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What are bacteria?

The origin of the term ‘bacteria’ can be traced to 19th

century German botanist ,Ferdinand Cohn (1828-98),

though initially they were described as microscopic

animalcules by Antony van Leeuwenhoek (1632-1723) in

the 17th century. Bacteria comprise a group of single-

celled prokaryotic microorganisms, existing either

independently, or as parasites virtually in all

environments, including those which apparently are

inhospitable for rest of the life forms. They are the most

abundant and diversified group of differently shaped

micro-organisms on Earth, adapted for different living

conditions (air, soil, water), ranging from the most

ancient lineage of extreme thermophilic chemosynthetic

autotrophs to the lineages of photosynthetic autotrophs

represented by cyanobacteria. They are immensely

important for their life-sustaining ecological and

economic roles in different aspects of human well being.

Phylogenetic analysis through molecular approaches

reveals 12 major lineages or kingdoms of Bacteria. A

pinch of rich soil in your palm contains billions of

bacteria, representing around 10,000 different species.

However, this immensely diverse world still remains

largely shrouded in mystery because of cultural,

technological and taxonomic impediments. Nevertheless,

recent molecular techniques and metagenomic

approaches have helped in getting useful insights about

their structural organization and community profiling.

Bacteria: Shape and size of bacteria

Due to the presence of a rigid cell wall, bacteria maintain

a definite shape, though they vary as shape, size and

structure. When viewed under light microscope, most

bacteria appear in variations of three major shapes (Fig.

1). the rod (bacillus), the sphere (coccus) and the spiral

type (vibrio). In fact, structure of bacteria has two

aspects, arrangement and shape. So far as the

arrangement is concerned(Fig. 2), it may Paired (diplo),

Grape-like clusters (staphylo) or Chains (strepto). In

shape they may principally be Rods (bacilli), Spheres

(cocci), and Spirals (spirillum).

Bacillus: The length of the rod or bacillus may be from

as long as 20µm to as short as 0.5 µm. Rods vary in

shape too, from slender (e.g. Typhoid fever causing

bacterium), rectangular (e.g. Anthrax agents), to club

shaped (e.g. Diptheria bacillus

). While most rods occur

singly, some form long chains called Streptobacilli, e.g.

the bacterium causing a particular form of the rat bite

fever.

Fig. 1. Common shapes of bacteria

Coccus: The diameter of the spherical bacteria is

approximately of the order of 0.5µm, and the spheres

are usually round in shape, though they may vary from

oval, elongated or indented on one side. Cocci may be of

three types:

Diplococci :{cocci that remain in pairs after

reproduction}, e.g. bacteria causing gonorrhoea.?

Streptococci :{cocci consisting of chains of diplococcic},

e.g. those found in intestines or involved in strep throat.

Sarcina type :{a cube like packet of 8 cocci}, e.g

Micrococcus luteus.

Staphylococcus :{irregular grape like cluster}, e.g.

Staphylococcus aureus.

Fig. 2: Different arrangements of cocci bacteria.

Vibrio :{spiral bacteria which may either be curved rods,

e.g. Vibrio cholrea, or corkscrew shaped, e.g.

Spirochetes}. Spiral bacteria may be from 1-3 μm in

length and 0.3-0.6 μm in width. (Fig. 3)

Fig. 3. A view of the spiral bacteria.

Ultra-structural details of bacteria

The simple structural organization of a typical bacterial

cell essentially includes cell wall, cell membrane,

cytoplasm, and chromosome with the genetic material.

The other special components of a bacterial cell include

capsule, flagella, and pili. In addition, bacterial cells also

possess some specialized inclusion bodies varying in

composition and function.

Cell walls

It is situated as the outermost covering of the bacterial

cell, ranging in thickness from 15-30nm and comprising

about 10%-25% of dry weight of the bacterium. Cell wall

is characteristically composed of a common

peptidoglycan, a huge polymer of interlocking chains of

identical monomers (Fig. 4a). N-acetylglucosamine (NAG)

and N-acetlymuramic acid (NAM), basically the two

derivatives of glucose molecule that are connected by

interpeptide bridges, comprise the backbone of

peptidoglycan layer. Peptidoglycan occurs in multiple

layers, connected by side chains of 4 amino acids, and

forms a supporting net around the bacterium (Fig. 4b) .

