NAME: DR. RUTH AFUNWA
DEPARTMENT: MICROBIOLOGY
FACULTY: NATURAL AND APPLIED SCIENCES
SESSION: 2023/2024
SEMESTER: FIRST
COURSE: MCB 201 GENERAL MICROBIOLOGY I
CREDIT UNIT: 2
COURSE OUTLINE:

History of the science of Microbiology

Sterilization and Disinfection

Pure and Applied aspects of Microbiology

General characteristics of microorganisms including
growth, reproduction and microbial techniques
Microbiology is the study of small living things. Generally this means living things that are too
small to see without the use of a microscope. These life forms are called microorganisms or
microbes. Microorganisms include bacteria, archaea (a type of prokaryote a bit like bacteria but
they have a distinct evolutionary origin), viruses, protozoa (single-cell eukaryotes like amoeba),
microscopic fungi and yeasts, and microscopic algae (plant-like organisms). Microorganisms
were discovered over three hundred years ago and it is thought that many new microbes have yet
to be discovered. Microbiology is a wide area of science that includes bacteriology, virology,
mycology, phycology, parasitology, and other branches of biology.
Most living things can be classified into prokaryotes or eukaryotes depending on whether their
nuclear material (for example DNA) is surrounded by a membrane or not. Archaea and bacteria
are prokaryotes. Most animals (including humans) and plants are eukaryotes. Protozoa, fungi,
yeasts, and algae are eukaryotes. Viruses are a little different. Traditional classification systems
do not classify viruses as living organisms. However in practise they are considered
microorganisms. The study of viruses is called virology.
Microbiology therefore includes the study of both prokaryotic and eukaryotic microorganisms. In
practise the majority of microbiology is concerned with bacteria and/or viruses although
eukaryotic microbiology is also a very important branch of microbiology. Many diseases of
animals (including humans) and plants are caused by bacteria, viruses, amoeba, and fungi.
Bacteria are important in probiotics, they are used in food production (e.g. yoghurt and cheese)
and biotechnology. Yeasts and fungi are important in food and drink production (e.g. wine, beer,
bread) and are also used to produce important pharmaceuticals (e.g. antibiotics).
DISINFECTION
Disinfection is the process, which involves the elimination of most pathogenic microorganisms
(excluding bacterial spores) on inanimate objects. •Chemicals used in disinfection are called
disinfectants. Different disinfectants have different target ranges, not all disinfectants can kill all
microorganisms.
Importance of disinfection
The method of disinfection is used internationally for the safety of humans, to decrease the scale
of transmission of diseases. A large emphasis of sterilisation and disinfection has been placed in
the food industry, water sanitisation and medical care and hospitals. These sectors have been
found to be largely affected with microorganisms in varying modes of transmission amongst the
population. Different disinfectants are used in different industries, which target the specific flora.
Disinfection techniques are classified according to
Consistency
• Liquid (Alcohols, Phenols)
• Gaseous (Formaldehyde vapor, Ethylene oxide)
Spectrum of activity
• High level
• Intermediate level
• Low level
Mechanism of action
• Action on membrane (Alcohol, detergent)
• Denaturation of cellular proteins (Alcohol, Phenol)
• Oxidation of essential sulfhydryl groups of enzymes (H2 O2, Halogens)
• Alkylation of amino-, carboxyl- and hydroxyl group (Ethylene Oxide, Formaldehyde)
