Characteristics of different microorganisms
Prokaryotes:
Physical characteristics:
Prokaryotes have a particular cellular structure. This is common in organisms like bacterial cells.
prokaryotes consist of a plasma membrane to control what comes in and out of the cell for cell
communication¹. It also has a cell wall to protect the cell from the environment and give the cell
structure and support. In case of bacterial cells, they can also have a capsule made of polysaccharides
which is like a slime layer that keeps the cell hydrated (hydrophilic)². The role of the bacterial capsule is
to ensure the bacteria is safe from being engulfed by phagocytes in an immune response. It does this by
preventing the phagocytes from adhering to the bacteria (smooth surface)²⁵. Gram positive and gram-
negative bacteria differ in their structure of cell which gives them different properties. For example, gram
positive bacteria are easier to breakthrough compared to gram negative. This is because it has a single
plasma membrane whilst gram negative has two plasma membranes enveloping the cell wall. This makes
it difficult for antibiotics such as penicillin to break through to attack the cell’s DNA. Although gram
positive bacteria have a thicker peptidoglycan wall, it is still easier to breakthrough³. Bacterium cells
don’t have membrane bound DNA, so it lies freely in the cytoplasm of the cell. The DNA is called the
nucleoid. Bacteria can have plasmids which are extra loops of DNA. They can hold genes for antibiotic
resistance. Bacteria also contains ribosomes to make proteins. Most bacteria also have flagella to help
the cell move. They have pili to infect other host cells¹. A prokaryote’s genome is all of their genetic
information. They are generally unicellular and so their genome consists only of their double stranded
nucleoid which lies freely in the cytoplasm⁴.
Growth characteristics:
Bacteria has a special way of reproducing. This is through a process called binary fission. It is an asexual
process and happens in a few steps: first the loop of DNA uncoils and is replicated to produce a second
identical loop of DNA. Then the size of the cell increases and the other structures like DNA plasmids and
ribosomes replicate. Next the loops of DNA are pulled to opposite ends and then the cell membrane
divides and a wall forms around each loop of DNA. Then the cell separates from two genetically identical
daughter cells. Each time this happens the bacterial population doubles². Some bacteria use different
reproduction strategies. Some bacteria can reproduce by a slightly different process where the parents
cell grows to more than double the starting size. Then it performs multiple divisions to form multiple
cells. Another type can be budding in bacteria. This is where offspring develops inside the parent cell⁵.
Bacteria have growth curves which occur when they are grown in a lab. When they’re grown in a closed
system their growth follows a particular pattern. This consists of lag, exponential, stationary, and death.
The lag phase is where bacteria is added to a growth medium which contains nutrients for the bacteria
to grow. The bacteria develop enzymes to use these nutrients. The next phase is the exponential phase.
This is when the cells divide by binary fission. The generation size doubles each time at this stage. After
this is the stationary phase. Here binary fission falls as there’s less nutrients and space and more waste.
,The death and reproduction rates stay equal. The last stage is death. The reproduction rate isn’t
supported due to nutrients running out, so the population of bacteria decreases, and they die¹.
Temperature has an impact on growth rates of different kinds of bacteria. Different kinds work at
different temperatures. Temperatures between -5 degrees and 20 degrees are suitable for growing
psychrophilic bacteria. The enzymes involved in these bacteria are adapted to work at optimum levels at
low temperatures. Another kind of bacteria is mesophilic bacteria which can work at 20 degrees Celsius
to 45 degrees Celsius. These kinds of bacteria invade human cells as they work within human body
temperature. Another kind is thermophilic and works between 50 degrees Celsius and 70 degrees
Celsius. They work at very high temperatures, so their enzymes are resistant to heat damage. They won’t
denature at high temperatures¹.
Phenotypic methods to classify bacteria:
Bacteria can be classified by their shape. There are multiple shapes that bacteria can take form of. They
are: cocci (bead shaped), bacilli (rod shaped), flagellate rods, spirilla (spiral shaped) and vibrios (curved
rod). Cocci bacteria can have a few ways of being further classified. When they are in pairs, they are
called diplococci, when they’re in chains, they are called streptococci and when they are in bunches,
they are called staphylococci¹. They can also be classified by structure like gram positive and gram-
negative bacteria. gram positive bacteria have their organelles enveloped in a plasma membrane made
of phospholipid bilayer which is covered in a thick cell wall (peptidoglycan). It is also coated in a capsule
which is made of polysaccharides, and it is hydrophilic to prevent dehydration. They have an overall cocci
shape When viewed under a microscope with gram staining.
Gram positive bacteria appear to be a purple color under the microscope. This is because it has a thick
cell wall which allows the dye to soak into the thick cell wall and allow the color to show through. Gram
negative bacteria have a slightly different structure. They have their organelles enveloped by a plasma
membrane and then that’s covered in a thin cell wall. Then on top of this is another plasma membrane
which is then enclosed in a capsule. Gram negative bacteria has a bacilli shape when viewed under a
microscope using gram staining. Gram negative bacteria appear to be pink or red in colour under a
microscope using gram staining. This is because it has a thin cell wall which gives the dye less room to
grab onto and so it just rinses off which then leaves it with a pink, red color. The second plasma
membrane and the thin cell wall is what sets gram positive and gram-negative bacteria apart
structurally³.
