UNIT 4: Introduction to Bacteriology
Microbiology
Microbes: living organisms of microscopic size- bacteria, fungi, algae, protozoa and
viruses.
Microbes:
Prokaryotes
Eukaryotes
Prokaryotic Cell
Eukaryotic cell
Microbiology: It’s the study of their form, structure, reproduction, physiology, metabolism
and classifications.
Father of Microbiology: Antony van Leeuwenhoek.
Why to study Microbiology?
It’s closely associated with the health and welfare of the human beings. Some of them are
beneficial while others are detrimental.
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 animals.
Koch formulated a rule that came to be known as Koch’s rule, and that the disease if
caused by microbes must follow this sequence.
Koch’s rules:
1. Organism must be found in each case.
2. Be able to isolate organism from diseased individual
3. 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.
Introduction to Bacteriology
Bacteriology: Is the branch of biology that deals with the study of minute organisms called
bacteria (singular bacterium).
Bacteria: a single-celled, often parasitic microorganism without distinct nuclei or
organized cell structures. Various species are responsible for decay, fermentation, nitrogen
fixation, and many plant and animal diseases. They are:
1. Prokaryotes
2. Single-celled organisms
3. Size: microscopic.
E.g. E. coli is 1.3 um wide x 1.0 um long.
6250 E. coli to make 1 inch.
Exceptions: Epulopiscium is half a millimeter found in red sea.
Structural parts of Bacteria
Slime layer (capsule): Many bacteria secrete extracellular polymers outside of their cell
walls. These polymers are usually composed of
polysaccharides and sometimes protein. Capsules are relatively
impermeable structures that cannot be stained with dyes such
as India ink. They are generally involved in attachment of
bacteria to other cells or inanimate surfaces. Slime layers are
somewhat looser structures that help protect bacteria from
phagocytosis and desiccation.
Flagella
A-Monotrichous;
B-Lophotrichous;
C-Amphitrichous;
D-Peritrichous;
Flagella are whip-like structures protruding from the bacterial cell wall and are responsible
for bacterial motility (i.e. movement). The arrangement of flagella about the bacterial cell
is unique to the species observed. Common forms include:
Peritrichous - Multiple flagella found at several locations about the cell
Polar - Single flagella found at one or both cell poles
Lophotrichous - A tuft of flagella found at one cell pole
Flagella are complex structures that are composed of many different proteins.
Fimbrae and Pili
Fimbrae are protein tubes that extend out from the outer membrane in many members of
the Proteobacteria. They are generally short in length and present in high numbers about
the entire bacterial cell surface. Fimbrae usually function to facilitate the attachment of a
bacterium to a surface or to other cells.
Pili
Pili are similar in structure to fimbrae but are much longer and present on the bacterial cell
in low numbers. Pili are involved in the process of bacterial conjugation. Non-sex pili also
aid bacteria in gripping surfaces.
Classification of bacteriology
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.
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 (if there are two
fish in a lake, and one of them is dead, that’s called binary fishin’).
Classification of bacterial organism
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. Based on the
biochemical and physical factors influencing their growth, bacteria are classified as
follows:
Based on pH requirements
pH
The pH scale is a measure of hydrogen ion (H+) concentration. Low pH corresponds 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
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.
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
Obligate anaerobe
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
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
The bacterial cell wall
As in other organisms, the bacterial cell wall provides structural integrity to the cell. In
prokaryotes, the primary function of the cell wall is to protect the cell from internal turgor
pressure caused by the much higher concentrations of proteins and other molecules inside
the cell compared to its external environment. The bacterial cell wall differs from that of all
other organisms by the presence of peptidoglycan (poly-N-acetylglucosamine and Nacetylmuramic acid), which is located immediately outside of the cytoplasmic membrane.
Peptidoglycan is responsible for the rigidity of the bacterial cell wall and for the
determination of cell shape. It is relatively porous and is not considered to be a permiability
barrier for small substrates. While all bacterial cell walls (with a few exceptions e.g.
intracellular parasites such as Mycoplasma) contain peptidoglycan, not all cell walls have
the same overall structures. There are two main types of bacterial cell walls, Gram positive
and Gram negative, which are differentiated by their Gram staining characteristics.
The Gram positive cell wall
The Gram positive cell wall is characterized by the presence of a very thick peptidoglycan
layer, which is responsible for the retention of the crystal violet dyes during the Gram
staining procedure. It is found exclusively in organisms belonging to the Actinobacteria
and the Firmicutes. Imbedded in the Gram positive cell wall are polyalcohols called
teichoic acids, some of which are lipid-linked to form lipoteichoic acids. Because
lipoteichoic acids are covalently linked to lipids within the cytoplasmic membrane they are
responsible for linking the peptidoglycan to the cytoplasmic membrane.
