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Bacterial Morphology and Structure Xiao-Kui Guo PhD SIZE OF BACTERIA 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.

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bacterial morphology and structure

Bacterial Morphology and Structure

Xiao-Kui Guo PhD

size of bacteria
  • 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.

  • 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
structure of bacteria
Structure of Bacteria

Essential structures cell wall 细胞壁cell membrane 细胞膜Cytoplasm 细胞质nuclear material核质

Particular structures

capsule 荚膜

flagella 鞭毛

pili 菌毛





Cell membrane

Cell wall

Gram +


Gram -



Cell (inner) membrane

Outer membrane


Cell wall

1884: Christian Gram: First publication for the Gram stain method)

Editor's note: I would like to testify that I have found the Gram method to be one of

the best and for many cases the best method which I have ever used for staining


Gram, C. 1884. Ueber die isolirte Farbung der Schizomyceten in SchnittÄund Trockenpraparaten. Fortschritte der Medicin, Vol. 2, pages 185-189.

cell wall
Cell wall
  • Situation: outmost portion. 15-30nm in thickness, 10%-25% of dry weight.
cell wall common peptidoglycan layer
Cell wall :Common peptidoglycan layer
  • A backbone of N-acetyl glucosamine and N-acetylmuramic 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
functions of cell wall
Functions of Cell Wall
  • Maintaining the cell's characteristic shape- the rigid wall compensates for the flexibility of the phospholipid membrane and 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 determinantsof the cell surface。
  • Resistance of Antibiotics
wall less forms of bacteria
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-less bacteria are often produced.
  • Usually these treatments generate non-viable organisms. Wall-less bacteria that can not replicate are referred to as spheroplasts (when an outer membrane is present) or protoplasts (if an outer membrane is not present).
  • Occasionally wall-less bacteria that can replicate are generated by these treatments (L forms).
cell membrane
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 are specialized structures formed by convoluted inveigh-nations of cytoplasmic membrane, and divided into septal and lateral mesosome.
  • 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: extrachromosomal genetic elements
  • Inclusions: sources of stored energy, e,g volutin

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.


Inclusions of Bacteria

  • Inclusions are aggregates of various compounds that are normally involved in storing energy reserves or building blocks for the cell. Inclusions accumilate when a cell is grown in the presence of excess nutrients and they are often observed under laboratory conditions.


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


  • Protection from phagocytic engulfment.
  • Resistance to drying.
  • Depot for waste products.
  • Reservoir for certain nutrients.
  • protection
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.

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.


  • Identification of Bacteria
  • Pathogenesis
  • Motility of bacteria
  • 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
  • Common pili or fimbriae: fine , rigid numerous, related to bacterial adhesion
  • Sex pili: longer and coarser, only 1-4, related to bacterial conjugation
endospores spores
Endospores (spores)
  • Identification of Bacteria
  • Pathogenesis
  • Resistance
  • Dormant cell
  • Resistant to adverse conditions
  • - high temperatures
  • - organic solvents
  • Produced when starved
  • Contain calcium dipicolinate
  • DPA, Dipicolinic acid
  • Bacillus and Clostridium



  • Light Microscope
  • Electron Microscope
  • Darkfield Microscope
  • Phase Contrast Microscope
  • Fluorescence Microscope
  • Cofocal Microscope)

Staining Methods

  • Simple staining;
  • Differential staining ( Gram stain, Acid-fast stain),
  • Special staining( Negative stain, Spore stain, Flagella stain)