bacterial structure

Lec. 1 Dr. Fatima Malik

Microbiology: is the science that deals with the study of microorganisms (M.O) which are a large and diverse group of microscopic organisms that exist as single cells or cell clusters e.g, Bacteria, protozoa and fungi. It also includes viruses, which are microscopic but not cellular.
The two oral infections caused by oral M.O are dental caries and periodontal disease.
M.O divided to non-cellular (virus) &cellular which includes
1-prokaryotes(bacteria)
2- eukaryotes (alge, fungi and parasites)
)

Eukaryote Prokaryote
1- have true nucleus 1- don’t have nucleus
2- DNA is complex with histone proteins, have nuclear membrane. 2-chromosome, is a single, circular molecule of double-stranded DNA, lacking a nuclear membrane
3- Peptidoglycan is not present in the cell wall. 3-peptidoglycan is present in the cell wall
4- divided by mitosis and meiosis
4- divided by binary fission.
5- 80S ribosomes
5-70S ribosomes
6- Have mitochondria and chloroplasts. 6- not have mitochondria and chloroplasts

In prokaryotes the bacterial genome, or chromosome, is a single, circular molecule of double-stranded DNA, lacking a nuclear membrane.(smaller circular DNA molecules called plasmids may also be present in bacteria) whereas the eukaryotic cell has a true nucleus with multiple chromosomes surrounded by a nuclear membrane. Bacteria comprise the vast majority of human pathogens.

Morphology (Shape and size)
The shape of a bacterium is determined by its rigid cell wall.
Bacteria are classified by shape into three basic groups
1. cocci (spherical)
2. bacilli (rod-shaped)
3. spirochaetes (helical).
Some bacteria with variable shapes, appearing both as coccal and bacillary forms, are called pleomorphic (pleo: many;morphic: shaped) in appearance



STAINING: stains combine chemically with the bacterial protoplasm. The commonly used stains are salts.
Basic stains consist of a colored cation with a colorless anion (eg, methylene blue+ chloride-); acidic stains are the reverse (eg, sodium+eosinate-).
Bacterial cells are rich in nucleic acid, has negative charges as phosphate groups. These combine with the positively charged basic dyes. Acidic dyes do not stain bacterial cells and hence can be used to stain background material a contrasting color (Negative Staining).
The basic dyes stain bacterial cells uniformly
. Special staining techniques
can be used to differentiate flagella, capsules, cell walls, cell membranes, granules, nucleoids, and spores.
The Gram Stain
The Gram-staining is a fundamental one because the Gram reaction is correlated with many other morphologic properties
Gram-stain classified bacteria into two major subgroup according to their cell walls. Gram-negative cells will appear red or pink color while gram-positive cells appear purple

Gram-staining characteristics
In clinical microbiology, bacteria can be classified into two major subgroups according to the staining characteristics of their cell walls. The stain used, called the Gram stain (first developed by physician, Christian Gram), divides the bacteria into Gram-positive (purple) and Gram-negative (pink) groups. The Gram-staining property of bacteria is useful both for their identification and in the therapy of bacterial infections because, in general, Gram-positive bacteria are more susceptible to penicillin than Gram-negative bacteria.








Structures external to the cell wall


Bacteria cell include the following structures:
Flagella, Pili, Capsule, Plasma Membrane, Cytoplasm, Cell Wall, Lipopolysaccharides, Teichoic Acids, Inclusions & Spores

Flagella
Flagella are whip-like filaments that act as motor and guide the bacteria towards nutritional sources. The filaments are composed of many subunits of a single protein, flagellin. Flagella may be located
At one end (monotrichous, a single flagellum; lophotrichous, many flagella) or all over the outer surface (peritrichous).
Many bacilli (rods) have flagella, but most cocci do not and are therefore non-motile.
Fimbriae and pili
Fimbriae and pili are fine, hair-like filaments, shorter than flagella, that extend from the cell surface. Pili, found mainly on Gram-negative organisms, are composed of subunits of a protein, pilin, and mediate the adhesion of bacteria to receptors on the human cell surface – a necessary first step in the initiation of infection. A specialized type of pilus, (the sex pilus), forms the attachment between the male (donor) and the female (recipient) bacteria during conjugation, when genes are transferred from one bacterium to another.
Capsule is gelatinous layer (usually more substantial than the glycocalyx) surrounds the entire bacterium; it is composed of polysaccharide, and sometimes protein (e.g.anthrax bacillus). The sugar components of the polysaccharide vary in different bacterial species and frequently determine the serological type within a species.
Glycocalyx (slime layer) is a polysaccharide coating that covers the outer surfaces of many bacteria and allows the bacteria to adhere firmly to various structures, e.g. oral mucosa, teeth, heart valves and catheters, and contribute to the formation of biofilms. This is especially true in the case of Streptococcus mutans, a major cariogenic organism, which has the ability to produce vast quantities of extracellular polysaccharide in the presence of dietary sugars such as sucrose.
Cell wall
The cell wall confers rigidity upon the bacterial cell. It is a multilayered structure outside the cytoplasmic membrane. It is porous and permeable to substances of low molecular weight. The inner layer of the cell wall is made of peptidoglycan and is covered by an outer membrane that varies in thickness and chemical composition, depending upon the Gram staining property of the bacteria. The term ‘peptidoglycan’ is derived from the peptides and the sugars (glycan) that make up the molecule. (Synonyms for peptidoglycan are murein and mucopeptide.)
The cell walls of Gram-positive and Gram-negative bacteria have important structural and chemical differences.
• The peptidoglycan layer is common to both Gram-positive and Gram-negative bacteria but is much thicker in the Gram-positive bacteria.
• In contrast, the Gram-negative M.On have a complex outer membrane composed of lipopolysaccharide (LPS), lipoprotein and phospholipid. These form porins, through which hydrophilic molecules are transported in and out of the organism.
The O antigen of the LPS and the lipid A component are also embedded in the outer membrane. The periplasmic space is Lying between the outer membrane and the cytoplasmic membrane of Gram-negative bacteria
• The LPS of Gram-negative bacteria, which is extremely toxic, has been called the endotoxin. (Hence, by definition, endotoxins cannot be produced by Gram positive bacteria as they do not have LPS in their cell walls.





Cell wall of G+ve and G-ve bacteria

The cytoplasm contains 70S ribosomes, and they do not have mitochondria and chloroplasts.
• The major functions of the cell membrane of prokaryotic
cells are
(1) Selective permeability and transport of
Solutes
(2) Electron transport and oxidative phosphorylation,
in aerobic species
(3) Excretion of hydrolytic enzymes and other proteins
(4) Bearing the enzymes and carrier molecules that function in the biosynthesis of DNA, cell wall polymers.

• The formation of endospores is a characteristic of the genera Bacillus and Clostridium. It is triggered by depletion of nutrients in the environment. Endospores (spores) are resting cells, highly resistant to desiccation, heat, and chemical agents; when returned to favorable nutritional conditions and activated, the spore germinates to produce a vegetative cell.