Download 1 Introduction to Bacteriology Early theories of the cause of diseases

Introduction to Bacteriology
Early theories of the cause of diseases
Theurgical theory. According to this theory it is said that the disease is due to the
wrath of the divine spirit because of the individual sin. It is governed by the
superstition followed by a community.
Miasmatic theory: this theory explains that all diseases originate from the earth
under the influence of stars, the moon, wind, water and season of the year.
Germ theory: Jacob Henle explained that minute organism not visible to the naked
eyes causes the disease. This theory was based on the finding of Leeuwenhook
who made the visualisation of microbe possible by inventing the first microscope.
F. A. Brauell demonstrated that anthrax could be transmitted to healthy animal by
injecting blood from anthrax infected anuimals.
Koch formulated rules that came to be known as Koch’s rule, and that the disease
if caused by microbes must follow this sequence.
Koch’s rules:
Organism must be found in each case.
Be able to isolate organism from diseased individual
Must reproduce same disease when isolated organism is used on healthy animal.
This lead to the study of microorganism by various scholars, which later on led to
the origin of bacteriology.
Bacteriology: Is the branch of biology that deals with the study of minute
organisms called bacteria (singular bacterium).
Microbial growth
"Because individual cells grow larger only to divide into new individuals,
microbial growth is defined not in terms of cell size but as the increase in the
number of cells, which occurs by cell division." This emphasis has practical
application since it is typically far easier to measure increases in cell number than
it is to measure increases in cell size. Bacterial organisms multiply by a process
called binary fission.
Binary fission
The majority of bacteria reproduce by a mechanism termed binary fission. Binary
fission is much simpler than the mechanisms of cell division seen in eucaryotic
cells.
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Factors affecting bacterial growth
"The kinds of organisms found in a given environment and the rates at which they
grow can be influenced by a variety of factors, both physical and biochemical.
Physical factors include pH, temperature, oxygen concentration, moisture,
hydrostatic pressure, osmotic pressure, and radiation. Nutritional (biochemical)
factors include availability of carbon, nitrogen, sulfur, phosphorus, trace elements,
and, in some cases, vitamins."
Classification of bacterial organism
Based on the utility to humankind
Industrial bacteriology: is the study of bacterial organisms and their action
involved in various industries. Alcohol production, curing of leather.
Agricultural bacteriology: deals with bacterial organisms involved in relation to
soil fertility and those related to diseases in agricultural plants.
Dairy bacteriology: deals with study of microbes involved in manufacture of dairy
products. Eg cheese production.
Human bacteriology: is the branch of medical bacteriology that deals with
organisms affecting the human health.
Veterinary bacteriology: Branch of medical bacteriology that deals with microbes
affecting the health of domestic animals.
Based on pH requirements
The pH scale measures hydrogen ion (H+) concentration and that low pH
correspond with high concentrations of hydrogen ion, neutral pH with equal
numbers of hydrogen and hydroxyl ions (OH-), and high pHs correspond to low
concentrations of hydrogen ion
Optimum pH
Optimum pH is that pH at which a given organism grows best. The range over
which most organisms can grow tends to vary over no more than a single pH unit
in either direction (e.g., from pH 6 to pH 8 for an organism whose pH optimum is
pH 7)
Acidophiles
Organisms whose optimum pH is relatively to highly acidic, which means
growing best in acidic pH
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Neutrophiles
Organisms whose optimum pH ranges about pH 7, plus or minus approximately
1.5 pH units
Alkaphiles
Organisms whose optimum pH is relatively to highly basic, which grow best in
high pH
Based on temperature requirement
Optimum temperature
Optimum temperature is the temperature at which an organism grows best
Typically the range in temperature over which a bacterium can grow is about
30ºC
Psychrophile
Cold-adapted organisms are called psychrophiles. The cut-off temperature for a
psychrophile is a 20ºC or colder. Psychrophiles may additionally be termed
obligate or facultative with obligate psychrophiles unable to grow above 20ºC, but
facultative psychrophiles are able to grow above 20ºC.
Mesophile
Organisms whose optimum growth temperature is found between 20ºC to 40ºC
are termed mesophiles. Human and animal pathogens, which must be able to grow
at the approximately 37ºC body temperature, are mesophiles
Thermoduric
Mesophilic organisms that can endure brief exposures to relatively high
temperatures are termed thermoduric These are one category of the organisms that
survive following inadequate heating of foods and may thereby contribute to the
spoilage of foods that have been heated (e.g pasteurization) to kill microorganisms
Thermophil
High-temperature-adapted organisms are called thermophiles. Examples organism
that grow in hot springs
Based on Oxygen requirements
Organisms differ in their requirements of molecular oxygen (i.e., O2) as well as
other atmospheric gasses (e.g., carbon dioxide). Categories of organisms as per
their oxygen requirements include:
Obligate aerobe
Organisms that are unable to grow in the absence of oxygen or they require
oxygen for their growth. Some times this group of organism may be called strict
aerobes as they can not grow without oxygen.
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Facultative aerobe
Organisms that can grow in the absence normal level of oxygen that is other wise
required.
