Transcript 0-bacterial-structure-short

Batterjee Medical College
Batterjee Medical College
Bacterial Introduction
and Structure
Dr. Manal El Said
Head of Microbiology Department
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Microbiology
• Microbiology is the study of microorganisms, which are
microscopic, unicellular, and cell-cluster organisms.
•Medical
microbiology
deals
with
the
study
of
microorganisms which are of medical importance and
causing diseases in human beings.
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Taxonomic Rank
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Bacteria Compared with Other
Microorganisms
The agents of human infectious diseases belong to five major
groups of organisms:
1-Bacteria :
2-Fungi
Prokaryote kingdom
:
3-Protozoa:
Protists kingdom
4-Helminths: Animal kingdom
NB. Taken together, helminths and protozoa are commonly called parasites.
5-Viruses: they are not cells but can replicate only within cells
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Eukaryotes & Prokaryotes
Cells have evolved into two types:
- Eukaryotic
- Prokaryotic
Characteristic
Prokaryotic
Bacterial Cells
Eukaryotic
DNA within a nuclear membrane
No
Yes
Mitotic division
No
Yes
DNA associated with histones
No
Yes
Chromosome number
One
More than one
Membrane-bound organelles, such No
as mitochondria and lysosomes
Yes
Size of ribosome
8OS
70S
Cell wall containing peptidoglycan Yes
No
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Structure of Bacterial Cells
Shape & Size
• Bacteria have three shapes:
- cocci (spheres)
- bacilli (rods)
- spirochetes (spirals)
• Cocci are arranged in three patterns:
- Pairs (diplococci)
- Chains (streptococci)
- Clusters (staphylococci)
• The size of most bacteria ranges from 1 to 3 μm.
• Mycoplasma, smallest bacteria (smallest cells) are 0.2 μ m.
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Structure of Bacterial Cells
Bacterial Cell Wall
•All bacteria have a cell wall composed of peptidoglycan
except Mycoplasma, which are surrounded only by a cell
membrane.
•Gram-negative bacteria have a thin peptidoglycan covered
by an outer lipid-containing membrane
•Gram-positive bacteria have a thick peptidoglycan & no
outer membrane.
•These differences explain why:
-Gram-negative bacteria lose the stain when exposed to a
lipid solvent in the Gram stain process
- Gram-positive bacteria retain the stain and remain purple.
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Structure of Bacterial Cells
The outer membrane of gram-negative bacteria
Peptidoglycan:
• It is found only in bacterial cells.
• It is a network that covers entire bacterium & gives
organism its shape.
• It is composed of a sugar backbone (glycan) and peptide
side chains (peptido).
• The side chains are cross-linked by transpeptidase
(enzyme that is inhibited by penicillins & cephalosporins)
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Structure of Bacterial Cells
Comparison of Cell Walls of Gram-Positive and Gram-Negative Bacteria
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Structure of Bacterial Cells
Cell Walls of Acid-Fast Bacteria
• Mycobacteria are acid-fast because it resist decolorization
with acid–alcohol after staining with carbolfuchsin.
• It is due to high concentration of lipids in cell wall.
• The dyes used in the Gram stain do not stain mycobacteria.
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Structure of Bacterial Cells
The outer membrane of gram-negative bacteria
Endotoxin (lipopolysaccharide, LPS):
• The main inducer of septic shock.
• Endotoxin consists of:
- lipid A
fever & hypotension in septic shock
- polysaccharide(O antigen)
use in laboratory identification
Periplasmic space:
•It lies between inner cell membrane & outer membrane of
gram-negative bacteria
(It is the location of β–lactamases-the enzymes that degrade β lactam antibiotics, such as penicillins and cephalosporins).
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Structure of Bacterial Cells
The outer membrane of gram-negative bacteria
Lysozymes
They kill bacteria
peptidoglycan.
by
cleaving
glycan
backbone
of
The cytoplasmic membrane of bacteria :
• It consists of phospholipid bilayer (without sterols) located
just inside peptidoglycan.
• It regulates:
- Active transport of nutrients into cell.
- Secretion of toxins out of cell.
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Structure of Bacterial Cells
Gram stain
• Gram-positive bacteria stain purple, whereas gram-negative
bacteria stain pink.
• Gram-positive bacteria has ability to retain crystal violet–
iodine complex in presence of lipid solvent as alcohol.
• Gram-negative bacteria lose purple dye when treated with
alcohol (have outer lipid-containing membrane & thin
peptidoglycan) & become colorless & then stain pink when
exposed to red dye such as safranin.
• Not all bacteria can be visualized using Gram stain.
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Structure of Bacterial Cells
Gram stain
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Structure of Bacterial Cells
ZN stain
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Structure of Bacterial Cells
Bacterial DNA
The bacterial genome
It consists of a single chromosome
of circular DNA located in nucleoid.
Plasmids
They are extrachromosomal pieces of circular DNA that
encode both exotoxins & enzymes
antibiotic resistance
Transposons
•They are small pieces of DNA that move
chromosomal DNA and plasmid DNA.
• They carry antibiotic-resistant genes.
between
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Structure of Bacterial Cells
Structures External to the Cell Wall
Capsules
• They
are antiphagocytic, i.e., they limit ability of
neutrophils to engulf bacteria.
• They are composed of polysaccharide
(the polypeptide capsule of anthrax bacillus is exception).
• Capsules are antigens in several vaccines, such as
pneumococcal vaccine.
• Antibodies against capsule neutralize antiphagocytic effect
& allow bacteria to be engulfed by neutrophils.
• Opsonization is process by which antibodies enhance
phagocytosis of bacteria.
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Structure of Bacterial Cells
Structures External to the Cell Wall
The glycocalyx
• It is polysaccharide "slime layer" secreted by certain
bacteria.
• It attaches bacteria firmly to:
- Surface of human cells
- Surface of catheters
- Prosthetic heart valves & hip joints.
Pili
•They are filaments of protein that extend from bacterial
surface & mediate attachment of bacteria to surface of
human cells.
• A different kind of pilus, sex pilus, functions in conjugation.
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Structure of Bacterial Cells
Structures External to the Cell Wall
Spores
• They are highly heat resistant & are not killed by many
disinfectants.
• Boiling will not kill spores.
• They are formed by certain gram-positive rods, especially
Bacillus & Clostridium species.
• They have a thick, keratinlike coat that allows them to
survive for many years, especially in the soil.
• Spores are formed when nutrients are in short supply
• Spores germinate to form bacteria that can cause disease
when nutrients are restored.
• They are metabolically inactive but contain DNA, ribosomes
& other essential components.
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Structure of Bacterial Cells
Structures External to the Cell Wall
Spores