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Medical Microbiology I
Lecture 6Lecture 6
Structure of Bacteria, Metabolism
and Genetical Adaptation to Stresses
Size and Shape of Bacteria
Bacteria are prokaryotes and come in many
sizes and several shapes
Most bacteria range from 0.2 to 2.0 m in
lengthlength
They have few basic shapes: spherical coccus
(plural: cocci, meaning berries), rod-shaped
bacillus (plural: bacilli, meaning little staffs),
and spiral
Size and Shape of Bacteria
Cocci are usually round but can be oval,
elongated, or flattened on one side
When cocci divide to reproduce, the cells can
remain attached to one anotherremain attached to one another
Cocci that remain in pairs after dividing are
called diplococci; those that divide and
remain attached in chain-like patterns are
called streptococci
Size and Shape of Bacteria
Those that divide in two planes and remain in groups
of four are known as tetrads
Those that divide in three planes and remain
attached in cube-like groups of eight are called
sarcinaesarcinae
Those that divide in multiple planes and form grape-
like clusters or broad sheets are called staphylococci
These group characteristics are frequently helpful in
identifying certain cocci
Size and Shape of Bacteria
Bacilli divide only across their short axis, so there are fewer groupings of bacilli than cocci
Most bacilli appear as single rods
Diplobacilli appear in pairs after division, and Diplobacilli appear in pairs after division, and streptobacilli occur in chains
Some bacilli like straws, while others have tapered ends, like cigars
Others are oval and look so much like cocci that they are called coccobacilli
Size and Shape of Bacteria
Bacillus has 2 meaning: Bacillus refers to a
specific genus, while bacillus refers to a
bacterial shape
For example, the bacterium Bacillus anthracis For example, the bacterium Bacillus anthracis
is the causative agent of anthrax
Bacillus cells often form long, twisted chains
of cells
Size and Shape of Bacteria
Spiral bacteria have one or more twists; they
are never straight
Bacteria that look like curved rods are called
vibriosvibrios
Other, called spirilla, have a helical shape, like
a corkscrew and fairly rigid bodies
There is another group of spirals that are
helical and flexible called spirochetes
Size and Shape of Bacteria
Unlike spirilla, which use propeller-like external appendages called flagella to move, spirochetes move by means of axial filaments, which resemble flagella but are contained within a flexible external sheathwithin a flexible external sheath
In addition to the three basic shapes, there are star-shaped cells (genus Stella), rectangular, flat cells (halophilic archaea of the genus Haloarcula), and triangular cells
Size and Shape of Bacteria
The shape of a bacterium is determined by
heredity
Genetically, most bacteria are monomorphic,
that is, they maintain a single shapethat is, they maintain a single shape
However, a number of environmental
conditions can alter that shape
If the shape is altered, identification becomes
more difficult
Size and Shape of Bacteria
Moreover, some bacteria, such as Rhizobium
and Corynebacterium are genetically
pleomorphic, which means they can have
many shapes, not just onemany shapes, not just one
Structure of Bacteria
1. Structures External to the Cell Wall
Among the possible structures external to the bacteria cell wall are the glycocalyx, flagella, axial filaments, fimbriae, and pili
I. Glycocalyx I. Glycocalyx
Many prokaryotes secrete on their surface a substance called glycocalyx
Glycocalyx (meaning sugar coat) is the general term used for substances that surround cells
Structure of Bacteria
The bacterial glycocalyx is a viscous (sticky), gelatinous polymer that is external to the cell wall - composed of polysaccharide, polypeptide, or both - the chemical composition varies widely with the specieswith the species
It is usually made inside the cell and secreted to the cell surface
If the substance is organised and is firmly attached to the cell wall - a capsule - can be determined by using negative staining
Structure of Bacteria
If the substance is unorganised and only loosely
attached to the cell wall - a slime layer
Function of capsules:
contribute to bacterial virulence (the degree to contribute to bacterial virulence (the degree to
which a pathogen causes disease).
