Anatomy of bacteria pdf - أ.م.د.وئام سعد عبد الحمزة

Course: Medical Microbiology

Lecturer: Dr. Weam Saad

Subject: Medical Bacteriology

Medical Bacteriology

Anatomy of bacteria

All bacteria, either pathogenic or saprophytic, are unicellular organisms

that  reproduce  by  binary  fission.  Most  bacteria  are  capable  of  independent
metabolic  existence  and  growth,  but  species  of  Chlamydia  and  Rickettsia  are
obligate  intracellular  organisms.  Bacterial  cells  are  extremely  small  and
measured  in  microns  (10-6µm).  They  range  in  size  from  large  cells  such  as
Bacillus  anthracis  (1.0  to  1.3  µm  X  3  to  10  µm)  to  very  small  cells  such  as
Pasteurella tularensis (0.2 X 0.2 to 0.7 µm), Mycoplasmas (atypical pneumonia
group) are even smaller, measuring 0.1 to 0.2 µm in diameter.

Bacteria have characteristic shapes. The common microscopic

morphologies  are  cocci  (round  cells,  such  as  Staphylococcus  aureus  or
Streptococcus  sp.;  rods,  such  as  Bacillus  and  Clostridium  species;  long,
filamentous  branched  cells,  such  as  Actinomyces  species;  and  comma-shaped
and spiral cells, such as Vibrio cholerae and Treponema pallidum.

The arrangement of cells is also typical of various species or groups of

bacteria. Some rods or cocci characteristically grow in chains; some, such as
Staphylococcus aureus, form grapelike clusters of spherical cells; some round
cocci form cubic packets. Bacterial cells of other species grow separately. The
microscopic appearance is important in classification and diagnosis.

 Surface Appendages

Two types of surface appendage can be recognized: Flagella, which are

organs of movement, and Pili (in Latin =hairs), which are also known as
fimbriae ( in Latin = fringes).

Flagella occur on both Gram-positive and Gram-negative bacteria, and their

presence can be useful in identification. For example, they are found on many
species of bacilli but rarely on cocci. In contrast, pili occur almost on all Gram-
negative bacteria and are found on only a few Gram-positive organisms (e.g.,
Corynebacterium renale). Some bacteria have both flagella and pili. The
electron micrograph in Fig. below shows the characteristic wavy appearance of
flagella and two types of pili on the surface of Escherichia coli.

1. Flagella

Bacterial flagella are long (3 to 12 µm), filamentous surface appendages

about 12 to 30 nm in diameter. A flagellum consists of three parts:

(1) The long filament, which lies external to the cell surface.

(2) The hook structure at the end of the filament.

(3) The basal body, to which the hook is anchored and which imparts

motion to the flagellum.

The ability of bacteria to swim by flagella provides them with the

mechanical means to undergo chemotaxis (movement in response to attractant
and repellent substances in the environment).

Chemically, flagella are constructed of a class of proteins called flagellins.

Flagellins are immunogenic, these antigens are called the H antigens, which are
characteristic  of  a  species  or  strain  of  bacteria.  The  species  specificity  of  the
flagellins reflects differences in the primary structures of the proteins. Antigenic
changes  of  the  flagella  known  as  the  phase  variation  of  H1  and  H2  occurs  in
Salmonella  typhimurium.  The  number  of  flagella  on  bacterial  surface  is  a
characteristic  for  classification.  Flagella  formation  can  be  inhibited  by
chloramphenicol  it  blocks  regeneration  of  flagella  and  the  protein  (flagellin)
synthesis.

Figure below illustrates typical arrangements of flagella on or around the

bacterial surface. For example, V. cholerae has a single flagellum at one pole of
the cell (i.e., it is monotrichous), whereas Proteus vulgaris and E. coli have
many flagella distributed over the entire cell surface (i.e., they are peritrichous).
The flagella of a peritrichous bacterium must aggregate as a posterior bundle to
propel the cell in a forward direction.

2. Pili

The terms pili and fimbriae are usually used to describe the thin, hairlike

appendages on the surface of many Gram-negative bacteria and proteins of pili
are referred to the pilins. Pili (plural of pilus) are more rigid than flagella. In
some bacteria, such as Shigella species and E coli, pili are distributed over the
cell surface as 200 per cell.

As in E coli, pili can come in two types: short, most common pili, and sex

pili

(one to six of very long pili). The sex pili attach male to female bacteria

during conjugation.

