all anti buotics word formet

7/27/2019 All Anti Buotics Word Formet

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GENERAL CONSIDERATIONS:

The causative agents of human infections diseases belong to 5 major groups of

organisms:

1. Bacteria2. Fungi3. Protozoa4. Helminths5. Viruses

The first 3 groups are members of kingdom of PROTISTS, one of the primary biologic

sub-divisions along with plants and animals. The protists are distinguished from plants andanimals by being either unicellular or relatively simple multicellular. Helminths are complex

multicellular which are classified as metazoa within animal kingdom. Helminths and protozoa

are commonly called parasites. Viruses are not cells but can replicate only within cells. Cellular

organisms differ from viruses as described below.

1) Structure:-

Cells possess nucleus or nucleoid containing DNA which is surrounded by

cytoplasm. Within cytoplasm proteins are synthesized and energy is produced. Viruses

have an inner core of genetic material (either DNA or RNA), surrounded by protein coat

called capsid, which is made up of sub units called capsomers but no cytoplasm. Hence

they (viruses) depend on host cells for protein synthesis and energy generation.

2) Method of Replication:-

Cells replicate by binary fission in which one parent cell divides into 2 progeny

cells retaining its cellular structure. In contrast, viruses disassemble to produce many copies of

their nucleic acid and protein and then reassemble into multiple progeny viruses. Viruses must

replicate in cells.

3) Nature of the Nucleic Acid:-

Cells contain both DNA and RNA whereas viruses contain either DNA or RNA

but not both.


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EUKARYOTES AND PROKARYOTES:

Cells are evolved into 2 fundamentally different types, eukaryotic and

prokaryotic. Fungi and protozoa are eukaryotic whereas bacteria are prokaryotic.

1- The eukaryotic cell has a true nucleus with multiple chromosomessurrounded by a nuclear membrane and uses mitotic apparatus to ensure equal

allocation of chromosomes to progeny cells. The nucleoid of prokarytoic cell

consists of a single circularmolecule of loosely organized DNA lacking a nuclear

membrane and mitotic apparatus.

2. Eukaryotic cells contain organelles, like mitochondria and lysosomes

and larger (80 S) ribosomes having 60S and 40S subunits whereas prokarytoes

contain no organelles and smaller (70S) ribosomes with 50S and 30S subunits.

Most prokaryotes contain a rigid external cell wall that contains peptidoglycan, a

polymer of amino acids and sugars as its unique structural unit. On the other handeukaryotes do not contain peptidoglycan.

3. Eukaryotic cell membrane contains sterols whereas no prokaryote

(except the wall-less mycoplasma) has sterols in its membranes.

STRUCTURE OF BACTERIAL CELL:

Bacteria are probably the simplest and the smallest living organisms

possessing cellular organization. Depending upon shape they are classified into 3 basic groups.

1. Cocci (single coccus) spherical in form e.g. shapes cluster

staphylococci or streptococci (chain) pair.

2. Bacilli (single bacillus) rod shaped (a) with square ends, (b) with

rounded ends, (c) club shaped, (d) fusiform, (e) coma shaped.

3. Spirochetes or spirilla (spiral, cork or screw shaped)

(a) relaxed coiled (b) tightly coiled. Some bacteria are variable in shape in

shape and they are called pleomorphic (many shaped). In addition to

shapes, the arrangement of bacteria is also important. For example certain

cocci occur in pairs (Diplococci) some in chains (streptococci) and otherin grapelike bunches (staphylococci)

SIZE:-

Bacteria range in size from about 0.2 to 5 m. The smallest bacteria are about the

same size as largest viruses poxviruses. The longest bacterial rod approaches the

size of some yeast and human red cell i.e. 7 m.