protect pathogenic bacteria from phagocytosis
(the ingestion and digestion of microorganisms and
other solid particles) by the cells of the host, and
helps bacteria to adhere and colonise the host cells
Structure of Bacteria
The glycocalyx is a very important component of
biofilms (In nature, microorganisms seldom live
in the isolated single-species colonies that are
seen on laboratory plates. They more typically seen on laboratory plates. They more typically
live in communities called biofilms or informally
called slime)
A glycocalyx that helps cells in a biofilm attach to
their target environment and to each other is
called an extracellular polymeric substance (EPS)
Structure of Bacteria
The EPS protects the cells within it, facilitates
communication among them, and enables the
cells to survive by attaching to various surfaces
in their natural environment (rocks, plant in their natural environment (rocks, plant
roots, human teeth, medical implants, water
pipes, and even other bacteria)
Structure of Bacteria
II. Flagella
Some prokaryotic cells have flagella (singular:
flagellum), which are long filamentous
appendages that propel bacteriaappendages that propel bacteria
Bacteria that lack flagella - atrichous (without
projections)
Flagella may be peritrichous (distributed over
the entire cell) or polar (at one or both poles
or ends of the cell)
Structure of Bacteria
If polar, flagella may be monotrichous (a single
flagellum at one pole), lophotrichous (a tuft of
flagella coming from one pole), or
amphitrichous (flagella at both poles of the cell)amphitrichous (flagella at both poles of the cell)
Structure of Bacteria
A flagellum has three basic parts:
a. The long outermost region, the filament is
constant in diameter and contains the protein
flagellinflagellin
b. The filament is attached to a slightly wider
hook, consisting of a different protein
c. The third portion is the basal body, which
anchors the flagellum to the cell wall and
plasma membrane
Structure of Bacteria
An advantage of motility:
enables a bacterium to move toward a favourable environment or
away from an adverse one
Motile bacteria contain receptors in various Motile bacteria contain receptors in various locations, such as in or just under the cell wall
The flagellar protein called H antigen is useful for distinguishing among serovars, or variations within a species, of gram negative bacteria
e.g. There are at least 50 different H antigens for E. coli
Structure of Bacteria
III. Axial filaments
Spirochetes are a group of bacteria that have
unique structure and motility - one the best-
known spirochetes is Treponema pallidum, known spirochetes is Treponema pallidum,
the causative agent of syphilis
Spirochetes move by means of axial
filaments, or endoflagella - bundles of fibrils
that arise at the ends of the cell beneath an
outer sheath and spiral around the cell
Structure of Bacteria
Axial filaments, which are anchored at one end of the spirochetes, have a structure similar to that of flagella
The rotation of the filaments produces a movement of the outer sheath that propels the movement of the outer sheath that propels the spirochetes in a spiral motion - similar to the way a corkscrew moves through a cork
This corkscrew motion probably enables a bacterium such as T. pallidum to move effectively through body fluids
Structure of Bacteria
IV. Fimbriae and Pili
Many Gram negative bacteria contain hair-
like appendages that are shorter, straighter,
and thinner than flagella and are used for and thinner than flagella and are used for
attachment and transfer of DNA
These structures, which consist of a protein
called pilin arranged helically around a
central core, are divided into 2 types,
fimbriae and pili (having different functions)
Structure of Bacteria
Fimbriae (singular: fimbria) can occur at the poles of the bacterial cell or can be evenly distributed over the entire surface of the cell
They can number from a few to several hundred per cellper cell
Fimbriae have a tendency to adhere to each other and to surfaces
As a result, they are involved in forming biofilms and other aggregation on the surfaces of liquids, glass, and rocks
Structure of Bacteria
Fimbriae can also help bacteria adhere to epithelial surfaces in the body
e.g. fimbriae on the bacterium Neisseria gonorrhoeae, the causative agent of gonorrhea, help the microbe colonise the mucous membraneshelp the microbe colonise the mucous membranes
Once colonisation occurs, the bacteria can cause disease
When fimbriae are absent (because of genetic mutation), colonisation cannot happen, and no disease arises
Structure of Bacteria
Pili (singular: pilus) are usually longer than fimbriae and number only one or two per cell
Pili are involved in motility and DNA transfer
Pili are used to bring bacteria together allowing the transfer of DNA from one cell to another - in the transfer of DNA from one cell to another - in a process called conjugation
Such pili are called conjugation (sex) pili
The exchanged DNA can add a new function to the recipient cell, such as antibiotic resistance or the ability to digest its medium more efficiently
Structure of Bacteria
2. Cell Wall
Complex, semi-rigid structure responsible for the shape of the cell
It surrounds the underlying, fragile plasma (cytoplasmic) membrane and protects it and the (cytoplasmic) membrane and protects it and the interior of the cell from adverse changes in the outside environment
Functions:
Prevent bacterial cells from rupturing when the water pressure inside the cell is greater than that outside the cell
Structure of Bacteria
It also helps to maintain the shape of the bacterium
Serves as a point of anchorage for flagella
Clinically, the cell wall is important because it contributes to the ability of some species to cause disease and is the site of action of some antibiotics
In addition, the chemical composition of the cell wall is used to differentiate major types of bacteria
Although the cells of some eukaryotes, e.g.plants, algae, and fungi, have cell walls, their cell walls differ chemically from those of prokaryotes, are simpler in structure, and are less rigid
Structure of Bacteria
A. Composition and Characteristics
Composed of macromolecular network called
peptidoglycan (also known as murein), which
is present either alone or in combination with is present either alone or in combination with
other substances
Peptidoglycan consists of a repeating
disaccharide attached by polypeptides to form
a lattice that surrounds and protects the
entire cell
Structure of Bacteria
Peptidoglycan is only found in bacteria
The thickness of the cell wall and its exact
composition vary with the species of bacteria
The cell walls of certain bacteria, called gram The cell walls of certain bacteria, called gram
positive bacteria, have a thick layer of
peptidoglycan combined with teichoic acid and
lipoteichoic acid molecules
The cell walls of gram negative bacteria have a
much thinner layer of peptidoglycan
Structure of Bacteria
However, this layer is covered with a complex
layer of lipid macromolecules, usually referred
to as the outer membrane
i. Gram Positive Cell Wallsi. Gram Positive Cell Walls
Consists of many layers of peptidoglycan,
forming a thick, rigid structure
Contains teichoic acids, which consist
primarily of an alcohol (such as glycerol or
ribitol) and phosphate
Structure of Bacteria
Functions:
1. teichoic acids may bind and regulate the
movement of cations into and out of the cell.