Pili in many enteric bacteria offer the adhesive properties on the bacterial

cells,  enabling  them  to  adhere  to  various  epithelial  surfaces  and  to  the  RBCs
(causing hemagglutination). These adhesive properties play an important role in
bacterial  pathogenesis;  colonization  of  epithelial  surfaces  and  are  therefore
called colonization factors.

 Surface Layers
1. Capsules and Loose Slime

Some bacteria form capsules, they are thick layer of viscous gel. Capsules

may  be  up  to  10  µm  thick.  Some  organisms  lack  a  well-defined  capsule  but
have loose slime layers external to the cell wall or cell envelope. The hemolytic
Streptococcus  mutans,  the  primary  organism  found  in  dental  plaque  is  able  to
synthesis a large extracellular mucoid glucans from sucrose.

Not all bacterial species produce capsules; the capsules of encapsulated

pathogens are often important determinants of virulence. In both groups, Gram-
positive  and  Gram-negative  bacteria;  most  capsules  are  composed  of  high
molecular-weight  polysaccharides  outside  the  cell  wall  or  envelope  except  the
capsule of Bacillus anthracis (the pathogen of anthrax), it is unusual in that it is
composed  of  a  g-glutamyl  polypeptide.  Mutation  can  cause  loss  of  enzymes
involved  in  the  biosynthesis  of  the  capsular  polysaccharides  can  result  change
from the smooth-to-rough as seen in the pneumococci.

The exact function of capsule is resistance to phagocytosis and protection

of the bacterial cell against the host defenses during invasion. Some bacterial
capsules work as main virulence factor.

2. Cell Wall

The Gram stain differentiates bacteria into Gram-positive and Gram-

negative groups. Gram-positive and Gram-negative organisms differ in the
structures of their cell walls, see figure below.

Most Gram-positive bacteria have a thick (about 20 to 80 nm), cell wall

composed of peptidoglycan (also known as mucopeptide or murein). In thick
cell walls, some other cells have other wall polymers (such as the teichoic acids,
polysaccharides, and peptidoglycolipids) are attached to the peptidoglycan. The
peptidoglycan layer in Gram-negative bacteria is thin (about 5 to 10 nm thick).

The basic differences in surface structures of Gram-positive and Gram-

negative bacteria explain the results of Gram staining.

 Peptidoglycan

Unique composition in all prokaryotic cells (except for mycoplasmas) is

the  peptidoglycan,  this  layer  help  in  mechanical  protection  and  there  are
specific enzymes involved in its biosynthesis. These enzymes are target sites for
inhibition  of  peptidoglycan  synthesis  by  specific  antibiotics.  The  primary
chemical  structures  of  peptidoglycans  consist  of  a  polymer  backbone  of
disaccharides of N-acetylmuramyl-N-acetylglucosamine and are linked through
the carboxyl group by amide linkage of muramic acid residues.

There are two groups of bacteria that lack the peptidoglycan, the

Mycoplasma species (causes atypical pneumonia and some genitourinary tract

infections) and the L-forms. The mycoplasmas and L-forms are all Gram-
negative and insensitive to penicillin.

 Teichoic Acids

The teichoic acids are found only in some Gram-positive bacteria (such as

Staphylococci, Streptococci, Lactobacilli, and Bacillus spp); they are not found
in gram- negative bacteria. Teichoic acids are polyol phosphate polymers, with
either ribitol or glycerol linked by phosphodiester bonds. It can act as a specific
antigenic determinant.

 Lipopolysaccharides

A characteristic of Gram-negative bacteria is the lipopolysaccharide (LPS),

only  one  Gram-positive  organism,  Listeria  monocytogenes,  has  been  found  to
contain  LPS.  The  LPS  are  also  called  endotoxins,  they  are  cell-bound,  heat-
stable toxins and differ from heat-labile, protein exotoxins secreted into culture
media. Endotoxins possess an array of powerful biologic activities and play an
important role in the pathogenesis of many Gram-negative bacterial infections.

In addition LPS is pyrogenic and causes endotoxic shock, can activate

macrophages  and  complement  system,  it  is  mitogenic  for  B  lymphocytes,
induces interferon production, causes tissue necrosis and tumor regression, and
has adjuvant properties. The endotoxic properties of LPS  is due to the lipid A
components.  Usually,  the  LPS  molecules  have  three  regions:  The  lipid  A
attached to the core composed of polysaccharide chains which are linked to the
O-antigens responsible for serologic specificity of the Gram-negative bacteria.

