Topic Two

Shapes of microbes:

Bacterial cells are referred to as procaryotic (meaning early nucleus) . They lack a nuclear membrane and mitochondria or chloroplasts. Cell with these are called eucaryotic cells (meaning true nucleus) and occure in plants, animals and fungi. The functions that normally occur in a chloroplast or mitochondrion in eucaryotic cells occures in the cytoplasm of bacteria. Bacterial DNA is found in a region of the cell known as the nucleoid . Within the bacterial cytoplasm there are many small loops of DNA known as plasmids .

A Bacteria Cell

DNA.

Bacteria have their DNA (double stranded) contained in a single circular chromosome in a nucleoid

Most Bacteria also have smaller circular DNA fragments called plasmids. These carry genes that can be transferred to other bacteria

Cell membranes.

Cell membranes are composed of phospholipid bi-molecular membrane material similar to animal cells

All Bacteria that have cell walls containing have peptidoglycan.. The amount of the peptidoglycan is the main determinant of whether an organisms is gram +ve ( >80%) or gram -ve. (<20%).

The peptidoglycan provides streath to resist presure caused by osmosis. Cell wall must be strong to protect cells from lysis in hypotonic solutions

Peptidoglycan has a structure containing a polymer of amino sugars (N-acetylglucosamine (NAG) and N-acetyl muramic acid (NAM)) linked by b 1-4 bonds like those in cellulose.

Peptidoglycan can be broken by an enzyme called lysozyme that is present in your tears, saliva, other bodily fluids and egg white.

Antibiotics like penicillin and cephalosporin inhibit the formation of peptidoglycan. This is why penicillins do not affect eucaryotic cells.

Gram +ve cell walls have >80% peptidoglycan and <20% lipid content.

Gram –ve cell walls are quite different in that they have much less peptidoglycan (less than 20) The peptidoglycan is located in an inner layer of the cell wall and protected from penicillin by an outer lipid layer. This why they are resistant to natural penicillin

Fungal cell walls.

The main component of fungal cell walls is chitin which is a polymer consisting of amino sugars

.

Layers outside the cell wall.

Many bacteria produce extracellular layers that serve different functions in different critters.

Capsules are slimy layers that are made of or polypeptides . Capsules help bacteria attach to surfaces - e.g. Streptococcus mutans attaches to your teeth using a capsule. Capsules also function as a protective layer . In pneumonia only the capsulated causing S. pneumoniae can avoid phagocytosis in the blood and cause the diease. Naked strains are non-pathogenic.

Slim layers: The capsule material is not always attached well and may ooze off a slime forming material.

Fimbriae (sing. = fimbria) are short appendages made of. Some are called pili). The main functions of fimbriae are attachment. 

Flagella (sing. = flagellum) are protein appendages that are used in bacterial locomotion. Many bacteria show chemotaxis and phototaxis when they can move towards a source of food or light.

The End

Return to Program Outline