Bacterial Morphology and Structure

SIZE OF BACTERIAUnit for measurement : Micron or micrometer,m: 1m=10-3mm Size: Varies with kinds of bacteria, and also related to their age and external environment.

Shape of BacteriaCocci: sphere, 1m Bacilli: rods , 0.5-1 m in width -3 m in length Spiral bacteria: 1~3 m in length and 0.3-0.6 m in width

CoccusBacillusSpiral Bacterium

Structure of Bacteria

Essential structures cell wall cell membrane Cytoplasm nuclear material Particular structures capsule flagella pili spore

Cell wallSituation: outmost portion. 15-30nm in thickness, 10%-25% of dry weight.

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 ofthe best and for many cases the best method which I have ever used for staining Schizomycetes.Gram, C. 1884. Ueber die isolirte Farbung der Schizomyceten in Schnittund Trockenpraparaten. Fortschritte der Medicin, Vol. 2, pages 185-189.

Cell wall :Common peptidoglycan layerA 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

NAM CH2OH H

CH C=O H N LAla DGlu LLys GlyGlyGlyGlyGlyN DAla C=OOOHHNHH3CHOHOHO=C CH3OOHHNHHO=C CH3CH2OHHHHNAG NAM CH2OH H

CH C=O H N LAla DGlu LLys GlyGlyGlyGlyGlyN DAla C=O

OOHHNHH3CHOHOHO=C CH3OOHHNHCH2OHHHHNAGO=C CH3H

GGGGMMDABDAB

Special components of Gram positive cell wall -Teichoic acid

Special components of Gram negative cell wall

Functions of Cell WallMaintaining 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 pressureProviding attachment sites for bacteriophagesProviding a rigid platform for surface appendages- flagella, fimbriae, and pili all emanate from the wall and extend beyond itPlay an essential role in cell divisionBe the sites of major antigenic determinants of the cell surface

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).

Bacteria L formBacteria with dfective cell wall-bacterial L form: protoplast, spheroplast

Cell membrane

Function of Cell membrane Selective permeability and transport of solutes into cellsElectron transport and oxidative phosphorylationExcretion of hydrolytic exoenzymesSite of biosynthesis of DNA, cell wall polymers and membrane lipids.

MesosomesMesosomes are specialized structures formed by convoluted inveigh-nations of cytoplasmic membrane, and divided into septal and lateral mesosome.

Cytoplasm Composed largely of water, together with proteins, nucleic acid, lipids and small amount of sugars and saltsRibosomes: numerous, 15-20nm in diameter with 70S; distributed throughout the cytoplasm; sensitive to streptomycin and erythromycin site of protein synthesisPlasmids: extrachromosomal genetic elementsInclusions: sources of stored energy, e,g volutin

RibosomesRibosomes are the protein synthesizing factories of the cell.They translate the information in mRNA into protein sequences.

PlasmidPlasmids are smallcircular/lineextrachromosomaldouble-stranded DNA moleculesThey are capable of self-replication and contain genes that confer some propertiessuch as antibiotic resistancevirulence factorsPlasmids are not essential for cellular survival.

Inclusions of BacteriaInclusions 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.

granulose

Nucleus

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

Capsules and slime layersThese are structures surrounding the outside of the cell envelope. When more defined, they are referred to as a capsule when less defined as a slime layer. 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.

Capsules and slime layers

Capsules and slime layers

Function of Capsules and slime layers(1)Attachment :These structures are thought to help cells attach to their target environment. Streptococcus mutans produces a slime layer in the presence of sucrose. This results in dental plaque and many bacteria can stick to tooth surfaces and cause decay once S. mutans forms a slime layer. Vibrio cholerae, the cause of cholera, also produces a glycocalyx which helps it attach to the intestinal villi of the host.

Function of Capsules and slime layers(2)Protection from phagocytic engulfment. Bacterial pathogens are always in danger of being "eaten" by phagocytes. (Host cells that protect you from invaders.) Streptococcus pneumoniae, when encapsulated is able to kill 90% of infected animals, when non-encapsulated no animals die. The capsule has been found to protect the bacteria by making it difficult for the phagocyte to engulf the microbe.

Function of Capsules and slime layers(3)Resistance to drying. Capsules and slime layers inhibit water from escaping into the environment.

Function of Capsules and slime layers(4)Reservoir for certain nutrients. Glycocalyx will bind certain ions and molecules. These can then be made available to the cell.

Function of Capsules and slime layers(5)Depot for waste products. Waste products of metabolism are excreted from the cell, and will accumulate in the capsule. This binds them up, and prevents the waste from interfering with cell metabolism.

Flagella.Some bacterial species are mobile and possess locomotory organelles - flagella. Those that do are able to taste their environment and respond to specific chemical foodstuffs or toxic materials and move towards or away from them (chemotaxis). Flagella are embedded in the cell membrane, extend through the cell envelope and project as a long strand. Flagella consist of a number of proteins including flagellin. They move the cell by rotating with a propeller like action. Relative Speeds of OrganismsOrganism Kilometers per hour Body lengths per second Cheetah 111 25 Human 37.5 5.4 Bacteria 0.00015 10

FlagellaThe 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.

Flagella Monotrichate AmphitrichateLophotrichatePeritrichate

Flagella

Function of FlagellaIdentification of BacteriaPathogenesisMotility of bacteria

PiliPili are hair-like projections of the cell , They are known to be receptors for certain bacterial viruses.Chemical nature is pilinClassification and FunctionCommon pili or fimbriae: fine , rigid numerous, related to bacterial adhesionSex pili: longer and coarser, only 1-4, related to bacterial conjugation

Sex piliA donor bacteria will attach to a recipient via the sex pilus. Then a copy of part of the donor bacterium's genome passes through the sex pilus into the recipient. Conjugation, as it is called, is one explanation for the rapid spread of drug resistance in many different species of bacteria.

Common pili or fimbriaePili have also been show to be important for the attachment of some pathogenic species to their host. Neisseria gonorrheae, the causative agent of gonorrhea, has a special pili that helps it adhere to the urogenital tract of its host. The microbe is much more virulent when able to synthesize pili.

EndosporesEndospores are highly resistant resting structures produced within cells. They are common to organisms which live in soil and may need to wait out some rough times such as >100C heat, radiation, drying or chemical agents ; under favourable conditions , a spore germinates into a vegetative cellSpores are commonly found in the genera Bacillus and Clostridium.

DPA and surviveDipicolinic acid,DPA. Spores can survive for a very long time, and then regerminate. Spores that were dormant for thousands of years in the great tomes of the Egyption Pharohs were able to germinate and grow when placed in appropriate medium. There are even claims of spores that are over 250 million years old being able to germiinate when placed in appropriate medium. These results have yet to be validated.

SporesThe mechanisms that acount for this include the dehydration of the protoplast and the production of special proteins that protect the spores DNA. are capable of detecting their environment and under favorable nutrient conditions germinating and returning to the vegetative state.

SporeIdentification of BacteriaPathogenesisResistance

MicroscopeyLight MicroscopeElectron MicroscopeDarkfield MicroscopePhase Contrast MicroscopeFluorescence MicroscopeCofocal MicroscopeMethods

Staining Methods

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