inside the plant; viruses or bacteria cannot enter through the cuticles unless there is a wound on the surface of the leaf or stem, such as wounds made by herbivores.

Many fungi grow into roots and penetrate as far as the endodermis; they cannot grow any further because of the impenetrable Casparian strip.

Plants produce chemicals to prevent the growth of pathogens by secreting substances onto the surface of leaves and roots; they can make the environment too acidic for the growth of a pathogen.

Active defense mechanisms

Plants have active defence mechanisms that are activated when pathogens invade.

The most drastic is hypersensitivity- the almost immediate death of tissues surrounding the site of infection by a pathogen.

This is highly effective because many pathogens require living host tissues to survive and spread- if the cells die, there are no nutrients, and there is no energy for continued growth.

Plants also respond by making physical barriers- when bacteria or fungi attempt to penetrate cell walls, this stimulates production of compounds such as callose and lignin to thicken and reinforce cell walls, making them harder to penetrate.

Callose

Callose is deposited between the cell surface membrane and the cell wall.

Callose is a polysaccharide that forms a matrix in which antimicrobial compounds can be deposited, such as hydrogen peroxide and phenols, which kill any pathogens attempting to enter the cells.

Callose also reduces the diameter of plasmodesmata, the membrane-lined channels that run through cell walls linking the cytoplasm of neighbouring plant cells.

By narrowing these channels, callose reduces the spread of viruses from cell to cell.

Some pathogens move through plants inside the vascular tissue; tyloses are ingrowths into xylem vessels to prevent the movement of pathogens within the vessels.

Callose is often deposited into sieve pores to block transport of phloem sap and impede movement of pathogens.

cell signalling

Cell signalling plays a large part in plant responses to disease.

Bacterial and fungal pathogens secrete cellulase to digest a pathway into cells.

The breakdown products of cellulose hydrolysis act as signals that are detected by receptors on the surfaces of cells.

Once detected, the breakdown products stimulate the production of phytoalexins, which are defence chemicals that act in a variety of different ways and:

disrupt the cell surface membranes of bacteria

stimulate the secretion of chitinases that break down the cell walls of the hyphae of fungal pathogens

disrupt metabolism in the pathogen

delay reproduction of the pathogen

Signalling molecules such as salicylic acid travel through plants to activate defence mechanisms in uninfected areas, giving them protection against many pathogens for some time after original infection.

This long-term response to infection is called systemic acquired resistance.

Ethylene is another signalling compound; when secreted by plants under attack from pathogens, it vapourises to stimulate other leaves of the same plant and also other plants in the immediate surroundings.