1
Microbiology
Is a science that deals with study of organisms which unable to be seen by naked eye
such as bacteria, viruses etc.
Medical microbiology
Is a branch of microbiology deals with micro-organism that causes the diseases for
human & other organisms and its divided several branch e.g:
1 – bacteriology.
2 – virology.
3 – parasitology.
4 – mycology.
5 – immunology.
Microorganisms
belong to the Protista biologic kingdom include some eukaryotes and prokaryotes,
viruses, viroids, and prions, these are classified according to their structure, chemical
composition, and biosynthetic and genetic organization.
B. Eukaryotic cells
contain organelles and a nucleus bounded by a nuclear membrane these contain complex
phospholipids, sphingolipids, histones, and sterols, the lack a cell wall (plant cells and
fungi have a cell wall) & have multiple diploid chromosomes and nucleosomes. have
relatively long-lived mRNA formed from the processing of precursor mRNA, which
contains exons and introns. have 80S ribosomes and uncoupled transcription and
translation.
C. Prokaryotic cells
•
have no organelles, no membrane-enclosed nucleus, and no histones; in rare cases,
they contain complex phospholipids, sphingolipids, and sterols.
•
have 70S ribosomes.
•
have a cell wall composed of peptidoglycan-containing muramic acid.
•
are haploid with a single chromosome.
•
have short-lived, unprocessed mRNA.
•
have coupled transcription and translation.
o
Typical bacteria
▪
have normal peptidoglycan.
▪
may be normal flora or pathogenic in humans.
▪
do not have a sexual growth cycle; however, some can produce asexual spores.
o
Mycoplasmas
▪
are the smallest and simplest of the bacteria that are self-replicating.
▪
lack a cell wall.
▪
are the only prokaryotes that contain sterols.
2
o
Rickettsia organisms
▪
are obligate intracellular bacteria that are incapable of self-replication.
▪
depend on the host cell for adenosine triphosphate (ATP) production.
o
Chlamydia
▪
are bacteria-like obligate intracellular pathogens with a complex growth cycle
involving intracellular and extracellular forms.
▪
depend on the host cell for ATP production.
Bacteriology
Bacteria, along with blue-green algae, are prokaryotic cells. That is, in contrast to
eukaryotic cells, they have no nucleus; rather the genetic material is restricted to an area of
the cytoplasm called the nucleoid. Prokaryotic cells also do not have cytoplasmic
compartment such as mitochondria and lysosomes that are found in eukaryotes. However,
a structure that is found in prokaryotes but not in eukaryotic animal cells is the cell wall
which allows bacteria to resist osmotic stress. These cell walls differ in complexity among
bacteria that are usually divided into two major groups, the gram-positive and gram-
negative organisms, which reflect their cell wall structure. The possession of this cell wall,
which is not a constituent of animal cells, gives rise to the different antibiotic sensitivities
of prokaryotic and eukaryotic cells.
Limited number of bacterial species are responsible for the majority of infectious
diseases in healthy individuals. Due to the success of vaccination, antibiotics, and effective
public health measures, until recently, epidemics were felt to be a thing of the past. Due to
the development of antibiotic resistant organisms, this situation is changing rapidly.
All humans are exposed to bacteria (the normal flora) living on their surfaces (including
the skin, gut and lungs). We are constantly also exposed to bacteria (including those that
live in air, water, soil and food). Normally due to our host defenses most of these bacteria
are harmless. In compromised patients, whose defenses are weakened, these bacteria often
cause opportunistic infectious diseases when entering the bloodstream (after surgery,
catheterization or other treatment modalities). When initiated in the hospital, these
infectious diseases are referred to as nosocomial. Some common bacteria found in the
normal flora include Staphylococcus aureus, S. epidermidis and Propionibacterium acnes
(found on the skin) and Bacteroides and Enterobacteriaceae found in the intestine.
Koch's postulates (modified)
1. The organism must always be found in humans with the infectious disease but not
found in healthy ones.
2. The organism must be isolated from humans with the infectious disease and grown
in pure culture.
3. The organism isolated in pure culture must initiate disease when re-inoculated into
susceptible animals.
4. The organism should be re-isolated from the experimentally infected animals.
3
Postulates 3. and 4. are extremely important in definite proof of the role of agent in human
disease. However, this depends on the ability to develop animal models that resemble the
human disease. In many cases such models do not exist.