However, different bacteria can have different amino

acids in the tetrapeptide chain, as well as different cross

links. While there is a set of identical tetrapeptide side

chain attached to N-acetyl-muramic acid, but there may

be different components and binding modes in Gram

positive and Gram negative bacteria. In Gram positive

bacteria,the third amino acid is lysine, while in Gram

negative bacteria, it is diaminopalmelic acid. Important

differences between the cell wall of Gram positive and

Gram negative bacteria are given in Table 1.

a.

b.

Fig.4a,b: N-acetylglucosamine (NAG) and N-acetlymuramic acid (NAM), the backbone of peptidoglycan layer connected by interpeptide bridges.? Table 1: Differences between cell wall of Gram positive and Gram negative bacteria.

Gram positive bacteria

Gram negative bacteria

Peptidoglycan layer very thick (25 nm)

Peptidoglycan layer thin (3 nm)

Peptidoglycan contains Tiecholic acid, an additional polysaccharide

Tiecholic acid absent

About 60-90% of cell wall is peptidoglycan

Only 10-20% of cell wall is peptidoglycan

Cell wall contains very little lipids and proteins

Cell wall contains many lipids and

proteins They retain crystal

violet iodine complex in Gram staining due to

plenty of peptidoglycan and high thickness

Gram stain is lost due to thinnes of cell wall

and abundance of lipo-proteins and lipopolyaccharides

Outer membrane absent

Outer membrane present

Periplasmic space absent

Periplasmic space present

Fig.5. Depiction of the outer membrane and periplasmic space in the cell wall. Functions of Cell Wall The most important functions of the cell wall in bacteria

include:

a. Maintaining the characteristic shape of the cell. In

fact, the rigid wall compensates for the flexibility of

the phospholipid membrane, and keeps the cell from

assuming a spherical shape ,

b. Countering the effects of osmotic pressure,

c. Providing attachment sites for bacteriophages,

d. Providing a rigid platform for surface appendages.

Flagella, fimbriae, and pili all emanate from the wall

and extend beyond it,

e. Plays an essential role in cell division,

f. Offers sites of major antigenic determinants of the

cell surface,and

g. Impart resistance to antibiotics, except those which

are wall

specific

in

nature.

Gram positive bacteria

Gram negative bacteria

Peptidoglycan layer very thick (25 nm)

Peptidoglycan layer thin (3 nm)

Peptidoglycan contains Tiecholic acid, an additional polysaccharide

Tiecholic acid absent

About 60-90% of cell wall is peptidoglycan

Only 10-20% of cell wall is peptidoglycan

Cell wall contains very little lipids and proteins

Cell wall contains many lipids and

proteins They retain crystal Gram stain is lost due

Flagella

A

flagellum (plural- flagella) is a locomotory tail-like

projection, emanating from the cell body of certain

bacteria and facilitates specific types of movement in

them. While flagella are also found in some eukaryotes,

bacterial flagella differ markedly from them in respect of

their composition, structure, and mechanism of

propulsion. The prokaryotic flagellum is about 1/10th of

the eukaryotic flagellum and is about 10-20µm in length.

Bacterial flagella also differ from eukaryotic flagella in

lacking microtubules with (9+2) arrangement and a

plasma membrane. About half of all the known bacteria

are motile due to the presence of flagella. Flagella vary in

number and placement, and their arrangement is an

important basis for bacterial classification. Depending

upon the flagellar arrangements (Fig. 6), bacteria can be:

violet iodine complex in Gram staining due to

plenty of peptidoglycan and high thickness

to thinnes of cell wall and abundance of

lipo-proteins & lipopolyaccharides

Outer membrane absent

Outer membrane present

Periplasmic space absent

Periplasmic space present

Monotrichous: with just one flagellum at one end, e.g.