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STRUCTURE

CELL WAL:-

The function of the cell wall is to support under lying plasma membrane which is

subject to an internal osmotic pressure of about 5 atmospheres in G-negative and about 20atmospheres in G-positive bacteria. The cell wall is multilayered structure located external to

cytoplasmic membrane. Plasma membrane and cell wall together comprise envelope. It is

composed of an inner layer of peptidoglycan, surrounded by an outer layer (outer membrane)

that varies in thickness and chemical composition. The peptidoglycan provides structural shape

and maintains characteristic shape of cell. The cell wall of Gram Positive bacteria differs from

G-Negative bacteria as under.

The peptidoglycan layer is much thicker in Gram positive (50-100

molecules thick) than in Gram negative bacteria (1-2 molecules thick).

Some Gram positive bacteria have a layer of teichoic acid on the outside

of the peptidoglycan. Whereas G-negative bacteria do not have this layer. In

contrast, the G-negative bacteria have a complex outer layer (out membrane)

consisting of lipopolysaccharide, lipoprotein and phospholipid.

There is no outer Membrane present in G-positive bacteria. The out

membrane in gram negative bacteria contains porins (pore molecules) through

which hydrophilic drugs pass more readily.

Lying between the outer membrane layer and the cytoplasmic membrane

in gram negative bacteria is the periplasmic space. This is the site, in somebacteria, of enzymes that degrade penicillins and other-lactam drugs. The cell

wall has other properties as well. e.g.

In gram negative bacteria:-

i. The cell wall conatins endotoxins, alipopolysacharide ii. Its

polysaccharides and proteins are antigens, that are useful in lab.

Identification and

ii- Its porin proteins play a role in regulating the passage of molecules into

the cell.

CELL WALL of ACID FAST BACTERIA:

Mycobacteria have unusual cell wall resulting in their inability to be

Gram-stained. These are called Acid-Fast since they resist decolonization with

acid alcohol after being stained with carbofuchsin. The property is related to high

concentration in the cell wall of lipids called mycolic acids.


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GROWTH OF BACTERIA

Growth Cycle:

Bacteria reproduce by binary fission a process by which a parent cell

divides to form two progeny cells. As one cell gives rise to two daughter cells,bacteria are said to undergo exponeutial growth (Logrithmic growth) i.e.

Number of cells 1 2 4 8 16

Exponential 20

21

22

23

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The doubling time for bacteria ranges from as little as 20 minutes (E. Coli) to

more than 24 hr (Mycobacterium tuberculosis). For example

E. Coli after 3hrs 1000

E. Coli after 7 hrs over one million

The doubling time varies not only with species but also with amount of

nutrient, pH, temperature and so many other factors. The growth curve fig:

1. In lag phase vigorous metabolic activity occurs but no division takes place.

2. In log phase rapid cell division occurs

3. In stationary phase, depletion of nutrients or production of toxic substance casegrowth to show until the number of new cell produced balance the cells that die.

4. Death phase is marked by a decline in number of viable bacteria.


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PATHOGENESIS:

A microorganism is apathogen if it is capable of causing disease. Some

organisms are frequently pathogens, whereas other cause disease rarely.

Opportunistic are those, which rarely if ever cause disease in immune competent

people but can cause serious disease in those individuals who have reduceddefense.

VIRULENCE:-

Virulence is quantitative measure of pathogenecity. In the context of

virulence the 50 % lethal dose (LS50) is the number of organisms needed to kill

half of the hosts. In the context of 50 % infections dose (ID50) is the number of

organisms needed to cause infection if half the hosts.

EPIDEMIC:-

An infection is epidemic if it occurs much more frequently than usual. It

is pandemic if it has a worldwide distribution. An endemic infection is constantly

present at a low level in a specific population.

GRAM STAIN:

This staining procedure is the most important in microbiology. It was

developed in 1884 byDanish Physician CHRISTIAN GRAM.

It separates most bacteria into 2 groups: Gram Positive bacteria which stain blue, and the

gram negative bacteria which stain red. The gram stain involves the following 4-step procedure

1- Crystal violet dye all cells blue.

