Infectious disease
What are Infectious Diseases?
Infectious diseases are illnesses caused by pathogenic microorganisms invading one's body. This can spread
from organism to organism. Common types of pathogens include bacteria, viruses, fungi, parasites and
protozoa. These pathogens can then multiply by their different methods of reproduction inside the host's body.
The transmission of these pathogens can happen in many ways which include, direct contact to the infected
person or their bodily fluid, particulates exiting the body through coughing sneezing, through contaminated
food and water and in some cases even vectors (1).
Infectious Diseases: Pathogens
Section 1: Bacteria
What is bacteria?
Bacteria are unicellular microorganisms that lack a nucleus and can reproduce asexually. Infectious bacterial
cells have many characteristics that aid in transmitting and survival (2). Here are common characteristics of
bacteria.
Unicellular Bacteria are characterized to be unicellular. This means it consists
of only one cell. This contrasts with multicellular organisms like
humans, which consist of many cells. Bacteria vary in size, shape,
and structure, but consist of a single cell that performs all necessary
functions (2,3).
Size Another notable feature of bacteria is that they are very small, with
most species being 0.5-5 micrometer in size. This means they are
too small to be seen with the naked eye and can only be observed
with a microscope (2,6).
Adaptation to extreme temperatures Bacteria also have a wide range of optimum temperatures meaning
different bacterial cells have adapted. Bacteria can grow and
reproduce in a wide range of temperatures, from freezing cold to
scorching hot. Bacteria that grow optimally at low temperatures are
called psychrophiles, while those that grow optimally at high
temperatures are called thermophiles. Bacteria that grow best at
moderate temperatures are called mesophiles (7). This allows
bacteria to thrive in a variety of environments, including extreme
environments like hot springs and deep-sea hydrothermal vents
(4,5).
, Prokaryotes Another feature is that bacteria are prokaryotes. It lacks a nucleus
and other membrane-bound organelles. The genetic material is
contained in a single circular chromosome in your cytoplasm. This
contrasts with eukaryotes like humans, which have cell nuclei
containing genetic material. (2,3).
Aerobic and anaerobic respiration Bacteria cells are also known to aerobically and anaerobically
respire. This means they can obtain energy through two main types
of respiration. Aerobic respiration requires oxygen and produces
more ATP (adenosine triphosphate), which breaks down to ADP to
release energy for the cell. Anaerobic respiration occurs in the
absence of oxygen and produces less ATP (2,5).
Asexual reproduction Bacterial cells reproduce asexually. That means, they do not need a
partner to reproduce. Instead, they replicate their DNA and divide
into two identical daughter cells through a process called binary
fission. This allows bacteria to grow quickly and efficiently (3,4).
Toxins Another main characteristic of bacteria is the fact that they produce
toxins. These toxins are usually used by the bacterial cells to infect
the host. Two types off toxins released by bacterial cells can be
classified as either exotoxins or endotoxins end the toxins remain
part of the bacterial cells under released when cell death whereas
exotoxins a part of the outer membrane of the bacterial cell under
release to cause harm to the host leading to an infection. (11)
Structure and Function of Bacteria
The structure of the infectious bacterial cell is
enhanced to adapt to its role to cause an infection.
The structure of the bacterial cell is made of
complex components such as, pili, capsule, cell
wall, plasma membrane, ribosomes, nucleoid,
plasmid, cytoplasm, and flagellum. These
components' structure and function serve a vital
role in enhancing the pathogens' main aim to cause
infection. Unlike other pathogens bacteria have a
simple lifecycle ending with cell death. (2)
Pilli Pilli are thin hair-like structures that can be found on the outside of bacteria cells. These structures are
made of proteins. The main function of these pili differs based on the type of bacteria, the most common
function of which includes its ability to transfer genetic material between cells. (9)
Capsule Capsule in bacteria is a protective layer that can be found on the outer bacterial wall. The membrane is
made of polysaccharides. The capsule's main function is to act as a barrier which protects the bacteria
from physical damage or the host's immune response. (9)
, Cell wall The cell wall in bacteria surrounds the cell membrane and has main functions to support and protect the
cells. In addition, the cell walls maintain the structure of the bacterial cell. The cell wall present in
bacteria is made of peptidoglycan. (9)
Cell The cell wall in bacteria surrounds the cell membrane and has main functions to support and protect the
membrane cells. In addition, the cell walls maintain the structure of the bacterial cell. The cell wall present in
bacteria is made of peptidoglycan. (9)
Ribosomes Ribosomes are another component of the bacterial cell. Ribosomes are found within the cytoplasm and
are responsible for making protein through protein synthesis. (9)
Nucleoid In nucleoid is complex structure where bacterial genome is present. This bacterial genome isn't contained
within a membrane. Therefore, genetic information can be described as in a loop like structure. (9)
plasmid The plasmid is a circular piece of DNA which can be found away from the nucleoid the main function of
plasmid is to carry genetic material which is not essential for survival of the bacterial cell therefore
provides bacteria with genetic advantages against immune responses by the host through factors such as
antibiotic resistance. (9,10)
cytoplasm Cytoplasm present in bacteria is found within the interior of the cell. The cytoplasm is where most of the
cell's organelles are found. In addition, the site of cytoplasm is where many cellular processes such as
metabolism protein synthesis and DNA replication take place. (9)
flagellum The flagellum in bacterial cells is a tail like structure found on the outer wall of the bacterium cell. The
main function of the flagellum is to aid in movement benefiting the bacteria to flow within its host. (9)
Pathogenesis of bacteria
Pathogenesis is the multi-step process by which an infectious micro-organism encounters the host and infects
them in their own adapted ways. There are four stages of pathogenesis of bacteria. All these stages together
conclude in the formation of the infection. The four stages of pathogenesis are contact colonization invasion and
infection. Initially, to start off the infection, the bacteria must contact the host. This can happen in many ways
such as through air particulates, physical contact and in some cases also ingestion through the eyes, nose, ears
and mouth. Then there is colonization this is where the cells start to grow and in the life cycle of bacteria is
known as the exponential phase here, we see the growth of the bacteria species spreading through the hosts'
tissues This can happen in different ways depending on the type of bacteria and the environment bacteria is
growing in These bacteria's start to form biofilms which overall help the bacteria protects itself from the host's
immune system. then comes invasion in this stage of pathogenesis the bacteria start to execute their function
caused the infection this could either be producing enzymes that breakdown the host tissue or using specialized
cells to penetrate the host cells this helps the exponential growth of bacteria as the invasion of the cells and
tissues allow the bacteria to gather nutrients to further divide. finally, is the last stage of pathogenesis the
infection if the bacteria have successfully colonized and invaded the host cells this thought to cause the
infection bacterial cells have characteristics that allow them to form toxins this can be either exotoxins or
endotoxins both these toxins help cause damage to the interior of the hosts body leading to the infection. The
exotoxins are released by the bacteria which cause damage to the tissue leading to the infection the endotoxins
however found within the cell and are released upon cell death leading to a post bacterial damage. (14)
Life cycle of bacteria