2. they may also have a role in cell growth
3. preventing extensive wall breakdown and
possible cell lysis
4. they also provide much of the walls antigenic
specificity
5. make it possible to identify Gram positive
bacteria by certain laboratory tests
Structure of Bacteria
ii. Gram Negative Cell Walls
Consist of one or very few layers of peptidoglycan and an outer membrane
The peptidoglycan is bonded to lipoproteins The peptidoglycan is bonded to lipoproteins (lipid covalently linked to proteins) in the outer membrane and is in the periplasm, a gel-like fluid between the outer membrane and the plasma membrane
The periplasm contains a high concentration of degradative enzymes and transport proteins
Structure of Bacteria
Gram negative cell walls do not contain
teichoic acids
Gram negative bacteria more susceptible to
mechanical damage because the cell walls mechanical damage because the cell walls
contain only a small amount of peptidoglycan
The outer membrane of the Gram negative cell
consists of lipopolysaccharides (LPS),
lipoproteins, and phospholipids
Structure of Bacteria
Functions:
It is strongly negative charged, an important factor in evading phagocytosis and the actions of the complement (lyses cells and promotes phagocytosis) - 2 components of the host phagocytosis) - 2 components of the host defenses
Provides a barrier to certain antibiotics (e.g.penicillin), digestive enzymes such as lysozyme, detergents, heavy metals, bile salts, and certain dyes
Structure of Bacteria
However, nutrients must pass through the outer membrane to sustain the metabolism of the cell
Part of the permeability of the outer membrane is due to proteins in the membrane, called porins, that form channels
Porins permit the passage of molecules such a s nucleotides, disaccharides, peptides, amino acids, vitamin B12, and iron
The lipopolysaccharide (LPS) of the outer membrane is a large complex of molecule that contains lipids and carbohydrates
Structure of Bacteria
It consists of 3 components: lipid A, a core polysaccharide, and an O polysaccharide
Lipid A is the lipid portion of the LPS and is embedded in the top layer of the outer membrane
When Gram negative bacteria die, they release When Gram negative bacteria die, they release lipid A, which function as an endotoxin (responsible for the symptoms associated with infections with Gram negative bacteria such as fever, dilation of blood vessels, shock, and blood clotting)
Structure of Bacteria
The core polysaccharide is attached to lipid A and contains unusual sugars
Its role is structural- to provide stability
The O polysaccharide extends outward from the core polysaccharide and is composed of sugar core polysaccharide and is composed of sugar molecules
It functions as an antigen and is useful for distinguishing species of Gram negative bacteria
This role is comparable to that of teichoic acids in Gram positive bacteria
Structure of Bacteria
3. Structures Internal to the Cell Wall
I. Plasma (cytoplasmic) membrane - a thin structure lying inside the cell wall and enclosing the cytoplasm of the cell and composed primarily of phospholipidsprimarily of phospholipids
II. Cytoplasm - refers to the substance of the cell inside the plasma membrane. It is thick, aqueous, semitransparent, and elastic. It is about 80% water and contains primarily proteins (enzymes), carbohydrates, lipids,
Structure of Bacteria
inorganic ions and many low molecular weight compounds
III. Nucleoid - usually contains a single, long, continuous, and frequently circularly arranged thread of dsDNA called the bacterial thread of dsDNA called the bacterial chromosome. It is not surrounded by nuclear envelope (membrane) and do not include histone. The nucleoid can be spherical, elongated, or dumbbell-shaped. In actively growing bacteria, as much as 20% of the cell
Structure of Bacteria
volume is occupied by DNA because these cells presynthesise nuclear material for future cells
IV. Ribosomes - sites of protein synthesis. They are composed of 2 subunits, each of which are composed of 2 subunits, each of which consists of protein and a type of RNA called ribosomal RNA (rRNA). Bacterial ribosome are called 70S ribosomes (small 30S subunit containing one molecule of rRNA and a larger 50S subunit containing 2 molecules of rRNA)
Structure of Bacteria
V. Inclusions - within the cytoplasm are several kinds
of reserve deposits known as inclusions. Cells may
accumulate certain nutrients when they are
plentiful and use them when the environment is
deficient. Evidence suggests that macromolecules deficient. Evidence suggests that macromolecules
concentrated inclusions avoid the increase in
osmotic pressure that would result if the
molecules were dispersed in the cytoplasm. Some
inclusions are limited to a small number of species
and serve as a basis for identification
Structure of Bacteria
VI. Endospores - when essential nutrients are
depleted, certain Gram positive bacteria e.g.