 Intracellular Components
1. Plasma (Cytoplasmic) Membranes

Bacterial plasma membranes is similar to eukaryotic plasma membranes in

function, are referred to cytoplasmic or protoplast membranes, they are
composed primarily of proteins and lipids (phospholipids). Protein-to-lipid
ratios of bacterial plasma membranes are approximately 3:1, close to those for
mitochondrial membrane.

Plasma membranes are the site of active transport, respiratory chain

components, energy-transducing systems, the ATPase of the proton pump, and
membrane stages in the biosynthesis of phospholipids, peptidoglycan, LPS, and
capsular  polysaccharides.  The  bacterial  cytoplasmic  membrane  is  a
multifunction  structure  similar  to  mitochondrial  transport  and  biosynthetic
functions  of  eukaryotic  cells.  The  plasma  membrane  is  also  the  anchoring site
for the bacterial DNA.

2. Mesosomes

The mesosomes are tubular-vesicular membrane structures found in Gram-

positive bacteria which are formed by an invagination of the plasma membrane.
These structures equivalent to bacterial mitochondria; and may be related to
events in the cell division cycle.

3. Other Intracellular Components

Ribosomes of the 70S type; ribonucleoprotein particles are not arranged on a
membranous rough endoplasmic reticulum as they are in eukaryotic cells, they
are found in the cytoplasm.

Endospores are highly heat-resistant, dehydrated cells formed

intracellularly  in  some  bacteria  like  Bacillus  and  Clostridium.  Sporulation,  is
the  process  of  forming  endospores  because  of  many  biochemical  and
morphologic changes begins in the stationary phase of the vegetative cell cycle
due  to  decrease  nutrients  (sources  of  carbon  and  nitrogen).  Also  formation  of
unusual  peptidoglycan  which  contains  calcium  dipicolinate,  help  in  resistance
to heat, radiation, pressure, and organic solvents.

The spore protoplast, or core, contains a complete chromosome,

ribosomes, and energy generating components. During germination, the spore
wall becomes the vegetative cell wall and cortex will be released.

 Genetic Information In Bacteria (Genome)

Genetic material, genome or the genotype replicate in the parental organism

to  give  the  daughter  organism  a  copy  of  the  genetic  material.  Expression  of
genetic  material  determines  the  observable  characteristics  (phenotype)  of  the
organism.  Some  bacteria  have  the  DNA  chromosome  as  the  only  genetic
material (genome)  in bacterial cells, like Escherichia coli. Other bacteria have
additional genetic materials, such as plasmids and bacteriophages.

1. Chromosomal DNA

Bacterial genomes differ in size from about 0.4 x 109 to 8.6 x 109 daltons

(Da), some of the smallest found in the obligate parasites (Mycoplasma) and the
largest  in  Myxococcus.  The  amount  of  DNA  in  the  genome  determines  the
maximum  amount  of  information  that  it  can  encode.  Most  bacteria  have  a
haploid genome, a single chromosome consisting of a circular, double stranded
DNA  molecule.  However  linear  chromosomes  have  been  found  in  Gram-
positive Borrelia and Streptomyces spp.,

The typical genome to be studied is the E coli genome, it is sufficient to

code  for  thousand  polypeptides  of  average  size  (40  kDa  or  360  amino  acids),
the  DNA  is  supercoiled  and  tightly  packaged  in  the  bacterial  nucleoid.    The
time  required  for  replication  of  the  entire  chromosome  is  about  40  minutes,
which is approximately twice the shortest division time for this bacterium.

The replication of chromosomal DNA in bacteria is complex and involves

many different proteins, in rapidly growing bacteria a new round of
chromosomal replication begins before an earlier round is completed. Thus, the
chromosome is replicating at more than one point. Bacterial chromatin does not
contain basic histone proteins, but low-molecular-weight polyamines and
magnesium ions may give a function similar to that of eukaryotic histones.

2. Plasmids

Plasmids are extrachromosomal genetic elements in bacteria. They are

smaller than the bacterial chromosome. Plasmids usually encode properties that
are not essential for bacterial viability, and replicate independently of the
chromosome.

Most plasmids are supercoiled, circular, double-stranded DNA molecules,

The Large plasmids (Conjugative plasmids) promote transfer of the bacterial
chromosome from the donor bacterium to other recipient bacteria are also called
fertility plasmids. The small plasmids are usually non-conjugative.

