Bacterial Cell Structure

Dr Shireen RafiqMBBS, M.Phil, Ph.D

2

Microbiology

• Agents of human infection:– Bacteria Prokaryotes– Helminths Eukaryotes– Protozoa Eukaryotes– Fungi Eukaryotes– Viruses Noncellular

• Difference between Eukaryotes and Prokaryotes

• Structure

• Replication

• Nucleic acid

4

Earliest Prokaryotes

• Most numerous organisms on Earth

• Include all bacteria• Earliest fossils date 2.5

billion years old

BACTERIA

• Bacteria are large group of single celled prokaryotic microorganisms

• 10 times as many bacteria cells in the human flora as there are human cell in the body

Shape of Bacteria

• Three basic shapes• Cocci: streptococci, Staphylococci,

Diplococci• Bacilli: E.coli, Klebsiella, Bacillus.• Spirochetes: Treponema, Borrelia

Bacterial Size • Bacteria range in size from about 0.2 to 5 µm.

• The smallest bacteria (Mycoplasma) are about the same size as the largest viruses (poxviruses) and are the smallest organisms capable of existing outside the host.

• The longest bacteria rods approach the size of some yeasts and human R.B.Cs

9

According to staining

• Gram positive Thick peptidoglycan layer and teichoic acid

• Gram negative Thin peptidoglycan layer and

lipopolysaccharide- endotoxin

• Acid fast bacilli Mycolic acid (lipids)

• Some bacteria are variable in shape

• PLEOMORPHIC----many shaped

• Shape of the bacteria is determined by its rigid cell wall

• The microscopic appearance of bacterium is most important criteria for its identification

12

pH requirements• Most grow best at pH of 6.5 to 7.0• Many act as decomposers

recycling nutrients• Some cause disease (Pathogenic)

The Prokaryote

• Structural Components MACROMOLECULE SUBUNIT POSITION IN CELL

PROTEIN Amino Acid Flagella, pili, cell wall, cytoplasmic membrane, ribosomes, cytoplasm

POLYSACCHARIDE Sugar/Carbohydrate Capsule, Inclusions, Cell wall

PHOPHOLIPID Fatty Acid Membranes

NUCLEIC ACID(DNA/RNA)

Nucleotide DNA, Nucleoid, Plasmids, Ribosomes,

Structural Components

Prokaryotes have 5 essential components• Nucleoid (DNA)• Ribosomes• Cell membrane• Cell wall• Surface layer (Capsule)• Appendages

Structural Components

sugars (carbohydrates)

BACTERIAL STRUCTURECOVERING LAYERS

• Cell wall• Peptidoglycan

Sugar back bone with peptide side chains,which are cross linked

Rigidity osmotic protection , site of action of antibiotic, lysozyme degrade.

Outer membrane Gram Negative bacteria

Lipid A

Polysaccharide

Toxic component of endotoxin.Surface antigen.

Surface fiber on Gram Positive bacteria

Teichoic acid Surface antigen

18

Protection

• Cell Wall made of Peptidoglycan

• May have a sticky coating called the Capsule for attachment to host or other bacteria

FUNCTION OF CELL WALL• Maintaining the cell's characteristic shape

• Countering the effects of osmotic pressure

• Providing attachment sites for bacteriophages-teichoic acids

• Providing a rigid platform for surface appendages- flagella, fimbriae

Peptidoglycan

COMPARISONProperty Gram Positive Gram Negative

Thickness of wall 20-80 nm 10 nm

Number of layers in wall 1 2

Peptidoglycan content >50% 10-20%

Teichoic acid in wall + -

Lipid and lipoprotein content 0-3% 58%

Protein content 0% 9%Lipopolysaccharide 0 13%Sensitive to penicliiin + - (not as much)

Digested by lysozyme + - (not as much)

Properties of cell wall

• Gram negative bacteria contains endotoxin---lipopolysaccharide

• Polysaccharides and proteins are antigens• Porin proteins helps entry of hydrophilic

molecules• Teichoic acid are fibers on outer surface of

gram positive ---ability to induces septic shock

Cell Membrane

• Composed of phospholipid bilayer

• FUNCTIONS• Active transport• Energy generation---oxidative phosphorylation• Synthesis of precursors of cell wall• Secretion of enzymes and toxins

25

• Infoldings of cell membrane carry on photosynthesis & cellular respiration

• Infoldings called Mesosomes

26

MesosomesMESOSOME

27

Sticky Bacterial Capsule

Plasmids

• Molecules of DNA that are found in bacteria separate from the bacterial chromosome.

• A circular molecule only much SMALLER than the genomic DNA

• REPLICATE AUTONOMOUSLY from the genomic chromosome. Often there are MANY PLASMID COPIES present in one cell. Further, a cell may contain SEVERAL DIFFERENT PLASMIDS or it may contain NO PLASMIDS at all. Plasmids generally carry genes that are NOT ESSENTIAL for a cell's survival

• May carry genes for ANTIBIOTIC RESISTANCE

Transposons

• Transposons are pieces of DNA move from one site to another ---- within or between the DNAs of bacteria plasmid or bacteriophage.

• Nick name as Jumping genes• Genes for one or more (usually more) proteins

imparting resistance to antibiotics. When such a transposon is incorporated in plasmid, it can leave the host cell and move to another. This is the way that the alarming phenomenon of multidrug antibiotic resistance spreads so rapidly.

Appendages

Flagella: FlagellinFunction: Motility/chemotaxis

31

Flagella• Bacteria that are

motile have appendages called flagella

• Attached by Basal Body

• A bacteria can have one or many flagella

32

Flagella• Made of Flagellin• Used for Classification• Monotrichous: 1 flagella• Lophotrichous: tuft at

one end• Amphitrichous: tuft at

both ends• Peritrichous: all around

bacteria

33

Pili• Short protein appendages PILIN• Smaller than flagella• Adhere bacteria to surfaces• Used in conjugation for Exchange of

genetic information• Aid Flotation by increasing buoyancy

34

Pili in Conjugation

35

Bacterial Shapes

36

Shapes Are Used to Classify• Bacillus: Rod shaped• Coccus: Spherical (round)• Vibrio: Comma shaped with flagella• Spirillum: Spiral shape• Spirochete: wormlike spiral shape

37

Grouping of Bacteria

• Diplo- Groups of two• Strepto- chains• Staphylo- Grapelike clusters

39

40

Bacillus and E. coli

Spirochetes

ACCORDING TO STAINING

GRAM STAINING

Crystal violet

Gram's iodine

Decolorise with acetone

Counterstain withe.g. methyl red

Gram-positives appear purple

Gram-negatives appear pink

The Gram Stain

Gram-positive rods

Gram-negative rods

Gram-positive cocci

Gram-negative cocci

FORMATION OF BACTERIAL SPORE

• Found in Gram positive bacteria

• Tough, heat resistant

• Peptidoglycan > Picolinic acid