Clostridium and Bacillus, form specialised resting
cells called endospores. These are unique to
bacteria, and are highly durable dehydrated cells bacteria, and are highly durable dehydrated cells
with thick walls and additional layers. They are
formed internal to the bacterial cell membrane.
When released into the environment, they can
survive extreme heat, lack of water, exposure to
many toxic chemicals and radiation.
Bacterial Metabolism
Breakdown of glucose to pyruvate
The Embden-Meyerhof pathway
The pentose-phosphate pathway
The Entner-Doudoroff pathway The Entner-Doudoroff pathway
The tricarboxylic acid cycle
The electron transport and oxidative
phosphorylation
Bacterial Metabolism
Aerobic respiration
Anaerobic respiration
Fermentation Fermentation
Protein, carbohydrate and lipid - anabolism
and catabolism
Synthesis of purines, pyrimidines and
nucleotides
Genetic Mutation in Bacteria
Mutations were initially characterised as altered phenotypes, later understood at the molecular level
Some mutations arise from the alteration of single pairs of nucleotides and from the addition single pairs of nucleotides and from the addition or deletion of 1 or 2 nucleotide pairs in the coding regions of a gene
Point mutation (microlesions)
Macrolesions - insertions, deletions, inversions, duplications and translocations
Spontaneous mutation
Induced mutation
Genetic Mutation in Bacteria
Spontaneous mutations arise without exposure to external agents and may result from errors in DNA replication or from the action of mobile genetic elements such as transposonstransposons
Some bacteria seem able to select which mutations occur so that they can adapt to their surroundings directed or adaptive mutation
Genetic Mutation in Bacteria
Mutagens - base analogs, DNA-modifying agents, intercalating agents and physical agents (e.g.radiation)
A mutation from wild type to a mutant form is called forward mutation called forward mutation
The second mutation is at the same site as the original mutation, is called reversion mutation
If the second mutation is at a different site than the original mutation, it is called a suppressor mutation (intra- and extra-genic suppressor mutation)
DNA Transformation
Transformation is uptake by a cell of a naked DNA molecule or fragmentation from the medium and the incorporation of this molecule into the recipient chromosome in a heritable formheritable form
In natural transformation, the DNA comes from a donor bacterium
The process is random, and any portion of a genome may be transferred between bacteria
DNA Transformation
When bacteria lyse, they release considerable amount of DNA into the surrounding environment
These fragments may be relatively large and These fragments may be relatively large and contain several genes
If a fragment contacts a competent cell, a cell that is able to take up DNA and be transformed, the DNA can be bound to the cell and taken inside
Transduction
Transduction is the transfer of bacterial genes by viruses
Bacterial genes are incorporated into a phage capsid because of errors made during the virus capsid because of errors made during the virus life cycle
The virus containing these genes then injects them into another bacterium, completing the transfer
It is a frequent mode of horizontal gene transfer in nature and is mediated by viruses
Transduction
Viruses are structurally simple, often composed of just a nucleic acid genome protected by a protein coat called the capsid
They are unable to replicate autonomously They are unable to replicate autonomously
Instead, they infect and take control of a host cell, forcing the host to make many copies of the virus
Viruses that infect bacteria are called bacteriophages, or phage
Transduction
After the number of replicated phages reaches
a certain number, they cause the host to lyse,
so they can be released and infect new host
cells (phage = virulent bacteriophages; process cells (phage = virulent bacteriophages; process
= lytic cycle)
Prophage - phage enter the host bacterium,
instead of replicating, insert their genomes
into the bacterial chromosome
Transduction
The host bacterium is unharmed by this, and
the phage genome is passively replicated as
the host cells genome is replicated
These bacteriophages are called temperate These bacteriophages are called temperate
bacteriophages and the relationship between
these viruses and their host is called lysogeny
2 types of transduction: generalised and
specialised
Transduction
Generalised transduction occurs during the lytic cycle of virulent and some temperate phages and can transfer any part of the bacterial genome
In specialised transduction, the transducing particle carries only specific portions of the bacterial genome
It is made possible by an error in the lysogenic life cycle of phages that insert their genomes into specific site in the host chromosome