2. The iodine solution ( mordant ) is added to form a crystal violet-iodine

complex; all cells continue to appear blue.

3. The organic solvent, such as acetone or ethanol extracts the blue dye complex

from the lipid rich, thin walled gram-negative bacteria to a greater degree than

from the lipid poor, thick walled gram positive bacteria.

The gram negative organisms appear colourless; the gram positive bacteria remainblue.

4- The red dye safranin stains the decolorized gram negative cells red, the gram

positive bacteria remain blue.


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Importance:-

Identification of bacteria for better choice of antibiotic.

NOTE:-

Immuno compromise:-

Having the immune response attenuated by administration of immuno suppressive agents,

by irradiation by malnutrition and by some disease process.

Cytoplasmic Membrane:-

Just inside the peptidoglycan, lies the cytoplasmic membrane which is composed

of phospholipid bilayer similar in microscopic appearance to that of eukaryotic cells.

They are chemically similar but eukaryotic membrane contains sterols whereas

prokaryotes generally do not the only prokaryotes that have sterols in their membrane are

mebers of the genus mycoplasma membrane has four important functions:

I. Active transport of molecules into cells

II. Energy generation by oxidative phosphoriylation

III. Synthesis of precursors of cell-wall

IV. Secretion of enzymes and toxins.

CYTOPLASM:

It has two distinct areas when seen under electron microscope:

1. An orphous matrix that contains ribosomes, nutrients, granules, metabolites and ions.

2. An inner nucleoid region composed of DNA.

A. Ribosome:

Bacterial ribosomes are the sites of protein synthesis as in eukaryotic cells

but they differ from eukaryotic ribosomes in size and chemical composition.

Bacterial ribosomes are 70 S in size having 50 S and 30 S in size, with 60 S and

40 S subunits. The difference in both ribosomal RNAs and protien constitute the

basis of the selective action of several antibiotics that inhibit bacterial but not

human protein synthesis.


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B. Granules:-

Granules serve as storage area for nutrients and stain characteristically

with certain dyes.

C. Nucleoid:-

It is the area of cytoplasm in which DNA is located. The DNA of

prokaryotes is a single, circular molecule that has molecule of approximately 2 x

109 and contains about 2000 genes. The nucleoid contains no nuclear membrane,

no mitotic apparatus and no his tones (simple protein)

D. Plasmids:-

Plasmids are extra chromosomal, double-stranded circular DNA molecules

that are capable of replicating independently of the bacterial chromosome.

Although they are extra-chromosomal, they can be integrated into the bacterialthey can be integrated into the bacterial chromosome. Plasmid can be

1. Transmissible:-Plasmids can be transferred from one cell to another by conjugation. They

are large because they contain about a dozen genes responsible for synthesis of

sex pilus and enzymes required for transfer.

2. Nontransmissible:-Nontransmissible are small since they do not contain transfer genes.

Plasmids occur in both Gram positive and the genes for the Gram negative

bacteria following functions and structures of medical importance are carried by

plasmids.

1. Antibiotic resistance that is mediated by variety of enzymes.

2. Resistance to heavy metals e.g. Hg.

3. resistance to UV light which is mediated by DNA repair enzymes.

4. pili which mediate adherance of bacteria to epithelial cells.

5. Oxotoxins:-

A toxic substance formed by bacteria that is found outside the bacterial cell or

free in the culture medium.


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Other plasmid-encoded products of interest are

1. bacteriocins toxins or enzymes produced by certain bacteria and one lethal to

other bacteria.

2. nitrogen fixation enzymes in Rhizopodium in the roots of legumes.

3. tumour caused by agrobacterium in plants.

4. certain antibiotics produced by streptomyces.

5. a variety of degradative enzymes.

E-Transposons:-

E-transposons are pieces of DNA that more readily from one place to another,

either within or between the DNAs of bacteria, plasmids and bacteriophages. They are

named jumping genes (nick name). They can code for drug resistance enzymes, toxins ordrug metabolizing enzymes.

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