• Damage to nucleic acids (Ethylene Oxide, Formaldehyde)
Chemical disinfectants
o
Alcohol
o
o
o
o
o
o
o
o
o
o
Chlorine and chlorine compounds
Formaldehyde
Glutaraldehyde
Hydrogen peroxide
Iodophors
Ortho-phthalaldehyde (OPA)
Peracetic acid
Peracetic acid and hydrogen peroxide
Phenolics
Quaternary ammonium compounds
Common uses of disinfectants:
Aldehydes: surface disinfection, fumigation of rooms, chambers and operating theatres.
Alcohol: 70% aqueous alcohol is more effective at microbial killing. 70% Ethyl alcohol is used
as antiseptic on skin.
Phenol: Is used in high concentrations as a disinfectant and in low concentrations as an
antiseptic.
Halogens: Iodine (antiseptic), Chlorine (bleach)
STERILIZATION
Sterilization is a process in which all the living microorganisms, including bacterial spores are
killed. It can be achieved by physical and chemical methods.
Sterilization is employed to minimize the growth of organisms and transmission of disease from
one individual to another. In the environment the use of disinfection techniques decreases the
growth of bacteria on surfaces, which leads to the decrease in transmission of organisms amongst
the population. These techniques are commonly used today in medical care and food industry.
Physical methods
• Heat (Dry and moist)
• Sunlight
• Vibration
• Radiation
• Filtration
Heat is considered to be most reliable method of sterilization of objects that can withstand heat.
Heat as Moist and Dry heat are the most common sterilizing methods used in hospitals and are
indicated for most materials.
Dry Heat: Causes denaturation of proteins and oxidative damage.
Techniques include:
• Red Heat (common uses: straight wires, bacterial loops and spatulas)
• Flaming (Common uses: bacterial loops, wires and spatula’s)
• Incernation (common uses: soil dressing, pathological bedding)
• Hot Air oven (discovered by Louis Pasteur, common uses: in dairy industry)
• Infra red rays (common uses: heat glassware and metallic instruments)
Moist Heat:
Moist heat is more efficient in contrast to dry heat; it causes coagulation and denaturation of
proteins.
At temperature below 100°C:
• Pasteurisation: Food (dairy) Industry
• Vaccine bath: (vaccine sterilisation)
• Serum bath: (serum contaminants, does not kill spores survive)
• Inspissation: (egg and serum containing media, can kill spores)
At temperature 100°C:
• Boiling: Boiling water (100°C)
• Steam (100°C)
At temperature above 100°C:
• Autoclave: works at 121oC at 15 psi
Radiation
There are 2 types of Radiation:
Non-ionizing: wavelength longer then visible light.
• UV Radiation has a wavelength of 200-280nm; it has a germicidal effect on microorganisms.
• Common uses: Surface disinfection, in hospitals, operating theatre and laboratories.
Ionising: 2 types:
• Particulate (Electron beam)
• Common uses: sterilization of instruments such as syringes, gloves, dressing packs, foods and
pharmaceuticals.
• Electromagnetic (Gamma rays)
• Common uses: sterilization of disposable Petri dishes, plastic syringes, antibiotics, vitamins,
hormones and fabrics.
FIELDS AND SCOPE OF MICROBIOLOGY
Microbiology has two major fields
1. Pure Microbiology
2. Applied Microbiology
Pure Microbiology: on the basis of Taxonimical classification
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Bacteriology
Mycology
Virology
Protozoology
Immunology
On the basis of integrative characteristics
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Microbial cytology
Microbial physiology
Microbial genetics
Microbial ecology
Microbial taxonomy
Cellular Microbiology
Molecular Microbiology
2. Applied Microbiology: on the basis of application