The Gram negative cell wall
Unlike the Gram positive cell wall, the Gram negative cell wall contains a thin
peptidoglycan layer adjacent to the cytoplasmic membrane, which is responsible for the
cell wall's inability to retain the crystal violet stain upon decolorisation with ethanol during
Gram staining. In addition to the peptidoglycan layer, the Gram negative cell wall also
contains an additional outer membrane composed by phospholipids and
lipopolysaccharides which face into the external environment.
Gram staining uses two dyes and based on what 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.
Based on presence of Capsule
Capsule is a 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
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.
Based on the shape of the 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

Filamentous
Elongated thread like
     
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
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 diseasecausing microorganisms, especially Gram-positive bacteria
Endoenzymes
To some extent equivalent to the exoenzymes used by Gram-positive bacteria, Gramnegative 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.
Gram positive bacteria
Staphylococcus
Streptococcus
Bacillus
Clostridium
Corynebacterium
Lactobacillus
Listeria
Erysepelothrix
Mycobacterium
Actinomyces
Gram negative bacteria
E.coli
Salmonella
Shigella
Leptospira
Pseudomonas
Brucella
Proteus
Pasteurella
Actinobacillus
Vibrio
Characteristics of some bacteria
Bacterial species
Gram positive Bacteria
Staphylococcus
Streptococcus
Listeria
Erysepelothrix
Clostridium
Bacillus
Gram negative Bacteria
Enterobacteriaceae
Aerobic/
nonaerobic
sporing/
non
sporing
facultative
anaerobic
cocci
Facultative
anaerobic
cocci
Facultative
anaerobic
rods
Aerobic
rods
Anaerobic
Aerobic
rods
Nonsporing
Non motile
Nonsporing
Nonmotile
Non
sporing
Motile
temperature
< 20ºC
Non-motile
Aerobic/
facultative
anaerobes
large rods
Nonsporing
Non
sporing
Sporing
Sporing
Motile/ nonmotile
–
at
Non-motile
Motile except B.
anthracis
Motile/nonmotile
Order: Rickettsiales
Kingdom:
Bacteria
Phylum:
Proteobacteria
Class:
Alpha Proteobacteria
Order:
Rickettsiales
-The Rickettsiales, also called rickettsias, are an order of small proteobacteria.
-Most survive only as endosymbionts.
-The Rickettsiales - most mysterious groups of Proteobacteria, owing largely to
difficulties in cultivating them.
Family: Rickettsiaceae
Genera: Rickettsia
Orientia
Wolbachia
Rickettsia felis
Rickettsia prowazekii
Rickettsia rickettsii
Rickettsia typhi
Rickettsia - non-motile, Gram-negative, non-sporeforming, highly pleomorphic
bacteria
Can present as cocci (0.1 μm in diameter), rods (1-4 μm long) or thread-like
(10 μm long).
Obligate intracellular parasites
Rickettsia depends on entry, growth, and replication within the cytoplasm of
eukaryotic host cells (typically endothelial cells).
Because of this, Rickettsia cannot live in artificial nutrient environments and are
grown either in tissue or embryo cultures (typically, chicken embryos are used).
The majority of Rickettsia bacteria are susceptible to antibiotics of the tetracycline
group.
Rickettsia species are carried as parasites by many ticks, fleas, and lice, and cause
diseases such as typhus, rickettsialpox, Boutonneuse fever, Rocky Mountain spotted
fever, and endemic typhus in human beings.
Orientia is a genus of bacteria in family Rickettsiaceae.
Orientia tsutsugamushi
They are obligate intracellular gram negative bacteria found in insects and mammals.
They are spread through the bites or feces of infected insects.
Family: Ehrlichiaceae
Genera: Aegytianella
Anaplasma
Cowdria
Ehrlichia
Neorickettsia
Growth and Culturing of Bacteria
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 siz
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
Standard bacterial growth curve
Bacteria added to fresh media typically go through four more-or-less distinct phases
of growth
Lag phase (A)
Log (logarithmic or exponential) phase (B
Stationary phase (C)
Decline (death) phase (D)
[Standard bacterial growth curve, bacterial growths curve (Google Search)]
Lag phase
(a) Transfers of bacteria from one medium to another, where there exist chemical
differences between the two media, typically results in a lag in cell division
(b) This lag in division is associated with a physiological adaptation to the new
environment, by the cells, prior to their resumption of division
(c) That is, cells may increase in size during this time, but simply do not undergo
binary fission
Log phase (logarithmic phase, exponential phase)
(a)
Lag phase is followed by log phase during which binary fission occurs
(b)
This phase of growth is called logarithmic or exponential because the rate
of increase in cell number is a multiplicative function of cell number
(c)
This can be seen in a graph of cell number versus time where cell numbers
increase at ever increasing rates with time or generation; that is, the rate of increase is
a function of absolute cell number such that the more cells present, the faster the
population of cells increases in size (at least, during log phase)
(e)
When graphed on semi-log graph paper (Figure 6.3, i.e., log cell number
versus time), log-phase growth produces a straight line
A means of keeping cultures in log phase can be accomplished either by employing a
chemostat or via serial transfer.
