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Bacteria
“Bacteria are unicellular organisms belonging to the prokaryotic group where the organisms
lack a few organelles and a true nucleus”
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They are among the earliest known life
forms on earth
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Some are airborne and others are most
prevalent in water, soil, plants, animals, and even people
...
There are few (less
than 1% of all bacteria types) that cause illness in humans
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The global problem of infectious and deadly diseases caused by bacteria are presently
major scientific and medical issues
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As a general rule, bacterial infections are easier to treat than viral infections, since we
have an extensive army of antimicrobial agents with activity against bacteria
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[3]

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The most deadly bacterial disease contracted by human beings is
mycobacterium tuberculosis, the world's leading infectious disease with more than
1,700,000 deaths per year
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Structure
Bacteria are prokaryotic organisms that carry their genetic information in a double-stranded
circular molecule of DNA
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The cell cytoplasm contains ribosomes and there is both a cell membrane and, in all species
except Mycoplasma, a complex cell wall
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Bacteria normally reproduce by binary fission
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Consequently, some infections require only a small number
of organisms to cause potentially overwhelming infectio
Another fascinating feature of bacteria is their protective cell wall, which is made up of a
special protein called peptidoglycan
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This particular protein isn’t found
anywhere else in nature except in the cell walls of bacteria
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On the outer layer, one or more flagella or pili is attached, and it functions as a
locomotory organ
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They do not contain any cell organelle as in animal or plant cell except for ribosomes
...
In addition to this DNA, they have an extra
circular DNA called plasmid
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coli)

Spirilla or spirochete (Spiral)

Spirillum volutans

Coccus (Sphere)

Streptococcus pneumoniae

Vibrio (Comma-shaped)

Vibrio cholerae

Classification of bacteria based on the Composition of the Cell Wall
Type of Classification

Examples

Peptidoglycan cell wall

Gram-positive bacteria

Lipopolysaccharide cell wall

Gram-negative bacteria

Classification of bacteria based on the Mode of Nutrition
Type of Classification

Examples

Autotrophic Bacteria

Cyanobacteria

Heterotrophic Bacteria

All disease-causing bacteria

Classification of bacteria based on the Mode of Respiration
Type of Classification

Examples

Anaerobic Bacteria

Actinomyces

Aerobic Bacteria

Mycobacterium

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Reproduction in Bacteria
Bacteria follow an asexual mode of reproduction, called binary fission
...
These are identical to the parent cell as well as to each other
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Eventually, cell elongates to form two daughter cells
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When there is a favourable condition, E
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Bacterial reproduction is strictly asexual, but it can undergo sexual reproduction in very rare
cases
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In such cases, the bacteria may become resistant to
antibiotics since there is variation in the genetic material (as opposed to asexual
reproduction where the same genetic material is present in generations)

bacteria replicate through a process called binary fission
...


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Phases of Bacterial Growth
The life cycle of bacteria under optimal growth conditions consists of four
phases: lag, log, stationary, and death
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They understand their
environment and try to adjust and metabolize
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Duration of this phase depends on
the availability of nutrients
...
Bacteria also make copies
of DNA during this stage
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If provided favorable conditions then they can get doubled in approximately 15
minutes but if conditions are not suitable then this stage can take time
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The process through
which bacteria gets multiplied is known as binary fission
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This decline depends on growth-inhibiting factors
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o

The Death Phase: – As the name implies, it is the last stage
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Factors that Influence Growth
Temperature, acidity, energy sources and the presence of oxygen, nitrogen, minerals and
water all affect bacterial growth, thus affecting the bacteria life cycle
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Psychrophiles, for example, thrive in
arctic conditions while hyperthermophiles grow best in hot environments, such as ocean
vents
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Of course, these are only two of many possible examples
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The lowest temperature that allows the growth is called minimum temperature and the
highest temperature that allows growth is called maximum temperature
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Below minimum temperature cell membrane solidifies and become stiff to transport
nutrients in to the cell, hence no growth occurs
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1
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Their optimum growth temperature is between -5C
and 15C
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2
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Their optimum growth
temperature is between 25C and 45C
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3
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Their optimum growth temperature is between
45C and 70C and are commonly found in hot springs and in compost heaps
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Hyperthermophiles are bacteria that grow at very high temperatures
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They are usually members of the
Archaea and are found growing near
hydrothermal vents at great depths in the
ocean
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Decrease in nutrient concentration decreases the growth rate
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In order to grow, bacteria need a minimum of nutrients: water, a carbon source, a nitrogen
source and some mineral salts
Water plays a fundamental role in solubilizing nutrients, transporting them and ensuring
hydrolysis reactions
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If evaporation occurs
during the incubation of the agar, there may be a loss of this water, resulting in a decrease in
colony size and inhibition of bacterial growth
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It is essential for bacteria to
produce carbon molecules, such as fats, carbohydrates, proteins and nucleic acids
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As for nitrogen sources, they are numerous and can be found in a large number of
compounds used in the composition of a culture medium
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Nitrogen allows bacteria to synthesize
their proteins
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There are two types of bacteria, phototrophic bacteria, such as Thiocapsa roseopersicina,
which uses light as an energy source by transforming it into an electrochemical gradient of
protons, and chemotrophic bacteria, which use the energy of oxidation of mineral or organic
compounds as energy sources
...




