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Dr. Tarek Mahbub KhanMBBS, M.Phil (virology)

Senior lecturerFaculty of Medicine, KUIN

BACTERIAL MORPHOLOGY

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TLO (students will be able to)

• describe different morphological form of bacteria and their arrangements.

• describe basic structures and functions of a bacterial cell.

• explain staining characteristics on the basis of structural components

• explain the role of bacterial structure in laboratory identification.

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SHAPE AND ARRANGEMENT OF BACTERIA

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STRUCTURES

ESSENTIAL COMPONENTS NONESSENTIAL COMPONENTSCell Wall CapsulePlasma membrane Pilus or FimbriaRibosome FlagellumMesosome SporeNucleoid PlasmidPeriplasm Granules

Glycocalyx

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BACTERIAL CELL

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CELL WALL• Multilayered structure outside the cytoplasmic

membrane• COMPOSED OF:– Peptidoglycans– Teichoic acid/ lipoteichoic acid– Lipopolysaccharide

• Function provides:– cell shape, osmotic stability, transportation of molecules,

antigenicity

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PEPTIDOGLYCANS

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A: E.coli B: Staphylococcus

Penicillinacts

Transpeptidase

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Gram positive and Gram negative cell wall

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LIPOPOLYSACCHARIDE

ENDOTOXIN

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CELL WALL OF ACID FAST BACTERIA

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TEICHOIC ACID• Present on gram positive bacteria• Chemically glycerol phosphate or ribitol phosphate• Glycerol teichoic acid are lipoteichoic acid• Ribitol teichoic acid attached to NAM acid of CHO

• Functions/effects:– Septic shock– Attachment to mucosa (e.g., Staphylococcus)

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BACTERIAL RIBOSOME (70S)

Antibiotic binds selectively

Selective toxicity

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CYTOPLASM AND GRANULES

• Cytoplasm has two distinct regions:– Amorphous (e.g, Nutrient granules, ribosome etc.)– Nucleoid

• Granules often contains high energy phosphates

Red stained metachromatic granule

Corynebacterium diphtheriae

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NUCLEOID

Not a true nucleusNo nuclear membraneNo nucleolusNo mitotic spindleNo histonesNo intronsBacterial genes are arranged in ‘Operons’

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PLASMIDExtra chromosomalDouble strandedCircularDNA moleculesSelf replicableTransmissibleNon transmissible

Contains Antibiotic R genesGenes for sex pilusGenes for exotoxins

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BACTERIAL CAPSULE

A gelatinous layerPolysaccharide in natureAntiphagocyticAntigenicHelps bacterial adherenceAids lab diagnosis

Negative staining shows unstained capsule

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FLAGELLA: Organ of motility

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ARRENGEMENT OF FLAGELLA

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PILI (Fimbria)Hair like filamentsComposed of pilin proteinsShort and straight than flagella

FUNCTIONSAttachment (Ordinary pili)Conjugation (Sex pili)

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GLYCOCALYXOften known as ‘Slime layer’Covers surface as filmPolysaccharidesComponents of biofilmHelps bacterial adherenceThick glycocalyx forms capsule

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BACTERIAL SPORE

Highly resistance formForms in adverse condition

RESISTANT FACTORS:Dipicolinic acid (Ca+ Chelator)Keratin like Spore coatLess waterLess cytoplasm

Resistant to boilingKilled by autoclaving

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BACTERIA AND LOCATION OF SPORE

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DIRECT OBSERVATION OF A BACTERIUM

• Can not be observed in naked eye• Microscopy:• Direct evidence– Shape– Arrangement– Staining characteristics

• Indirect evidence– Motility

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HOW BACTERIAL STRUCTURE INFLUENCES STAINING METHOD

• GRAM STAIN– Primary stain (crystal violet) is retained in gram positive

bacteria due to thick cell wall. Hence bacteria stained purple or blue

– Gram negative bacteria loss the primary stain easily due to thin cell wall and take the counter stain color (red carbol fuchsin)

• ACID FAST STAIN– Due to mycolic acid in the cell wall of acid fast bacteria

primary stained (red carbol fuchsin) is applied in hot state. Hence the bacteria is red

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GRAM POSITIVE BACTERIA

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ACID FAST BACTERIA

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EXAMPLES OF BACTERIA WITH MORPHOLOGICAL AND BIOCHEMICHEL

PROPERTIESPROPERTIES EXAMPLESCapsulated Streptococcus pneumoniae, Neisseria

meningitidis, Klebsiella pneumoniaeFlagellated Escherichia coli, Salmonella, Vibrio, ProteusPiliated Neisseria gonorrhoeae, E.coli, Strict aerobe Mycobacterium tuberculosis, PseudomonasAnaerobe Clostridium, BacteriodesFacultative anaerobe

E.coli, Salmonella, Shigella other enteric bacteria

Spore forming Clostridium, Bacillus

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REFERENCE• Warren Levinson. Review of Medical Microbiology and

Immunology, 11th edition (2010). Appleton and Lange.

• Geo. F. Brooks, Karen C. Carroll, Janet S. Butel, Stephen A. Morse, Timothy A. Mietzner. Medical Microbiology, 25th edition (2010). Appleton & Lange.

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Thank you