Pseudomonas aeruginosa

Lophotrichous: with tuft of flagella at one end, e.g.

Pseudomonas fluorescence

Amphitrichous: with tufts of flagella at both ends, e.g.

Aquaspirillum serpens

Peritrichous

: with flagella all around bacteria, e.g.

Salmonella typhi.

Fig. 6. Different flagellar arrangements in bacteria.

Bacterial flagella are composed of long, rigid strands of

protein called Flagellin, arranged in chains and wound

around a triple helix, with a hollow central core. Each

flagellum is attached to cell membrane by a basal region

consisting of a protein other than flagellin. A flagellum is

composed of 3 parts (Fig 7):

a. basal body (associated with the cell membrane

and the cell wall), consisting of a central rod or

shaft surrounded by a set of rings.

b. A short hook,and

c. A helical filament (several times as long as the

cell), and

Gram negative bacteria have two pairs of rings (one in

cell membrane & one in cell wall). Gram positive bacteria

have just one pair (one in cell membrane & 1 in cell

wall). When flagella bundle together, they rotate counter

clock wise and allow bacteria to run in a straight line.

when flagella rotate clockwise, the flagellar bundles come

apart, causing the bacterium to tumble randomly.

Fig. 7. Structure of the flagellar components. Pilli

Pilli are hollow, non-helical, tiny, hair like filamentous

appendages, composed of protein subunits called pilin.

They are thinner, shorter and more numerous than

flagella. Pili are not generally involved in the movement,

but allow attachment to other bacteria and surfaces,

though some pilli called as IV pili, generate some motile

forces. They facilitate passage of genetic material

between bacteria (e.g. F-pilus), or help in their flotation

to increase buoyancy and reach oxygen rich surface

waters to form the Pellicle (scum on water). Pilli that help

in attachment are called as fimbrae, which are spread

over the surface ,or may be located at poles of the cell.

Fig. 8: Pilli around bacteria and facilitating bacterial copulation. Capsule and glycocalyx The cell wall in bacteria is generally surrounded by a

sticky, gelatinous, and protective layer, called as capsule,

which is formed mainly from polysaccharides and

polypeptides, or the both. It is found only in certain

bacteria, e.g. mostly in Bacilli and Cocci, but not in Spiral

bacteria. Composition of the capsule is specific to

bacteria that secretes it. A relatively thinner layer bound

less tightly to cell the wall, is called as Slime layer.

However, the currently accepted inclusive term for all

the polysaccharide containing substances found external

to the cell wall from the thinnest slime layer to the

thickest capsule is the Glycocalyx.

Functions of Glycocalyx The main functions of the glycocalyx include:

i. Serves as a buffer between the cell and its external

environment

ii. Prevents the cell against drying and dehydration due

to high water content,

iii. Helps bacteria trap nutrients and protects nutrients

from flowing away

iv. Sticky sugars allow attachment of bacteria to host

cells or tissues, rock surfaces, plant root hairs, etc.

v. Facilitates disease establishment, because

encapsidated bacteria can not be easily

phagocytosed, and

vi. It may block the attachment of the bacteriophages.

Cell Membrane

All bacteria do possess a lipoproteinaceous, bilayer

membrane (Fig. 9), which is selectively permeable,

flexible and dynamic. While the relative proportion of

lipids and proteins may vary between species, generally

membranes comprise of 60% proteins and 40% lipids.

However, there is a striking difference between

phospholipids of eubacterial and archaeobacterial

membranes, which mainly influences their differential

ability to thrive in different environmental conditions.

While in eubacteria phospholipids are phosphoglycerides,

in which straight chain fatty acids are ester linked to

glycerol. in case of archaeobacteria, the lipids are

polyisoprenoid, branched chain lipids, in which long

chain, branched alcohols (phytanols) are ether linked to

glycerol.

Fig. 9: Structure and components of the bacterial cell

membrane.