Most of the bacteria grow at neutral pH (60
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5)
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

relationship between pH and bacterial growth is given in figure below
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Neutrophiles grow best at a pH range of 5 to 8
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Acidophiles grow best at a pH below 5
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3
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5
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

Oxygen is required for aerobic respiration and obligate aerobic bacteria must require
O2 for growth
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Mycobacterium, Bacillus



For obligate anaerobes Oxygen is harmful or sometime lethal
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

Carbon-dioxide is needed for capnophilic bacteria
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

Most bacteria do not require NaCl in media however they can tolerate very low
concentration of salt
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Benefits of Bacteria
Bacteria provide vital ecosystem services
...
They are also
needed for the carbon and nitrogen cycles
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They help digest food, make vitamins, and play other important roles
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Making drugs, such as antibiotics and vaccines
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Cleaning up oil spills and toxic wastes
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Transferring normal genes to human cells in gene therapy
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Bacteria are used
to make a wide range of food products
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Infections and diseases caused by bacteria

Bacteria can be strictly pathogenic, which means that they will cause disease if they manage
to overwhelm the human immune system
...
coli
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Such infections are called zoonotic
infections
...

Most Deadly Bacterial Infections
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Tuberculosis
Anthrax
Tetanus
Leptospirosis
Pneumonia
Cholera
Botulism
Pseudomonas Infection
MRSA Infection
E
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In 40 per cent of cases, the type of bacteria
causing the infection isn’t identified in time, making it difficult to treat and causing the
cascade of bodily responses that can become fatal
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It remains a major killer because of a rise in drug-resistant strains
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Tuberculosis (TB) is a bacterial infection spread through inhaling tiny droplets from the coughs
or sneezes of an infected person
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In most healthy people, the body's natural defence against infection and illness (the immune
system) kills the bacteria and there are no symptoms
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You will not have any symptoms, but the bacteria will remain in your body
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People with latent TB are not infectious to others
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This is
known as active TB
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Symptoms include difficulty breathing,
cough, fever or chills and phlegm
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Urinary tract infections (UTI)
These infections occur when bacteria enter and multiply in any part of the urinary system,
most commonly in the bladder and urethra
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coli) and are the most common infection in humans worldwide
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While UTIs can usually be easily treated with antibiotics, they can cause death
if they develop into sepsis
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Some people are at higher risk of getting a UTI
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This makes it easier for bacteria to enter
the urinary tract
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While this disease can be caused by a virus or a fungus, bacterial meningitis is
the most severe type, and can turn fatal within hours
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An infected wound can swell, become red, feel warm, and ooze pus
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The
ageing population and increase in people with type 2 diabetes are drivers of an increase in
chronic wounds
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Diarrhoea
Previously, most deaths from diarrhoea were caused by the accompanying dehydration, but
sepsis resulting from bacterial infection represents a growing proportion
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Infection with E
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most of which are spread by faeces-contaminated water
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Infection in people usually results from skin contact but can result from inhaling
anthrax spores, eating contaminated meat, or rarely by injecting contaminated drugs
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Anthrax bacteria produce several toxins, which cause many of the symptoms
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Symptoms suggest the infection, and identifying the bacteria in samples taken from
infected tissue confirms the diagnosis
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Antibiotics and the vaccine must be given soon after exposure to reduce the risk of
dying
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A painless, itchy, red-brown bump appears 1 to 10 days
after exposure
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Nearby lymph nodes may swell, and people may feel
ill—sometimes with muscle aches, headache, fever, nausea, and vomiting
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 Inhalation anthrax (woolsorter’s disease)
Inhalation anthrax is the most serious
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Spores may stay in the lungs for weeks but eventually enter white blood cells called
macrophages, where they germinate, and the resulting bacteria multiply and spread to
lymph nodes in the chest
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Infected fluid
accumulates in the space between the lungs and the chest wall
 Gastrointestinal anthrax
Gastrointestinal anthrax is rare
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People have a fever, a sore throat, a swollen neck, abdominal pain, and bloody
diarrhea
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 Injection anthrax
Injection anthrax is rare
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The bump develops
into a painless sore that forms a black scab (eschar) with swelling around it
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Injection anthrax can spread throughout the body faster than and can be harder for doctors
to diagnose and treat than skin anthrax
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Anthrax spores are not easily killed by cold
or heat and can survive for decades
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25 million people have received the
anthrax vaccine without having a serious
adverse reaction

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