A chemostat involves adding fresh medium to a culture, mixing, and then allowing an
equal volume of culture to drain from the vessel; this is typically done continuously
(i.e., a steady stream of fresh medium is added)
Serial transfer means taking a volume of culture and diluting that volume into fresh
media
Generation time
(a) Generation time it takes a bacterial population to double in size (number) during
log-phase growth
(b) Note that the time it takes for the population to double in size does not change with
cell number (so long as cells remain in log phase)
(c) That is, with exponential growth, the absolute increase in cell number increases as
cell number increases while the relative increase remains invariant
(d) Typically, generation times range from 20 minutes to 20 hours depending on the
bacterial species/strain and the conditions during which log-phase growth is occurring
Stationary phase
(a) Stationary phase is a steady-state equilibrium where the rate of cell growth
(division) is exactly balanced by the rate of cell death (i.e., increase in cell number
due to cell divisions exactly balanced by a decrease in cell number due to death)
(b) Cell death (or, at least, lack of cell growth) occurs because of a loss of limiting
nutrients (due to their incorporation into cells during log-phase growth) or a build-up
of toxins (due to their release during log-phase growth, e.g., fermentative products)
(c) Note that the simplest conditions that will result in a stationary phase is when both
the rate of cell increase and the rate of cell death together equal zero (i.e., cells neither
die nor are born)
Decline phase (death phase)
(a) Stationary phase, in a standard bacterial growth curve, is followed by a die-off of
cells
(b) Cell death in bacteria cultures basically means that the cells are unable to resume
division following their transfer to new environments
(c) Typically this die-off occurs exponentially, i.e., such that cell number graphed
against time, using a semi-log scale for cell number, results in a straight line (i.e., see
Figure 6.3)
(d). This death occurs because vegetative cells can survive exposure to harsh
conditions (few nutrients or too-many toxins) for only so long.
Media for bacterial culture/growth
For any bacterium to be propagated for any purpose it is necessary to provide the
appropriate biochemical and biophysical environment. The biochemical (nutritional)
environment is made available as a culture medium, and depending upon the special
needs of particular bacteria. A large variety and types of culture media have been
developed with different purposes and uses. Culture media are employed in the
isolation and maintenance of pure cultures of bacteria and are also used for
identification of bacteria according to their biochemical and physiological properties.
The manners in which bacteria are cultivated, and the purpose of culture media, vary
widely. Liquid media are used for growth of pure batch cultures while solidified
media are used widely for the isolation of pure cultures, for estimating viable bacterial
populations, and a variety of other purposes. The usual gelling agent for solid or
semisolid medium is agar, a hydrocolloid derived from red algae. Agar is used
because of its unique physical properties (it melts at 100 degrees and remains liquid
until cooled to 40 degrees, the temperature at which it gels) and because it cannot be
metabolized by most bacteria. Hence as a medium component it is relatively inert; it
simply holds (gels) nutrients that are in aquaeous solution.
Solid medium
Solid media contains agar, which is a compound that goes into water solution at
temperatures approaching boiling, and then, once in solution, solidifies the medium at
room (<40ºC) temperature. E.g., Mac Conkey’s agar.
Liquid medium
The liquid media used for microorganisms is called as are nutrient broths.
Selective media: selective media are used for the growth of only select
microorganisms. For example, if a microorganism is resistant to a certain antibiotic,
such as ampicillin or tetracycline, then that antibiotic can be added to the medium in
order to prevent other cells, which do not possess the resistance, from growing.
Some examples of selective media include:



eosin-methylen blue agar (EMB) that contains methylene blue – toxic to
Gram-positive bacteria, allowing only the growth of Gram negative bacteria
YM (yeast and mold) which has a low pH, deterring bacterial growth
MacConkey agar for Gram-negative bacteria
Differential media: Differential media or indicator media distinguish one
microorganism type from another growing on the same media. This type of media
uses the biochemical characteristics of a microorganism growing in the presence of
specific nutrients or indicators (such as neutral red, phenol red, eosin , or methylene
blue) added to the medium to visibly indicate the defining characteristics of a
microorganism. This type of media is used for the detection of microorganisms and by
molecular biologists to detect recombinant strains of bacteria.