Functions of the cell membrane

The most important functions of the membrane include:

a. It regulates movement of materials into and out

of the cells ,

b. It is a seat of cellular respiration ,

c. Thylakoids in photosynthetic bacteria occur here,

d. It provides location for enzymes used in cell wall

synthesis,

e. It serves as anchor for the attachment of DNA

during replication, and

f. Flagellar appendages are based in membrane.

There is also an extensive internal membrane system in

bacteria, discernible in the form of mesosomes.

Mesosomes are invaginations of the plasma membrane,

in the shape of vesicles, tubules or lamellae, present in

both gram positive and gram negative bacteria, being

more prominent in the former. So far as the functions of

mesosomes are concerned, they are ,not still exacrly

known though a few functions attributed to them are as

given below:

a. May be involved in cell wall formation during division,

b. May play a role in chromosome replication and

distribution to daughter cells,and

c. May be involved in some secretory processes

It is pertinent to mention here that some

bacteriologists consider mesosomes just as artifacts

Cytoplasm

The cytoplasm of a bacterial cell generally comprises

about 80% water and 20% salts, proteins,

carbohydrates, lipids, and nucleic acids, etc. Cytoplasmic

streaming as exhibited by eukaryotic cells, is

characteristically absent in bacteria. Being prokaryotes,

there are no organelles, such as mitochondria, golgi

bodies, etc, but ribosomes of 70S types are attached

either to plasma membrane or present in the matrix of

cytoplasm. There are also gas vesicles, especially in

aquatic bacteria, for buoyancy and optimal utilization,

and inhabitation of appropriate micro-environments.

Some inclusion bodies (avariety of small bodies, including

granules and vesicles) are found in bacteria. Granules

contain very densely packed, compact substances, which

do not dissolve in cytoplasm; each granule contains

specific substances, such as glycogen (for energy),

polyphosphates or volutin granules (for use in

starvation), polyhydroxy butyrate granules (PHBs) for

carbon storage, and carboxysomes ( the sites for CO2

fixation). Specific kinds of proteins, called Chaperones

(heat shock proteins) help bacteria withstand heat shocks

and even osmotic shocks.

Bacterial Genome Bacteria do not have membrane bound nucleus; instead

they have a nucleoid, the part where the genetic material

is generally concentrated in the cytoplasm. Bacterial

genome (Fig. 10) is haploid, and there are certain

advantages of having this, as manifested in terms of

more efficiency, quicker growth and fast rate of

mutations. In contrast to the linear chromosomes as

found in eukaryotic cells, bacteria generally have single

circular chromosomes in addition to Plasmids, the extra

circular DNA capable of independent replication.

However, not all bacteria do have a single circular

chromosome, some do have multiple circular

chromosomes, and many bacteria even have linear

chromosomes and linear plasmids. In case of a circular

DNA molecule there are no free ends (as seen in most

eukaryotes), which may be problematic to cells for their

DNA replication and stability. Plasmids vary in number as

well as size. In size they vary from 1 to over 1,000

kilobase pairs and in number they can range in the single

cell from one to even thousands in some cases. While

plasmids have assumed incredible significance in

biotechnology and genetic engineering as vectors, they

are mainly associated with horizontal gene transfer

during conjugation in bacteria.

Fig. 10. Bacterial genome

Endospores

These are the specialized round or oval structures

resistant to heat, irradiation, cold, and remain viable

even after boiling for one hour or more;they are formed

by bacteria generally to overcome harsh conditions. If

rest of cell dies because of harsh conditions, endospore

will survive and make a new cell when environment is

favourable. This is because endospores have a tough and

resistant outer covering made of keratin, (Fig. 11) which

also resists staining; so specialized procedures are

necessary to stain bacterial endospores. Endospores are

formed only by a few genera of bacteria, such as Bacillus

and Clostridium. The location (central, sub-terminal,

terminal) of endospores is an important taxonomic

character, and used in classification of bacteria.

Fig. 11. Endospores of bacteria