Bacterial Cell StructureA. L. Samer Faisal

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Microbiology

• Agents of human infection:– Bacteria Prokaryotes– Helminthes Eukaryotes– Protozoa Eukaryotes– Fungi Eukaryotes– Viruses Non-

cellular

• Difference between Eukaryotes and Prokaryotes

• Structure

• Replication

• Nucleic acid

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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

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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

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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

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

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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

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• Infoldings of cell membrane carry on photosynthesis & cellular respiration

• Infoldings called Mesosomes

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MesosomesMESOSOME

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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

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Flagella• Bacteria that are

motile have appendages called flagella

• Attached by Basal Body

• A bacteria can have one or many flagella

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

one end• Amphitrichous: tuft at

both ends• Peritrichous: all around

bacteria

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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

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Pili in Conjugation

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Bacterial Shapes

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

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Grouping of Bacteria

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

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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

Normal Flora• Definition

Normal flora is the mixture of microorganisms (bacteria and fungi) that are regularly found at any anatomical site of human body.

Symbiotic Relationship

• 1. Mutualistic– Both organisms benefit – “mutually benefical”

• 2. Commensalistic– One organism benefits, the other is neither helped

nor harmed

• 3. Opportunistic– Under normal conditions, microbe does not cause

disease, but if conditions become conducive , it can cause disease

Mutualistic

• Escherichia coli – Synthesizes Vitamin K & B complex Vitamins

– In return, we provide a warm, moist nutrient rich environment for E. coli

Commensalistic

• We have no Commensalistic relationships with Bacteria

• If Bacteria are in or on our body, they are either helping us (Microbial Antagonism) or harming us.

Opportunistic

• Escherichia coli - normally in our digestive tract where it causes no problems, but if it gets into the urinary tract it can become pathogenic.

• Staphylococcus aureus – commonly found in the upper respiratory tract, but if it gets into a wound or a burn it can become pathogenic

The Three Types of Symbiotic Relationships

Table 14.1

Normal Microbial Flora

• 1. Resident Flora– Microbes that are always present

• 2. Transient Flora– Microbes that live in or on your body for a period

of time (hours, days, weeks, months) then move on or die off

Transient Microbiota

• Remain in the body for only hours to months before disappearing

• Found in the same regions as resident microbiota

• Cannot persist in the body…because…– Competition from other microorganisms– Elimination by the body’s defenses cells– Chemical or physical changes in the body

Sterile tissuesIn a healthy human, the

internal tissues such as:• blood• brain• muscle• cerbrospinal fluid (csf.) are normally free of

microorganisms.

Normal Flora

Normal Flora• Skin• Eyes (i.e.Cunjunctiva)• Nose (i.e. Respiratory tract)• Mouth (i.e Human Oral

Cavity)• Ears• Urogenetal tract• Elementry tract

Normal Flora• Resident flora

– Acquired rapidly during & after birth

Normal Flora• Resident flora

– Reflects age of person

– Changes continuously through out life

– Reflects nutrition of person

– Reflects genetics of person

– Reflects environment of person

– Reflects sex of person

1. They constitute a protective host defense mechanism by occupying ecological niches.

Importance of The Normal Flora (Advantages)

Importance of The Normal Flora (Advantages)

2. They produce vitamin B and vitamin K in intestine.

Importance of The Normal Flora (Advantages)

3.The oral flora contribute to immunity by inducing low levels of circulating and secretory antibodies that may cross react with pathogens.

Importance of The Normal Flora (Advantages)

4. The oral bacteria flora exert microbial antagonism against nonindigenous species by production of inhibitory fatty acids, peroxides, bacteriocins, etc.

Importance of The Normal Flora (Advantages)

5. The normal flora may antagonize other bacteria through the production of substances which inhibit or kill nonindigenous species.

Importance of The Normal Flora (Disadvantages)

1. They can cause disease in the following:a) When individuals become immunocompromised.b) When they change their usual anatomic location.

Contamination vs. Infection

• Contamination – the mere presence of microbes in or on the body

• Infection – results when the organism has evaded the body’s external defenses, multiplied, and become established in the body

Portals of Entry

• Sites through which pathogens enter the body

• Four major types– Skin– Mucous membranes– Placenta– Parenteral route

Skin

• Outer layer of packed, dead, skin cells usually acts as a barrier to pathogens

• Some pathogens can enter through openings or cuts

• Others enter by burrowing into or digesting the outer layers of skin…flesh-eating bacteria

Mucous Membranes

• Line the body cavities that are open to the environment

• Provides a moist, warm environment that is hospitable to pathogens

• Respiratory tract is the most commonly used site of entry – entry is through the nose, mouth or eyes

• Pathogens able to survive the acidic pH of the stomach may use the gastrointestinal tract as a route of entry

Some Pathogens that Cross the Placenta

Table 14.3

Parenteral Route

• Pathogens deposited directly into tissues beneath the skin or mucous membranes

Infection vs. Disease

• Infection is the invasion of the host by a pathogen

• Disease results only if the invading pathogen alters the normal functions of the body

• Disease is also referred to as morbidity

Manifestations of Disease

• Symptoms – subjective characteristics of disease felt only by the patient

• Signs – objective manifestations of disease that can be observed or measured by others

• Syndrome – group of symptoms and signs that characterize a disease or abnormal condition

• Asymptomatic, or subclinical, infections lack symptoms but may still have signs of infection

Etiology

• Study of the cause of disease

• Germ theory of disease – disease caused by infections of pathogenic microorganisms

• Robert Koch developed a set of postulates one must satisfy to prove a particular pathogen causes a particular disease