Unit 12 – Osman Yousuf
Diseases and Infections
Types of Pathogens:
Bacteria - Type 1
Bacteria are prokaryotic microorganisms with no nucleus. They are found all over the natural world
and are adapted to extreme temperatures and pH levels. Their structure contains organelles of a
capsule, cell wall, plasma membrane, 70s ribosomes, nucleoid, cytoplasm (where DNA floats),
flagellum, food granules, plasmids (containing DNA). They also contain pili; these are spikes which
allow a transfer of genetic material for sexual reproduction. Bacteria have many characteristics
which include generation of energy in many ways such as by performing photosynthesis and
obtaining energy from the sun (photosynthetic bacteria). They contain no mitochondria, instead
they perform similar aerobic respiration themselves. They exchange genetic information across
species, random mutations can occur which may create and result in antibiotic resistance. Finally,
they reproduce asexually such as binary fission (multiplying). The bacteria growth cycle is of four
phases which depends on the environment and bacteria type. The lag phase is when bacteria adjust
to the environment and prepares to grow and divide, by producing vitamins and amino acids, as well
as copying their DNA. In the exponential phase, bacteria start to multiply rapidly, undergoing binary
fission. During the third phase – stationary, the rate of growth and division decreases, this is due to
limited space, lack of nutrients or source, and other limiting factors. This is changed if bacteria
migrate. The fourth and final phase is death. The bacteria no longer can grow or multiply; they die
out and decrease rapidly in quantity.
Bacteria causes disease by damaging cells and tissues by utilising cell nutrients by living
intracellularly, creating excess waste products, producing toxins which destroys cells or inhibits the
cell or organism correct function. The disease tuberculosis (TB) is caused by the mycobacterium
tuberculosis (MTB). This is a contagious bacterial infection which mainly affects the lungs of an
individual. The characteristics of MTB are that it utilises aerobic respiration, it is rod-shaped,
heterotrophic, non-motile due to no cilia nor flagella, contains a thick, hydrophobic cell wall of lipids,
and antibiotic tolerance. MTB has a slow growth rate. The life cycle begins with infectious droplets
inhaled by other individuals. They then reach the lungs, which enters the alveolar macrophage. MTB
undergo binary fission, they then replicate. By an immune response, phagocytosis occurs to control
the infection and granuloma is formed. This leads to granuloma necrosis and coughing leads to the
release into the air.
Parasites - Type 2
Parasites are organisms that live on or in a host’s organism such as within the digestive tract/system
and can cause the host harm. There are two types of parasites. This includes endoparasites which
live within the body with subgroups such as protozoa and helminths. The second type is
ectoparasites which live on the surface, this is exemplified with ticks. Parasites are diverse and
therefore adapted in many ways. Structural adaptations of parasites lead to feeding from blood, and
a limited digestive tract and nervous system (in worms). Physiological adaptations lead to parasites
not producing digestive enzymes as well as hiding and therefore evading an immune response.
Reproductive adaptations are exemplified with some parasites being hermaphrodites (reproducing
with themselves) and parasites producing great quantity of eggs. Parasites of adult worms’ life cycle
begins in the small intestine via ingestion. They produce eggs via asexual or ‘sexual’ reproduction.
The eggs are passed through faeces and fertilise. The fertilised eggs undergo further development
and multiply, forming embryonated eggs with larva. They then migrate around, specifically on areas
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such as grass (infective stage). The larvae are ingested into a host which results in them hatching and
developing in circulation, they then migrate to the gastrointestinal tract and mature in the small
intestine to produce eggs.
Parasites can cause diseases by having a variety of mechanisms. Helminths can block blood vessels
and burrow into tissues. Ectoparasites can also burrow into tissues. An example of a disease caused
by parasites is tapeworm infection (taeniasis). Taeniasis is caused by ingesting contaminated and
undercooked beef from animals who have eggs that have developed into cysticerci within the
muscles. These larvae are infective, thus causing the disease. This is caused by Taenia saginata (beef
tapeworm). They consist of a scolex, neck, and four suckers, many regions of the tapeworm are
segmented. The characteristics of taenia saginata are, production of lots of eggs/ proglottids that are
motile, lack of digestive system/tract, they obtain nutrients via microtrichia, they are
hermaphrodites, but can also reproduce ‘sexually’ The first step of the lifecycle is gravid proglottids
passed to the environment via faeces. This contaminates surrounding areas of vegetation where the
gravid proglottids have contacted. The cattle ingest the eggs and vegetation and becomes infected.
The oncospheres (embryonated eggs) hatch inside the body of cattle, circulate, penetrate the
intestinal walls, and migrate to muscles. In muscles, oncospheres develop into cysticerci (infective
stage). When humans ingest beef, they are infected due to raw meat or the cysticerci. In the small
intestine, the scolex and suckers attach itself to the walls, they develop into long, adult worms and
reproduce.
Protozoa – Type 3
Protozoa are single-celled eukaryotes that are widespread in nature. They contain organelles such as
a nucleus, cytoplasm, cilia, vacuole, pores and flagella which enables them to be motile. They lack a
cell wall to allow it to carry out their own functions. Protozoa are heterotrophic, they cannot
produce food on their own therefore they rely on other sources of nutrients to survive. As a result,
they undergo asexual reproduction - binary fission to divide and grow, or ‘sexual’ reproduction.
