Download Essential Medical Microbiology

Physiology of Bacteria
‫م رنامشعل‬.‫م‬
Bacterial chemical components

Water: free water and bound water.

Inorganic salt: phosphors, potassium ,magnesium, calcium, sodium, etc.

Protein: 50%-80% of dry weight according bacterial kinds and age.

Sugar: mainly distributing in cell wall and capsule.

Lipids: composed of lipid, fatty acid, wax, etc.

Nucleic acid: RNA and DNA.
Environmental factors affecting growth of bacteria
1. Nutrients
2. Temperature
3. hydrogen ion concentration ( pH )
4. Oxygen Requirements
5. Osmotic pressure
Nutrient requirements of bacterial growth:
1.Water 2. Carbon source 3. Nitrogen source 4. Minerals
5. Growth factors：that a cell must have for growth but cannot synthesize itself Such as some
amino acids, purines, pyrimidines, and vitamins.
Bacteria that require special growth factors are termed Fastidious bacteria. Haemophilus
influenza requires X and v factors .
Temperature:
Psychrophilic forms (15-20℃)
Mesophilic forms （30-37℃）include all human pathogens and opportunists.
Thermophilic forms （50-60℃）
pH
Neutrophilic （ 5 to 8）
Acidophilic （below 5.5）
Alkaliphilic （above 8.5）
O2 Requirements
Bacteria on the basis of their oxygen requirements can be classified into aerobic and
anaerobic bacteria.
Aerobic bacteria: They require oxygen for their growth. They may be:
■ Obligate aerobes—which can grow only in the presence of oxygen (e.g., P.
aeruginosa).
■ Facultative aerobes—which are ordinary aerobes but can also grow without oxygen
(e.g., E. coli). Most of the pathogenic bacteria are facultative aerobes.
■ Microaerophilic bacteria—those bacteria that can grow in the presence of low
oxygen and in the presence of low (4%) concentration of oxygen.
Anaerobic bacteria: Obligate anaerobes are the bacteria that can grow only in
the absence of oxygen (e.g., Clostridium tetani,). oxygen is lethal to these
organisms.
Bacterial Growth
Bacterial growth can be defined as an orderly increase of all the chemical components of
the cell. Cell multiplication is a consequence of growth that leads to an increase in the
number of bacteria. Most bacteria divide by binary fission in which the bacteria undergo
cell division to produce two daughter cells identical to the parent cell. .one bacterium
divides into two, these two produce four, and then eight, and so on..
Generation Time
Generation time is the time required for a bacterium to give rise to two daughter cells under
optimum conditions. The generation time for most of the pathogenic bacteria, such as E.
coli, is about 20 minutes. The generation time is longer (i.e.,
20 hours) for M. tuberculosis. A bacterium replicates and multiplies rapidly producing
millions of cells within 24 hours. For example, E. coli in about7 hours can undergo 20
generations and produce 1 million cells, However, in actual practice, the multiplication of
bacteria is arrested after a few cell divisions due to exhaustion of nutrients and
accumulation of toxic products

Bacterial Growth Curve
. The bacterial growth curve shows the following four distinct phases;
1. Lag phase. In this phase, the inoculated bacteria become adapted to the environment, switch on various
enzymes, and adjust to the environmental temperature and atmospheric conditions. During this phase,
there is an increase in size of bacteria but no appreciable increase in number of bacterial cells. The
cells are active metabolically..
2. Log phase: This phase is characterized by rapid exponential cell growth (i.e., 1 to 2 to 4 to 8 and so on).
They multiply at their maximum rate. The bacterial cells are small and uniformly stained. The
microbes are sensitive to adverse conditions, such as antibiotics and other antimicrobial agents.
3. Stationary phase: After log phase, the bacterial growth almost stops completely due to lack of
essential nutrients, lack of water oxygen, change in pH of the medium, etc. and accumulation of their
own toxic metabolic wastes. Death rate of bacteria exceeds the rate of replication of bacteria.
Endospores start forming during this stage,
4. Decline phase: During this phase, the bacterial population declines due to death of cells. The decline
phase starts due to (a) accumulation of toxic products and autolytic enzymes and (b) exhaustion of
nutrients. The continuous culture is a method of culture useful for industrial and research purpose.
This is achieved by using a special device for replenishing nutrients and removing bacterial population
continuously so that bacteria growth is not inhibited due to lack of nutrients or due to accumulation of
toxic bacterial metabolites .

the growth curve
Pathogenesis of Bacterial Infection
The pathogenesis of bacterial infection includes initiation of the infectious process and the mechanisms
that lead to the development of signs and symptoms of disease. Characteristics of bacteria that are
pathogens include transmissibility, adherence to host cells, invasion of host cells and tissues,
toxigenicity, and ability to evade the host's immune system. Disease occurs if the bacteria or
immunologic reactions to their presence cause sufficient harm to the person.
Transmission of Infection

Some bacteria that commonly cause disease in humans exist primarily in animals and incidentally
infect humans. For example, Salmonella species typically infect animals and are transmitted in food
products to humans. Other bacteria produce infection of humans that is inadvertent, a mistake in the
normal life cycle of the organism; the organisms have not adapted to humans, and the disease they
produce may be severe. For example, Yersinia pestis (plague) has a well-established life cycle in
rodents and rodent fleas, and transmission by the fleas to humans is inadvertent;

The clinical manifestations of diseases (eg, diarrhea, cough, genital discharge) produced by
microorganisms often promote transmission of the agents. Examples : Vibrio cholerae can cause
voluminous diarrhea which may contaminate salt and fresh water; drinking water or seafood such
as crabs may be contaminated; ingestion of contaminated water or seafood can produce infection
and disease. Mycobacterium tuberculosis (tuberculosis) naturally infects only humans; it produces
respiratory disease with cough , resulting in transmission of the bacteria from one person to
another.

Many bacteria are transmitted from one person to another on hands. Many opportunistic
pathogens that cause nosocomial infections are transmitted from one patient to another on the
hands of hospital personnel. Hand washing is thus an important component of infection control.

The most frequent portals of entry of pathogenic bacteria into the body are the sites where
mucous membranes meet with the skin: respiratory (upper and lower airways), gastrointestinal
(primarily mouth), genital, and urinary tracts. Abnormal areas of mucous membranes and skin (eg,
cuts, burns, and other injuries) are also frequent sites of entry. Normal skin and mucous
membranes provide the primary defense against infection. To cause disease, pathogens must
overcome these barriers