Microaerophile
These are organisms that grow best when small amounts of oxygen are present
That is, less than atmospheric concentrations, but more than those concentrations
tolerable by obligate anaerobes
Obilagate anarobe
Organisms that are strict anaerobe and require complete absence of oxygen for
their growth, which mean they cannot grow in the presence of oxygen.
Facultative anaerobe
Facultative anaerobes can grow either in presence or absence of oxygen. These
organisms tend to exist in environments in which oxygen concentrations are
uncertain.
Aerotolerant anaerobe
These are organisms that are able to grow in the presence of oxygen though they
do not require it for their growth.
Capnophiles
These are organisms whose optimum growth requires relatively high
concentrations of carbon dioxide
Effect of Osmotic pressure
The concentration of dissolved substances in the environment can impact on the
growth and survival of bacterial cells
Plasmolysis
Environments containing large concentrations of dissolved substances draw water
out of cells, causing shrinkage of the cytoplasm volume, a phenomenon termed
plasmolysis. Plasmolysis interferes with growth and this is why highly osmotic
environments prevent bacterial growth (e.g., brine, the high sugar concentrations
in jellies and jams, salting of meats)
Halophiles
Organisms that require high concentrations of dissolved salts to grow are termed
halophiles. Depending on organism, the salt concentrations required range from
those of seawater on up to those of brine
Based on gram staining
Gram staining uses two dyes and based on the type of dye an organism takes
It is classified as gram positive or gram negative. Gram-positive organisms get
blue stained while gram negative organism get red stained.
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Based on presence of Capsule:
Covering over the cell membrane present in certain bacteria. When capsule is
present the bacteria is called capsulated eg anthrax bacilli when capsule is absent
it is called uncapsulated.
1. Virulence factors, protecting bacteria from phagocytosis by immune
cells.
2. Permit bacteria to adhere to cell surfaces
3. Capsules can be a source of nutrients and energy to microbes.
4. Prevent cell from drying out (desiccation)
5. Polysaccharides from certain capsules can be the targets of protective
immune responses and have therefore been included in 'conjugated'
vaccines.
Based on ability to produce spore
Is the inactive form of bacteria formed under unfavourable condition to tide over
this condition. E.g. Clostridium tetani. Such organisms are called spore forming
while those that does not produce spore are called non-spore forming bacteria.
Spores are resistant to:
heat: withstand boiling for over one hour, desiccation: for a long period of time,
UV radiation, chemical disinfectants the resistance of these spores has serious
consequences and some very pathogenic bacteria have the ability to produce such
spores.
Shapes of bacteria:
Bacterial organisms are grouped according to their shape into the following:
Group name
Shape of bacteria
Appearance
Coccus (cocci)
Spherical / round
°°° ° °°••
Bacillus (bacilli)
Straight rod
−-_-==- -Spirillum (spirilla)
Curved rods
∼∼∼ιι,
Filamentous
Elongated thread like
⁄ √ ζ⎞ ⎞ ⎠ ⎠
Coccobacillus
Not perfectly round but ◘ ◘ ◘
have blunted
Vibrio
Comma shaped cells
‚’ ‘ “
Names in brackets mean plural term for each.
Groupings of cocci include:
Diplococci: pairs of cocci
Streptococci: chains of cocci.
Staphylococci: clusters of geometrically arranged cocci.
Tetrads: packets of 4 cells
Sarcinus: packet of 8 cocci
Groupings of bacilli include:
Diplobacilli : bacilli in pair
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Streptobacilli : bacilli in chain
Toxin
Are the substance released by certain bacteria during their multiplication and
growth, which produces adverse effect on the host animal and other bacteria.
Exoenzymes
These are enzymes employed to break down nutrients found outside of bacterial
cells (extracellularly) so that the breakdown products may be taken up into the
bacterial cell and used. Many of these enzymes are harmful and represent
exotoxins produced by disease-causing microorganisms, especially Gram-positive
bacteria
Endoenzymes
To some extent equivalent to the exoenzymes used by Gram-positive bacteria,
Gram-negative bacteria employ enzymes secreted into their periplasm to break
down large molecules before those molecules are brought across the plasma
membrane and into the cytoplasm of bacterium. This is in a way endotoxin that is
released after the death of bacterium.
Conditions affecting disease-producing ability of an organism:
Virulence of organism: when the organism is very virulent disease will be
produced very rapidly and when the organism is not virulent it will take long time
to produce disease in the host.
Resistance of host: resistance of the host can also influence the out come of an
infection. When the resistance of the host is good it will take longer time for the
infecting organism to produce disease on the contrary when the resistance of the
host is low or poor the organism produces disease rapidly.
Host barrier to infection:
Skin: covers the body and act as the barrier to infection.
Mucous membrane: internal organs are line by mucous membrane to protect from
infection.
Hairs in nostril: act as the filter and hold back dust and organism that are present
in the air.
Hair on animal body: act as the first barrier against the invasion by microbes and
parasites.
Tear: in the eyes tear is produced to wash away the microbes.
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