Examples of differential media include:

Eosin methylene blue (EMB), which is differential for lactose and sucrose
fermentation


MacConkey (MCK), which is differential for lactose fermentation
Mannitol Salt Agar (MSA), which is differential for mannitol fermentation
Sterilization and Disinfection
The destruction of microorganisms may be accomplished by physical and chemical
means. The use of physical means to destroy microorganisms is known as sterilization
while the use of chemicals is known as disinfection. However, this is not a set rule, for
the ultimate action which kills the organism may be the same when either a physical
or a chemical method is employed.
An instrument is considered sterilized when it is free of living micro organisms.
A. Sterilization
Sterilization is most commonly done through heat.
1. Moist heat
a. Autoclave: Moist heat in the form of steam under regulated pressure is
used for sterilization. The equipment used for this purpose in the
laboratory is called as autoclave. Both pressure and temperature are
applied for a definite period of time to kill the micro organisms. In
autoclave, most of the materials are sterilized under 15 lb pressure at
121°C temperature.
Cotton, wools, various culture media, solution are sterilized with this
instrument.
b. Tyndallization: Tyndallization is a process of moist heat application
some materials like amino acids, sugar solutions, etc . are required to
be heated at 90-100°C temperature on three successive days with
interval in between. During this interval, resistant spore germinate and
on subsequent exposure to heat, the vegetative stage get destroyed.
This method of successive at successive interval is called as fractional
sterilization or tyndallization.
c. Boiling: most of the vegetative forms of the microorganisms can be
destroyed through boiling, but spores cannot be destroyed with this
method. Surgical instruments are sterilized by this method.
d. Pastuerization: it is the process by which food and food products are
protected from putrifaction and fermentation. . it involves a brief
exposure to heat at a lower temperature than that employed in normal
sterilization. The temperature of pasteurization is selected based on the
thermal death time of most resistant type of microorganisms to be
destroyed by this process.
Milk is generally pasteurized at 72 °C for 27 minutes or 63°C for 30
minutes, followed by rapid cooling.
2. Dry heat: it is done by the following methods.
a. Red hot: inoculating wire, forceps, spatula, etc are sterilized by heating
them red hot.
b. Flamming: Scalpel, needle, cultural tube, cotton wool plug, glass slide,
etc are sterilized by heating without allowing it to become red hot.
c. Hot air oven: heating is done by electricity above chosen temperature
which is maintained by thermostat. Here sterilization is done at 160°C
for 1 hour.
Materials sterilized by this method are glass wares, surgical
instruments, glass syringe, etc.



Precautions:
All glass wares must be wrapped with a craft paper.
All articles must be free from moisture.
The whole system must be allowed to cool.
B. Disinfection
Disinfectants: Disinfectants are agents which are too toxic to be applied to the
tissues of the host but which can be used in destroying contaminated inanimate
objects. E.g., drains, fecal matter, building, vehicles, cooking materials, surgical
instruments, etc.
Disinfection is the process of freeing objects from harmful germs. Disinfectants
are to be used depending upon the nature of the organism. Some organisms are
killed in acid media and therefore acids have to be used. But most of the
organisms especially the virus like rabies, FMD virus are killed by alkali. Alkali
containing pH 9 has antiviral properties
4 % washing soda is very effective against FMD virus. Soda lime is used for the
disposal of carcasses. Poultry sheds can be disinfected through fumigation (250 g
KMnO4 in 500 ml of formalin).
C. Antiseptics
It is an agent that prevents sepsis i.e. prevents the growth of infective agents like
bacteria, virus, protozoa, etc. The substances are substantially nontoxic for
superficial application to living tissues. Therefore, these agents can be applied
externally on animals to kill or prevent the growth of microbial population. The
antiseptics belong to a variety of chemical substances e.g. Alcohol, phenol, dyes,
etc. They are used in the intact skin before surgical operation and injection. They
are applied to broken skin following wounds, burns, etc.
They are grouped as
i.
Oxidizing agents: H2O2, KmnO4, chlorine, iodine, etc.
ii.
Reducing agents: Sulphur dioxide, formaldehyde.
iii. Metallic compounds: mercuric chloride and iodide.
iv.
Acids and alkalis: boric acids,soda, etc
v.
Alcohol: 70 % ethyl alcohol.
vi.
Phenol and cresol
vii.
Dyes: Acriflavin, brilliant green, proflavine.
viii. Detergents: various soaps
ix. Cetrimides
x. Quaternary ammonium compounds.
xi. Antimicrobial agents: antibiotics and chemotherapeutics