II. Bacteria
A. Bacterial shape
•
can usually be determined with appropriate staining and a light microscope.
•
is usually round (coccus), rod-like (bacillus), or spiral with most species; cocci and
bacilli often grow in doublets (diplococci) or chains (streptococci). Cocci that grow
in clusters are called staphylococci.
•
may be pleomorphic with some species, such as Bacteroides.
•
is used, along with other properties, to identify bacteria.
•
is determined by the mechanism of cell wall assembly.
•
may be altered by antibiotics that affect cell wall biosynthesis (e.g., penicillin).
Bacterial structure
B. Genetic materials
•
is not surrounded by a nuclear membrane, nor does it contain a mitotic apparatus.
•
is generally called a nucleoid or nuclear body.
•
consists of polyamine and magnesium ions bound to negatively charged, circular,
supercoiled, double-stranded DNA; small amounts of RNA; RNA polymerase; and
other proteins.
C. Bacterial cytoplasm
•
contains ribosomes and various types of nutritional storage granules.
•
contains no organelles.
D. Bacterial ribosomes
•
have a sedimentation coefficient of 70S and are composed of 30S and 50S subunits
containing 16S, and 23S and 5S RNA, respectively.
•
are the sites of action of many antibiotics that inhibit protein biosynthesis.
•
have proteins and RNAs that differ from those of their eukaryotic counterparts.
•
form the basis for the selective toxicity of antibacterial protein synthesis inhibiting
agents, which affect 70S ribosomes (e.g., erythromycin) but not 80S ribosomes.
•
are membrane-bound if engaged in protein biosynthesis.
4
E. Cell (cytoplasmic) membrane
•
is a typical phospholipid bilayer.
•
contains the cytochromes and enzymes involved in electron transport and oxidative
phosphorylation.
•
contains carrier lipids and enzymes involved in cell wall biosynthesis.
•
contains enzymes involved in phospholipid synthesis and DNA replication.
•
contains chemoreceptors.
•
is responsible for selective permeability and active transport, which are facilitated by
membrane-bound permeases, binding proteins, and various transport systems.
•
is the site of action of certain antibiotics, such as polymyxin.
G. Plasmids
•
are small, circular, nonchromosomal, double-stranded DNA molecules.
•
are capable of self-replication.
•
are most frequently extrachromosomal, but may become integrated into bacterial
DNA.
•
contain genes that confer protective properties, such as antibiotic resistance,
virulence factors, or their own transmissibility to other bacteria.
H. Transposons
•
are small pieces of DNA that move between the DNA of bacteriophages.
•
are bacteria or plasmids not capable of self-replication.
•
code for antibiotic resistance enzymes, metabolic enzymes, or toxins.
•
may alter expression of neighboring genes or cause mutations to genes into which
they are inserted.
I. Cell envelope
•
is composed of the macromolecular layers that surround the bacterium.
•
always includes a cell membrane and a peptidoglycan layer.
•
includes an outer membrane layer in gram-negative bacteria.
•
may include a capsule, a glycocalyx layer, or both.
•
contains antigens that frequently induce a specific antibody response.
•
Cell wall
o
refers to that portion of the cell envelope that is external to the cytoplasmic membrane
and internal to the capsule or glycocalyx.
o
confers osmotic protection and gram-staining characteristics.
o
is composed of peptidoglycan, teichoic and teichuronic acids, and polysaccharides in
gram-positive bacteria.
o
is composed of peptidoglycan, lipoprotein, and an outer phospholipid membrane that
contains lipopolysaccharide in gram-negative bacteria.
o
contains penicillin-binding proteins
5
•
Peptidoglycan
o
is also called mucopeptide or murein and is unique to prokaryotes.
o
is found in all bacterial cell walls, except Mycoplasma.
o
is a complex polymer that consists of a backbone, which is composed of alternating N-
acetylglucosamine and N-acetylmuramic acid and a set of identical tetrapeptide side
chains, which are attached to the N-acetylmuramic acid and that are frequently linked
to adjacent tetrapeptide side chains by identical peptide cross-bridges or by direct
peptide bonds.
o
contains the glycosidic bond between N-acetylmuramic acid and N-acetylglucosamine,
which is cleaved by the bacteriolytic enzyme lysozyme (found in mucus, saliva, and
tears).
o
may contain diaminopimelic acid, an amino acid unique to prokaryotic cell walls.
o
is the site of action of certain antibiotics, such as penicillin and the cephalosporins.
o
comprises up to 50% of the cell wall of gram-positive bacteria but only 2%- 10% of
the cell wall of gram-negative bacteria.