Medical Microbiology

Veterinary Microbiology

Public Health Microbiology

Industrial Microbiology

Pharmaceutical Microbiology

Agriculture Microbiology

Plant Microbiology

Soil Microbiology

Food and Dairy Microbiology

Environmental Microbiology
BACTERIAL GROWTH
Bacterial growth is the asexual reproduction, or cell division, of a bacterium into two daughter
cells, in a process called binary fission. The resulting daughter cells are genetically identical to
the original cell. Hence, bacterial growth occurs. The measurement of an exponential bacterial
growth curve in batch culture requires bacterial enumeration (cell counting) by different methods
In the laboratory, under favorable conditions, a growing bacterial population doubles at regular
intervals. Growth is by geometric progression: 1, 2, 4, 8, etc. or 20, 21, 22, 23.........2n (where n =
the number of generations). This is called exponential growth. In reality, exponential growth is
only part of the bacterial life cycle, and not representative of the normal pattern of growth of
bacteria in Nature.
When a fresh medium is inoculated with a given number of cells, and the population growth is
monitored over a period of time, plotting the data will yield a typical bacterial growth curve
Bacterial growth curve
Bacterial growth in culture occurs in four different phases:
Lag phase (A),
Log phase or exponential phase (B),
Stationary phase (C), and
Death phase (D)
During lag phase, bacteria adapt themselves to growth conditions. It is the period where the
individual bacteria are maturing and not yet able to divide. During the lag phase of the bacterial
growth cycle, synthesis of RNA, enzymes and other molecules occurs. During the lag phase cells
change very little because the cells do not immediately reproduce in a new medium. This period
of little to no cell division is called the lag phase and can last for 1 hour to several days. During
this phase cells are not dormant.
The log phase (sometimes called the logarithmic phase or the exponential phase) is a period
characterized by cell doubling. The number of new bacteria appearing per unit time is
proportional to the present population. If growth is not limited, doubling will continue at a
constant rate so both the number of cells and the rate of population increase doubles with each
consecutive time period. For this type of exponential growth, plotting the natural logarithm of
cell number against time produces a straight line. The slope of this line is the specific growth rate
of the organism, which is a measure of the number of divisions per cell per unit time. The actual
rate of this growth (i.e. the slope of the line in the figure) depends upon the growth conditions,
which affect the frequency of cell division events and the probability of both daughter cells
surviving. Under controlled conditions, Exponential growth cannot continue indefinitely,
because the medium is soon depleted of nutrients and enriched with wastes.
The stationary phase is often due to a growth-limiting factor such as the depletion of an
essential nutrient, and/or the formation of an inhibitory product such as an organic acid.
Stationary phase results from a situation in which growth rate and death rate are equal. The
number of new cells created is limited by the growth factor and as a result the rate of cell growth
matches the rate of cell death. The result is a “smooth,” horizontal linear part of the curve during
the stationary phase. Mutations can occur during stationary phase.
At death phase (decline phase), bacteria die. This could be caused by lack of nutrients,
environmental temperature above or below the tolerance band for the species, or other injurious
conditions.
This basic batch culture growth model draws out and emphasizes aspects of bacterial growth
which may differ from the growth of macrofauna. It emphasizes clonality, asexual binary
division, the short development time relative to replication itself, the seemingly low death rate,
the need to move from a dormant state to a reproductive state or to condition the media, and
finally, the tendency of lab adapted strains to exhaust their nutrients. In reality, even in batch
culture, the four phases are not well defined. Near the end of the logarithmic phase of a batch
culture, competence for natural genetic transformation may be induced, as in Bacillus
subtilis and in other bacteria. Natural genetic transformation is a form of DNA transfer that
appears to be an adaptation for repairing DNA damages.
Batch culture is the most common laboratory growth method in which bacterial growth is
studied, but it is only one of many. It is ideally spatially unstructured and temporally structured.
The bacterial culture is incubated in a closed vessel with a single batch of medium. In some
experimental regimes, some of the bacterial culture is periodically removed and added to fresh
