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Lecture Two

Cell Structure of Microbes

Online Lecture references:

Introduction

Cell structure

Microbial Evolution

Bacterial cell structure

Shapes of microbes:

Cocci (sing. coccus)
rods = bacilli (sing. bacillus)
spirilla and spirochaetes
pleomorphic cells

Prokaryotic and Eukaryotic cells

Prokaryotes:

Bacterial cells are referred to as being prokaryotic (i.e. early nucleus) and are missing a nuclear membrane and the membrane-bound organelles like mitochondria and chloroplasts that are found in the eukaryotic cells (that have a  true nucleus) such as  in plants, animals and fungi. The activities that normally take place in chloroplasts or mitochondria of the eukaryotes occure in the cytoplasm or cell membranes of bacteria. Bacterial DNA is found in a region of the cell known as the nucleoid. Also bacterial cytoplasm has small loops of DNA known as plasmids. Throughout the cytoplasm are many ribosomes on which protein synthesis takes place.

 

Bacteria Cell

DNA.

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

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

Cell membranes.

Phospholipid bi-molecular membrane similar to animal cells

Figure 3.7

Cell Walls

Osmosis - cell wall must be strong to protect cells from lysis in hypotonic solutions

All Bacteria that have cell walls have peptidoglycan in them. The thickness of the peptidoglycan layer is the main determinant of whether an organisms is gram +ve or gram -ve.

 

Peptidoglycan

Peptidoglycan is a polysaccharide  consisting of alternating amino sugars (N-acetylglucosamine (NAG) and N-acetyl muramic acid (NAM)) linked by Beta 1-4 bonds like those in cellulose.

The Beta 1-4 bond can be broken by an enzyme called lysozyme that is present in your tears, saliva, blood, other bodily fluids and egg white. These polysaccharide chains are held together by peptide cross-links . These peptides  that they may contain D-amino acids.

Antibiotics like penicillin and cephalosporin  inhibit the enzymes that synthetise peptidoglycan. This is why these antibiotics are not affective against the eucaryotic cells of fungi and parsites.

Gram +ve cell walls have teichoic acids that helps stabilise the peptidoglycan layer and connect it with the plasma membrane.

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%) and no teichoic acids. The peptidoglycan is located in an inner layer of the cell wall and protected from penicillin by an outer lipid layer. They have an outer layer of lipopolysacchardide (LPS)

LPS  can function as a toxin  in gram -ve. pathogens such as Salmonella spp. and Yersinia pestis (plague). This toxin is refered to as  an endotoxin becuase it is only released when the bacterial cell dies and disintegrates

Fungal cell walls.

The main component of fungal cell walls is chitin which is a polymer consisting of N-acetylglucosamine (NAG) as the monomer. Chitin is also found in the exoskelitins of insects and crustacians and is used these days a an aditive in food to bind fat.

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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 polysaccharides (monomers include NAG and glucose (in dextran) or polypeptides (monomers of D-glutamic acid in some Bacillus spp.).

Capsules help bacteria attach to surfaces - e.g. Streptococcus mutans attaches to your teeth using a capsule. Capsules also function as a protective layer - as in pneumonia causing S. pneumoniae that can avoid phagocytosis in the blood only if it has a capsule.

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

Bacterial Appendages

Fimbriae  are appendages made of protein and are found on many types of bacteria. One type of Fimbria are the pili. These include the sex pili used in conjugation  The main functions of fimbriae are attachment and transport.

 

Flagella  are proteinacious appendages that are used in bacterial locomotion. bacterial flagella are siple  Many bacteria use flagella for  chemotaxis and/or phototaxis in which they can move towards a source of energy.

 

The End

Prepared by Barry Brazier