Bacterial Morphology and Structure

Bacterial cells Morphology

• Cocci: sphere, 1μm• Bacilli: rods , 0.5-1 μm in width -3 μm in length• Spiral bacteria: 1~3 μm in length and 0.3-

0.6 μm in width.• Unit for measurement Micron or micrometer, μm: • 1μm=10-3mm• Size: Varies with kinds of bacteria, and also related 

to their age and external environment

Bacterial Shapes and Arrangements

Structure of BacteriaEssential structure• Cell wall• cell membrane• Cytoplasm• Nuclear material Particular structures• Capsule•  flagella• pili • Spore

Gram Stain• 1884: Christian Gram: First publication for the Gram stain method.• Gram staining is a common technique used to differentiate two large 

groups of bacteria based on their different cell wall constituents. The Gram stain procedure distinguishes between Gram positive and Gram negative groups by coloring these cells red or violet.

Cell Wall of Bacteria

Cell Wall: common peptidoglycan layer

• A  back  bone  of  N-acetyl  glucosamine  and  N-acetyl muramic acid: Both discovered in Gram positive and Gram negative bacteria.

• A  set  of  identical  tetrapeptide  side  chain attached  to  N-acetyl-muramic  acid:  different   components  and  binding  modes  in  Gram positive and Gram negative bacteria.

• A set of identical peptide cross bridges: only in Gram positive bacteria

Special Components of Gram Positive Cell Wall

• Teichoicacid

Special Components of Gram Negative Cell Wall

• lipopolysaccharide

Functions of Cell Wall• Maintaining the cell's characteristic shape • The rigid wall compensates for the flexibility of the phospholipid 

membrane •  keeps the cell from assuming a spherical shape• Countering the effects of osmotic pressure• Providing attachment sites for bacteriophages• Providing a rigid platform for surface appendages- flagella, 

fimbriae, and pili all emanate from the wall and extend beyond it• Play an essential role in cell division• Be the sites of major antigenic determinants of     The cell surface• Resistance of Antibiotics

Wall-less forms of Bacteria• When bacteria are treated with 1) enzymes that are 

lytic for the cell wall e.g. lysozyme or 2) antibiotics that interfere with biosynthesis of peptidoglycan, wall-lles bacteria are often produced

• Usually these treatments generate non-viable organisms. Wall-less bacteria that can not replicate are referred to as sphero-plasts (when an outer membrane is present) or protoplasts(if an outer membrane is not present).

• L-form bacteria, also known as L-phase bacteria, L-phase variants, and cell wall-deficient (CWD) bacteria, are strains of bacteria that lack cell walls

Cell membrane

• Site of biosynthesis of DNA, cell wall polymers and membrane lipids. Selective permeability and transport of solutes into cells

• Electron transport and oxidative phosphorylation• Excretion of hydrolytic exoenzymes

Mesosomes

Mesosomes are folded invaginations in the plasma membrane of bacteria  that are produced by the chemical fixation techniques used to prepare samples for electron microscopy.

            Although  several  functions were  proposed  for  these  structures  in  the 1960s, and are no longer considered to be part of the normal structure of bacterial cells.

•  Cytoplasm Composed largely of water, together with proteins, nucleic acid, lipids and small amount of sugars and salts 

• Ribosomes: numerous, 15-20nm in diameter with 70S; Distributed throughout the cytoplasm; sensitive to streptomycin and erythromycin site of protein synthesis  

• Plasmids: extra-chromosomal genetic elements 

•  Inclusions: sources of stored energy, e,g volutin

Plasmid• Plasmids are small， circular/line，• extrachromosomal， double-stranded DNA  molecules。• They are capable of self-replication and contain genes that confer 

some properties， such as antibiotic resistance， virulence factors。• Plasmids are not essential for cellular survival. 

Cell Inclusions and Storage Granules

• Bacteria have different methods of nutrient        storage that are employed in times of plenty, for          use in times of want.• Inclusions are aggregates of various  compounds        that are normally involved in storing energy reserves          or building blocks for the cell • Inclusions accumulate when a cell is grown in        the presence excess nutrients and they are        often observed under laboratory conditions.

Nucleus• Lacking nuclear membrane•  absence of nucleoli, hence      known as nucleic material or nucleoid• one to several per bacterium.

Capsules and slime layers• These are structures surrounding the outside of the cell envelope.• They usually consist of polysaccharide; however, in certain bacilli they are composed of a polypeptide (polyglutamic acid). • They are not essential to cell viability and some strains within a species will produce a capsule, whilst others do not.• Capsules are often lost during in vitro culture.

Functions• Attachment• Protection from phagocytic engulfment.• Resistance to drying.• Depot for waste products.• Reservoir for certain nutrients.• Protection• A bacterial capsule is similar, but is a well ordered structure that is resistant to washing off

Flagella

• Some bacterial species are mobile and possess locomotory organelles - flagella. • Flagella consist of a number of proteins including flagellin• The diameter of a flagellum is thin, 20 nm, and 

long with some having a length 10 times the diameter of cell.• Due to their small diameter, flagella 

cannot be seen in the light microscope unless a special stain is applied.• Bacteria can have one or  more flagella arranged in clumps 

or spread all over the cell. Functions• Identification• of Bacteria• Pathogenesis•  Motility of bacteria

Flagellum Structure

Pili• Pili are hair-like projections of the cell • They are known to be receptors for certain     bacterial viruses • Chemical nature is pilin• Classification and Function   a. Common pili or fimbriae: fine , rigid numerous,      related to bacterial adhesion b. Sex pili: longer and coarser, only 1-4, related to      bacterial conjugation

Endospores(spores)• A bacterial spore is a spore or spore- like structure produced 

by bacteria • Spore formation in bacteria is not a method of reproduction 

but simply a method of surviving in an   unfavorable conditions• Produced when starved• Contain calcium dipicolinate• DPA, Dipicolinic acid• E.g.Bacillus and Clostridium• Dormant cell• Resistant to adverse conditions - high temperatures    - organic solvents

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