Protozoa alters in structure during different phases of their life cycle. They start off as cysts or
oocysts ingested or in a host body. Cysts are the dominant form where they have a thickened wall,
this is to protect them and aid for them to survive in harsh conditions such as lack of food and
nutrient source, or extreme temperatures. The cysts differentiate to bradyzoites and differentiates
again to trophozoites. In the form of trophozoites the structure and formation are irregular. This is
where protozoa actively feed and therefore replicate to kill the cell, they then differentiate back into
bradyzoites to then form cysts (encystation) in different areas of the body such as muscle tissue.
Protozoa causes diseases by infecting and overwhelming cells. An example of a protozoal disease is
malaria. This is an infection spread by mosquitoes. The malaria life cycle begins when an infected
female anopheles mosquito bites an individual. They inject sporozoites (plasmodium parasites). The
sporozoites migrate to the liver via the bloodstream and reproduce asexually and produce daughter
cells called merozoites. The merozoites migrate within vesicles to lung capillaries where the vesicles
break down and the merozoites are released into the bloodstream. They then invade erythrocytes
and multiply till erythrocytes burst. The individual is now infected. Some of the infected erythrocytes
stop multiplying and the merozoites within them form gametocytes (sexual form). When a mosquito
bites an individual, it ingests the gametocytes which develop into gametes. The sex cells within a
mosquito further develop into oocytes. The oocytes house and develops many sporozoites until it
bursts, and the cells are released, they migrate towards the mosquitos’ salivary glands. When this
mosquito bites another individual, malaria is spread and the cycle repeats.
Viruses – Type 4
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Viruses are extremely small infectious particles (akaryotes). They replicate only within living (host)
cells. They are not a living organism; they do not contain certain structures or organelles such as
nucleus or nucleoid. There are many types of viruses that depend on the shape. This includes helical,
polyhedral, spherical, and complex. There are many characteristics of viruses which include that they
cannot replicate on their own, they are composed of a protein coat and genetic material. The
protein coat provides the virus with its shape and protection. They also do not perform life functions
such as metabolic processes.
An example of a disease caused by viruses is bronchitis caused by adenovirus, which is a non-
enveloped, polyhedral type with double-stranded DNA genome, they can be spread person to
person through droplets and contact. The first step of the adenovirus lifecycle is attachment. The
virus cell binds itself to the host cell membrane, this forms a hole in the membrane, where the
particle can now enter the host cell. The protein coat of the virus is then digested upon entry, and
viral genome and nucleic acids are injected into the cytoplasm of host cell. Replication and gene
expression occurs in the nucleus. Viral particles then insert their own DNA (prophages) into human
DNA which results in the improper functioning of the human body, the virus will rapidly produce
copies of themselves, the virus causes diseases by releasing toxins, destroying cells, or affecting
cellular processes. They change proteins into misfolded proteins which no longer functions correctly.
When virus replicates and all assemble, this causes the membrane to burst and therefore form a
passage for the virus to leave and go on to infect other neighbouring host cells.
Fungi – Type 5
Fungi are eukaryotic organisms which are abundant worldwide. They contain membrane-bound
organelles such as a nucleus, 80s ribosomes, and mitochondria. Fungi are diverse; there are many
types of fungi including yeasts, mushrooms, and moulds. The characteristics of fungi include thread-
like bodies. They have a cell wall composed of chitin (a polymer). They are heterotrophic, they rely
on other organisms for food as they cannot create their own. They are non-motile are they do not
have the correct structures to move, they can also reproduce asexually or ‘sexually’ which is
dependent on their environment. When reproducing ‘sexually’ spores are produced, these spores
travel to an appropriate environment where they germinate and form mycelium (‘roots’) which find
and provide nutrients for the spores to grow. If the mycelium encounters other fungi that is
compatible, the cells of each fungi fuse together forming diploids. This is called sexual union. The
cells then undergo meiosis which maintains genetic diversity. When reproducing asexually, conidia
(mitospores) are formed which are identical to the parent. The conidia then germinate into
mycelium and may now undergo sexual reproduction.
Fungi causes diseases by having varied mechanisms, production of cell-damaging toxins, penetration
into tissues and spores that are easily inhaled. An example of a fungal disease is blastomycosis. This
is an infection that is caused from Blastomyces dermatitidis fungus. This pathogen causes the
disease by reproducing to release spores in the air. This is inhaled by individuals and enters the
respiratory tract and the human body. This fungus lives in moist environments as mould. The
lifecycle of Blastomyces begins with the mould with aerial hyphae (long, branching structures) they
reproduce asexually and give rise to conidia. These spores can be released into the air when
disturbed which results in individuals inhaling or inoculating the spores. They transform into large-
celled budding yeast due to a warm temperature inside the body and continue to colonise the
respiratory tract especially the lungs. They can also travel to other areas of the body via the
bloodstream.
Non-infectious diseases
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