BACTERIAL MORPHOLOGY I

Dr. Tarek Mahbub Khan

MBBS, M.Phil (virology)

Senior lecturer

Faculty of Medicine

Basic Medical Microbiology Semester 3, Year 2

Date: December 30, 2014 Time: 0930-1030

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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 COMPONENTS

Cell Wall Capsule

Plasma membrane Pilus or Fimbria

Ribosome Flagellum

Mesosome Spore

Nucleoid Plasmid

Periplasm 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

Penicillin acts

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 nucleus No nuclear membrane No nucleolus No mitotic spindle No histones No introns Bacterial genes are arranged in ‘Operons’

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PLASMID

Extra chromosomal Double stranded Circular DNA molecules Self replicable Transmissible Non transmissible

Contains Antibiotic R genes Genes for sex pilus Genes for exotoxins

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BACTERIAL CAPSULE

A gelatinous layer Polysaccharide in nature Antiphagocytic Antigenic Helps bacterial adherence Aids 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 filaments Composed of pilin proteins Short and straight than flagella FUNCTIONS Attachment (Ordinary pili) Conjugation (Sex pili)

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GLYCOCALYX

Often known as ‘Slime layer’ Covers surface as film Polysaccharides Components of biofilm Helps bacterial adherence Thick glycocalyx forms capsule

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BACTERIAL SPORE

Highly resistance form Forms in adverse condition RESISTANT FACTORS: Dipicolinic acid (Ca+ Chelator) Keratin like Spore coat Less water Less cytoplasm Resistant to boiling Killed 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

PROPERTIES

PROPERTIES EXAMPLES

Capsulated Streptococcus pneumoniae, Neisseria meningitidis, Klebsiella pneumoniae

Flagellated Escherichia coli, Salmonella, Vibrio, Proteus

Piliated Neisseria gonorrhoeae, E.coli,

Strict aerobe Mycobacterium tuberculosis, Pseudomonas

Anaerobe Clostridium, Bacteriodes

Facultative 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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