sterile medium. In the extreme case, this leads to the continual renewal of the nutrients. This is
a chemostat, also known as continuous culture. It is ideally spatially unstructured and temporally
unstructured, in a steady state defined by the rates of nutrient supply and bacterial growth. In
comparison to batch culture, bacteria are maintained in exponential growth phase, and the
growth rate of the bacteria is known.
Bacterial growth can be suppressed with bacteriostats, without necessarily killing the bacteria. In
a synecological, that is, true-to-nature situation in which more than one bacterial species is
present, the growth of microbes is more dynamic and continual.
Environmental factors influence rate of bacterial growth such as acidity (pH), temperature, water
activity, macro and micro nutrients, oxygen levels, and toxins. Conditions tend to be relatively
consistent between bacteria with the exception of extremophiles. Bacteria have optimal growth
conditions under which they thrive, but once outside of those conditions the stress can result in
either reduced or stalled growth, dormancy (such as formation spores), or death. Maintaining
sub-optimal growth conditions is a key principle to food preservation.
Temperature]
Low temperatures tend to reduce growth rate. Depending on temperature, bacteria can be
classified as:
Psychrophiles
Psychrophiles are extremophilic cold-loving bacteria or archaea with an optimal temperature for
growth at about 15 °C or lower (maximal temperature for growth at 20 °C, minimal temperature
for growth at 0 °C or lower). Psychrophiles are typically found in Earth's extremely cold
ecosystems, such as polar ice-cap regions, permafrost, polar surface, and deep oceans.
Mesophiles
Mesophiles are bacteria that thrive at moderate temperatures, growing best between 20° and
45 °C. These temperatures align with the natural body temperatures of humans, which is why
many human pathogens are mesophiles.
Thermophiles
Survive under temperatures of 45° - 60 °C
Acidity
Optimal acidity for bacteria tends to be around pH 6.5 to 7.0 with the exception of acidophiles.
Some bacteria can change the pH such as by excreting acid resulting in sub-optimal conditions.
Other factors include
Water activity
Oxygen
Micronutrients
Toxins
Toxins such as ethanol can hinder or kill bacterial growth. This is used beneficially
for disinfection and in food preservation.
Growth Rate and Generation Time
Bacterial growth rates during the phase of exponential growth, under standard nutritional
conditions (culture medium, temperature, pH, etc.), define the bacterium's generation time.
Generation times for bacteria vary from about 12 minutes to 24 hours or more. The generation
time for E. coli in the laboratory is 15-20 minutes, but in the intestinal tract, the coliform's
generation time is estimated to be 12-24 hours. For most known bacteria that can be cultured,
generation times range from about 15 minutes to 1 hour. Symbionts such as Rhizobium tend to
have longer generation times. Many lithotrophs, such as the nitrifying bacteria, also have long
generation times. Some bacteria that are pathogens, such as Mycobacterium
tuberculosis and Treponema pallidum, have especially long generation times, and this is thought
to be an advantage in their virulence.
Table 2. Generation times for some common bacteria under optimal conditions of growth.
Bacterium
Medium
Generation Time (minutes)
Escherichia coli
Glucose-salts
17
Bacillus megaterium
Sucrose-salts
25
Streptococcus lactis
Milk
26
Streptococcus lactis
Lactose broth
48
Staphylococcus aureus
Heart infusion broth
27-30
Lactobacillus acidophilus
Milk
66-87
Rhizobium japonicum
Mannitol-salts-yeast extract
344-461
Mycobacterium tuberculosis
Synthetic
792-932
Treponema pallidum
Rabbit testes
1980
Calculation of Generation Time
When growing exponentially by binary fission, the increase in a bacterial population is by
geometric progression. If we start with one cell, when it divides, there are 2 cells in the first
generation, 4 cells in the second generation, 8 cells in the third generation, and so on.
The generation time is the time interval required for the cells (or population) to divide.
G (generation time) = (time, in minutes or hours)/n(number of generations)
G = t/n
t = time interval in hours or minutes
B = number of bacteria at the beginning of a time interval
b = number of bacteria at the end of the time interval
n = number of generations (number of times the cell population doubles during the time interval)
b = B x 2n (This equation is an expression of growth by binary fission)
Solve for n:
logb = logB + nlog2
n = logb - logB
log2
n = logb - logB
.301
n = 3.3 logb/B
G = t/n
Solve for G
G=
t
3.3 log b/B
Example: What is the generation time of a bacterial population that increases from 10,000
cells to 10,000,000 cells in four hours of growth?
G=
t_____
3.3 log b/B
G = 240 minutes
3.3 log 107/104
G = 240 minutes
3.3 x 3
G = 24 minutes