Different species of bacteria have different oxygen requirements. Some species of bacteria are obligate
aerobes which means they need oxygen to survive. They cannot live without it. An example of an
obligate aerobe is mycobacterium tuberculosis. Some bacteria however are obligate anaerobes. This is
when bacteria can only grow with small amounts or no oxygen. They can be killed by contact with
oxygen. some examples are bacteroides spp. And clostridium spp There is also a facultatively anaerobic
bacteria. They are able to survive at high and low oxygen levels. Some examples are Staphylococcus
aureus and E. coli. There is another kind of bacteria called aerotolerant bacteria which doesn’t
particularly need oxygen but can survive with it. Tetanus and Clostridium tetani are some examples¹.
M1:
,Bacteria (prokaryotes) have methods of pathogenicity. They include¹:
• Exposure to the host
• Sticking to the host cell
• Producing endotoxins and exotoxins to damage cells and tissues
• Evading immune response
• Development of infection
Bacteria can be exposed to the host through multiple ways. This can be through the ear canal, nasal
passages, intravenous (from dirty needles), breakages in the skin, eyes/tear ducts, mouth and Urinary or
reproductive tracts.²⁶ When bacteria makes contact with an individual this way, they can become
infected. However, most pathogens that come into contact with an individual don’t cause infections.¹
The next stage that bacteria takes to infect its host is by attaching itself to the host cells. To do this, the
bacteria have pili which sticks to the surface of the host cell. This enables the bacteria to cause infection.
Some bacteria have biofilms which makes it more difficult for the host cell to remove itself from the
bacteria. This makes it difficult for the cell to evade the effects of the bacteria. The pili can also protect
the bacteria from immune responses and antibiotics.¹
Once the bacteria have stuck to the host, it releases endotoxins and exotoxins. They both cause damage
to cells and tissues. Endotoxins are lipopolysaccharides which make up part of gram negative bacterial
walls. Lipopolysaccharides are toxic to the host cell. When the bacteria is destroyed, endotoxins are
released. For endotoxins to cause infection in a person, a large amount of endotoxins must be secreted
as they’re not very potent. When endotoxins are released, they aren’t recognised by the immune system
and so don’t trigger an immune response. This causes the infected individual to get a fever. An example
of bacteria that releases endotoxin is E.coli.¹
Exotoxins, on the other hand, are made of proteins and are produced by both gram positive and gram
negative bacteria. However, they are only produced by certain types of these bacteria. Exotoxins are
released whilst the bacteria are growing and the exotoxins released are potent. So, only a small amount
of endotoxin is needed to cause infection in an individual. The endotoxin is recognised by the immune
system, triggering an immune response. An example of bacteria that produces exotoxin is vibrio
cholerae.
When toxins are released, the body will usually trigger an immune response to fight off the infection.
This is done when the immune system detects a foreign body in the individual that does not have the
normal antigens for that body. The immune system recognises it as non-self and so attacks them using
the process of phagocytosis, to bind to the non-self antigens on the bacteria and destroy it. However,
some bacteria evade the immune response by changing the shape of the antigens on its surface. When
the bacteria first comes into contact with the individual and the immune system fights it off, and keeps
memory cells to recognise if the bacteria comes again. It would be able to destroy the bacteria much
faster next time. But, if antigenic variation occurs in the same bacteria and it comes into contact with the
individual again, the body will not recognise it, causing it to have to create new antigens and new
memory cells. So, the bacteria stays in the individual for longer.¹
, Bacteria can also evade the immune system with its capsule (protein coat). The capsule prevents
immune cells from binding to the bacteria, so it can not be destroyed.¹ Some bacteria also release
protease enzymes to break down any immune cells that try to bind to antigens on the bacterial cells.
Through this process, phagocytosis isn’t triggered because immune cells aren’t able to bind to antigens
of the bacteria.¹ Mycolic acid is a waxy substance that is unique to Tuberculosis. Mycolic acid protects TB
from phagocytosis.¹
The development of infection, or incubation period is when the bacteria is developing in the body
between the stages of exposure and producing toxins that cause fever.²⁷
Bacteria like Mycobacterium tuberculosis can mutate which causes them to resist antibiotics³². There is a
correlation between mutation rate and the resistance to drugs, which means when there is more
mutation, there is more likelihood of antibiotic resistance development³³. In tb, it is the rpoB gene that
mutates causing the antibiotic resistance. Many strains of tb have resistance to rifampicin (RIF)³⁴. There
is a variant of tuberculosis that is extremely resistant to antibiotics. It is called XDR. As a result of
extremely resistant bacteria, a complex and thorough course of antibiotics is given. They are given across
a course of a few months. Antibiotic resistance to tb is becoming a public health concern because if this
continues, there is a chance that bacteria can become resistant to all drugs, meaning it may become
untreatable.
HIV is a virus that can be mutated. This causes the virus to become more resistant to antibiotics causing
treatment methods to become more complex.³⁵There are two main kinds of mutations in HIV. They are
caused by a mutation called CCR5-DELTA 32. This mutation allows the HIV to be recognised as immune
cells so that it cannot be targeted³⁶ If HIV has mutated, it is no longer treatable with the normal
medicine used for HIV which causes any treatment used with it to be useless³⁷. This means that the
course of medicine will need to be changed. If a person becomes resistant to the three main courses of
medicine given to HIV patients, they can be given an alternative. This can be any one of the following:
Protease inhibitors
NNRTI etravirine
Integrase inhibitors
Entry inhibitors
Fusion inhibitors
Using these alternatives, a treatment combination is made. The treatment combination is made by
judging which treatment was previously given. It’s decided by your doctor and its very important that the
next treatment combination is taken properly. If this isn’t done, it can lead to further resistance, making
the virus even more difficult to treat³⁸.