Many plasmids control medically important properties of pathogenic

bacteria, including resistance to antibiotics, production of toxins, and synthesis
of cell surface structures required for adherence or colonization. Plasmids that

determine resistance to antibiotics are called R plasmids (or R factors) like the
plasmid  in  Staphylococcus  aureus.  Some  toxins  encoded  by  plasmids  include
heat-labile  and  heat-stable  enterotoxins  of  E.  coli,  exfoliative  toxin  of
Staphylococcus aureus, and tetanus toxin of Clostridium tetani. Some plasmids
have no recognizable effects on the bacterial cells that have them.

3. Bacteriophages

Bacteriophages (bacterial viruses, phages, viruses that infect bacteria) are

infectious  agents  for  bacteria  that  live  as  obligate  intracellular  parasites  inside
bacteria,  consist  proteins  plus  nucleic  acid  (DNA  or  RNA,  but  not  both).  The
proteins  of  the  phage  particle  form  a  protective  shell  (capsid)  surrounding  the
tightly packaged nucleic acid genome.

Phage genomes are different in size, consist of double-stranded DNA,

single-stranded DNA, or RNA. Phage genomes, like plasmids, encode functions
required for the replication of virus in bacteria, they also encode capsid proteins
and nonstructural proteins required for phage life cycle.

Bacteriophages attached on bacteria surface under electron

microscope

For example of phages responsible of virulence: the production of

diphtheria toxin by Corynebacterium diphtheriae, erythrogenic toxin by
Streptococcus pyogenes (group A beta-hemolytic streptococci), botulinum toxin
by Clostridium botulinum, and Shiga-like toxins by E. coli.

Exchange of Genetic Information

A  phenomena  of  medical  importance  that  involve  exchanges  of  genetic
information  or  genomic  rearrangements  include  the  rapid  antibiotic  resistance
plasmids,  flagellar  phase  variation  in  some  bacteria  like  Salmonella,  and
antigenic  variation  of  surface  antigens  like  in  Neisseria  and  Borrelia.  These
processes involve Transformation, transduction, and conjugation

1. Transformation

In transformation, pieces of DNA (at least 500 nucleotides in length)

released  from  donor  bacteria  are  taken  up  directly  from  the  extracellular
environment by other recipient bacteria. Recombination occurs between single
molecules  of  transforming  DNA  and  the  chromosomes  of  recipient  bacteria.
Transformation  was  discovered  in  Streptococcus  pneumoniae  and  occurs  in
other  bacterial  genera  including  Haemophilus,  Neisseria,  Bacillus,  and
Staphylococcus.

2. Transduction

In transduction, bacteriophages function as vectors to carry a small segment

of  the  bacterial  genome  (DNA  genes)  from  donor  bacteria  into  other  recipient
bacteria  by  infection.  When  phage  infects  a  recipient  cell,  expression  of  the
transferred  donor  genes  occurs  and  complete  transduction  is  characterized  by
production  of  new  proteins  (donor  phenotype).  In  abortive  transduction  the
donor DNA fragment does not replicate.

3. Conjugation

In conjugation, direct contact between the donor and recipient bacteria leads

to establishment of a cytoplasmic bridge between them and transfer of part or
all of the donor genome to the recipient. Donor bacteria must have conjugative
plasmids called fertility+ plasmids, sex plasmids or F factor, e.g. F plasmid of
E. coli.

Recombination DNA and Gene Cloning (Genetic Engineering)

Recombination involves breakage and joining of DNA molecules to form

hybrid, recombinant molecules. Several kinds of recombination have been
identified by the help of specific enzymes that act on DNA (e.g., exonucleases,
endonucleases, polymerases, ligases) participate in recombination .

Many methods are available to make hybrid DNA molecules in vitro

(recombinant DNA). Cloned genes can be expressed in appropriate host cells,
and the phenotypes that they show can be determined.

Applications of DNA cloning are expanding rapidly in all fields of

biology and medicine :

 In medical genetics such applications range from the prenatal diagnosis of

inherited human diseases to the characterization of oncogenes and their
roles in carcinogenesis .

 Pharmaceutical applications include production from cloned human genes

of biologic products with therapeutic purposes, such as hormones,
interleukins, and enzymes .

 Applications in public health and laboratory medicine include

development  of  vaccines  to  prevent  specific  infections  by  polymerase
chain reaction (PCR). The PCR process uses oligonucleotide primers and
DNA  polymerase  to  amplify  specific  target  DNA  sequences  during
synthesis  in  vitro,  help  to  detect  target  DNA  sequences  in  clinical
specimens with great sensitivity.