•
Teichoic and teichuronic acids
o
are water-soluble polymers, containing a ribitol or glycerol residue linked by
phosphodiester bonds.
o
are found in gram-positive cell walls or membranes.
o
are chemically bonded to peptidoglycan (wall teichoic acid) or membrane glycolipid
(lipoteichoic acid), particularly in mesosomes.
o
contain important bacterial surface antigenic determinants, and lipoteichoic acid helps
anchor the wall to the membrane.
o
may account for 50% of the dry weight of a gram-positive cell wall.
•
Lipoprotein
o
cross-links the peptidoglycan and outer membrane in gram-negative bacteria.
o
is linked to diaminopimelic acid residues of peptidoglycan tetrapeptide side chains by
a peptide bond; the lipid portion is noncovalently inserted into the outer membrane.
•
Periplasmic space
o
is found in gram-negative cells.
o
refers to the area between the cell membrane and the outer membrane.
o
contains hydrated peptidoglycan, penicillin-binding proteins, hydrolytic enzymes
(including ²
β -lactamases), specific carrier molecules, and oligosaccharides.
•
Outer membrane
o
is found in gram-negative cells.
o
is a phospholipid bilayer in which the phospholipids of the outer portion are replaced
by lipopolysaccharides.
o
protects cells from many things, including harmful enzymes and some antibiotics, and
prevents leakage of periplasmic proteins.
o
contains embedded proteins, including matrix porins (nonspecific pores), some non-
pore proteins (phospholipases and proteases), and transport proteins for small
molecules.
6
•
Lipopolysaccharide
o
is found in the outer leaflet of the outer membrane of gram-negative cells.
o
consists of lipid A, several long-chain fatty acids attached to phosphorylated
glucosamine disaccharide units, and a polysaccharide composed of a core and terminal
repeating units.
o
is negatively charged and non-covalently cross-bridged by divalent cations.
o
is also called endotoxin; the toxicity is associated with the lipid A.
o
contains major surface antigenic determinants, including O antigen found in the
polysaccharide component.
J. External layers
•
Capsule
o
is a well-defined structure of polysaccharide surrounding a bacterial cell and is external
to the cell wall. The one exception to the polysaccharide structure is the poly-D-
glutamic acid capsule of Bacillus anthracis.
o
protects the bacteria from phagocytosis and plays a role in bacterial adherence.
•
Glycocalyx
o
refers to a loose network of polysaccharide fibrils that surrounds some bacterial cell
walls.
o
is sometimes called a slime layer.
o
is associated with adhesive properties of the bacterial cell.
o
is synthesized by surface enzymes.
o
contains prominent antigenic sites.
K. Appendages
•
Flagella
o
are protein appendages for locomotion.
o
consist of a basal body, hook, and a long filament composed of a polymerized protein
called flagellin.
o
may be located in only one area of a cell (polar) or over the entire bacterial cell surface
(peritrichous).
o
contain prominent antigenic determinants.
•
Pili (fimbriae)
o
are rigid surface appendages composed mainly of a protein called pilin.
o
exist in two classes: ordinary pili (adhesins), involved in bacterial adherence, and sex
pili, involved in attachment of donor and recipient bacteria in conjugation.
o
are, in the case of ordinary pili, the colonization antigens or virulence factors associated
with some bacterial species, such as Streptococcus pyogenes and Neisseria
gonorrhoeae.
o
may confer antiphagocytic properties, like the M protein of S pyogenes.
7
L. Endospores
•
are formed as a survival response to certain adverse nutritional conditions, such as
depletion of a certain resource.
•
are metabolically inactive bacterial cells that are highly resistant to desiccation, heat,
and various chemicals.
•
possess a core that contains many cell components, a spore wall, a cortex, a coat,
and an exosporium.
•
contain calcium dipicolinate, which aids in heat resistance in the core.
•
germinate under favorable nutritional conditions after an activation process that
involves damage to the spore coat.
•
are helpful in identifying some species of bacteria (e.g., Bacillus and Clostridium).
•
are not reproductive structures.