CLASSIFICATION & STRUCTURE
OF BACTERIA
Assistant Professor Microbiology
OBJECTIVES
1. Classify Bacteria
2. Identify Unicellular nature of Bacterial cell
3. Review selective toxicity of antimicrobials
1. CLASSIFICATION OF BACTERIA
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Nomenclature
Binomial Linnean System
Genus species
Italicize
Example Escherichia coli
Used for bacteria, fungi and
parasites
• Not used for viruses(single
name e.g. Poliovirus)
1. CLASSIFICATION OF BACTERIA
• Criteria for classification
1. Growth requirement
a. O2 requirements e.g. Aerobe, Anaerobe, Facultative
anaerobe, Strict anaerobe, Strict aerobe
b. CO2 requirements e.g. Carboxyphilic
c. Salt requirements e.g. Halophilics (Salt loving)
2. Microscopy
a. Gram staining e.g. Gram Pos & Gram Neg due to cell
wall differences
1. CLASSIFICATION OF BACTERIA
3. Biochemical Reactions
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Oxidase test, Catalase test
4. Serology
•
Strep pyogenes –Rheumatic fever nephritogenic
strain, M types 2, 42, 49, 56, 57, and 60
5. DNA-DNA Similarity
6. Ribosomal RNA analysis
1. CLASSIFICATION OF BACTERIA
• Bergey's Manual of Systematic Bacteriology
• Classification of medically imp bacteria based
upon criteria like Gram reaction, motility, cell
structure, etc
1. CLASSIFICATION OF BACTERIA
I. Gram-negative eubacteria that have cell
walls
Group 1: The spirochetes
Treponema, Borrelia, Leptospira
Group 2: Aerobic/microaerophilic, motile
helical/vibroid gram-negative bacteria
Group 4: Gram-negative
aerobic/microaerophilic rods and cocci
Group 5: Facultatively anaerobic gramnegative rods
Group 6: Gram-negative, anaerobic,
straight, curved, and helical rods
Group 9: The rickettsiae and chlamydiae
Campylobacter , Helicobacter, Spirillum,
Bordetella, Brucella, Legionella, Neisseria,
Pseudomonas
Escherichia (and related coliform bacteria)
Klebsiella, Proteus, Salmonella, Shigella,
Vibrio
Bacteroides, Fusobacterium
Rickettsia, Coxiella, Chlamydia
1. CLASSIFICATION OF BACTERIA
II. Gram-positive bacteria that have cell
walls
Group 17: Gram-positive cocci
Group 18: Endospore-forming grampositive rods and cocci
Group 19: Regular, nonsporing grampositive rods
Group 20: Irregular, nonsporing grampositive rods
Group 21: The mycobacteria
Enterococcus, Staphylococcus,
Streptococcus
Bacillus,Clostridium
Listeria
Actinomyces, Corynebacterium
Mycobacterium
Group 22–29: Actinomycetes
Nocardia
III. Cell wall-less eubacteria: The
mycoplasmas or mollicutes
Group 30: Mycoplasmas
Mycoplasma, Ureaplasma
STRUCTURE OF BACTERIAL CELL
• Shape, Arrangement,
Color & Size
– Three basic shapes- Cocci
(round), Bacilli (rod),
Spirochetes (spiral)
– Pleomorphism
– Shape due to cell wall
– Arrangement is due to
orientation at cell division
• Pairs-Diplococci
• Chains-Streptococci
• Clusters-Staphylococci
STRUCTURE OF BACTERIAL CELL
• Shape, Arrangement, Color
& Size
–Color-Gram Staining gives
purple or red color to bacteria
–Staining reaction-cell wall
differences
–Size-0.2 to 5µ
• Clinical importance
Above parameters help us
identify certain bacteria
BASIC STRUCTURE OF PROKARYOTE
2. Unicellular nature of Bacterial cell
• Prokaryotic cell is a unicellular
structure
• It has nucleoid, cytoplasm and
outer covering called Cell envelop
• Prokaryotic cell lacks nuclear
membrane- DNA is present
loosely in area called Nucleoid
• Cytoplasm contains ribosomesmaller size 70S with 30S &
50Ssubunits
• Insoluble granules stores-e.g.
metachromatic granules of
Corynaebacterium diphtheriae
Metachromatic granules
CELL ENVELOPE
– Structure that surrounds
the cytoplasm of bacteria
– In Gram positive, two
components:• Cytoplasmic membrane
• Thick peptidoglycan layer
(Cell wall)
– In Gram negative, three
components:• Cytoplasmic membrane
(Also called Inner
membrane)
• Thin peptidoglycan layer
• Outer membrane
Gram + cell envelope
CELL ENVELOPE IN GRAM +ve AND
GRAM –ve BACTERIA
BACTERIAL CELL STRUCTURE
• Teichoic Acid
– Outer layer of
Gram +ve bacteria
– Can induce septic
shock like
Endotoxin of Gram
–ve
– Attach
Staphylococci to
mucosal surface
BACTERIAL CELL STRUCTURE
• LIPOPOLYSACCHARIDE
– LPS of outer membrane of
Gram –ve is Endotoxin
– Produce hypotension, fever,
shock
– Three components
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Lipid A is toxic component
Core polysaccharide of 5
sugars
Outer polysaccharide of up
to 25 sugar- O somatic Ag
BACTERIAL CELL STRUCTURE
Outside Cell Wall
• Capsule
– Layer covering entire bacterium
– Polysaccharide except Bacillus
anthracis-polypeptide capsule
– Determine sero-type e.g. Strep
pneumoniae 84 serotypes
– Virulence factor- Negative charge
resist phagocytosis. Bacteria that
lose capsule-become avirulent
BACTERIAL CELL STRUCTURE
Outside Cell Wall
• Pili (Fimbrae)
–Hair-like filament extend
from cell surface
–Mainly in Gram –ve
–Two functions
• Attachment of bacterium to
specific receptor in human
tissue (Mutant Ns lose piliavirulent)
• Sex pili form attachment
during conjugation
BACTERIAL CELL STRUCTURE
Outside Cell Wall
• Flagella
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Long whip-like appendage-motility
Subunit-flagellin
Energy provided by ATP
Spirochetes have flagellum likeaxial filament
– Importance for two reasons
• UTI causing bacteria move up in
bladder
• Lab identification using anti-sera e.g.
Salmonella
BACTERIAL CELL STRUCTURE
Outside Cell Wall
• Glycocalyx (Slime Layer)
– polysaccharide coating
– allows the bacteria to adhere firmly to various
structures e.g. skin, heart valves, and catheters
– Pseudomonas aeruginosa- cystic fibrosis
– Staphylococcus epidermidis- endocarditis
– Streptococcus mutans-dental plaque and caries
BACTERIAL CELL STRUCTURE
• Spores
– Highly resistant structures
– In response to adverse conditions
– Sporulation occurs when sources of
carbon and nitrogen are depleted
– Inside the cell
– bacterial DNA, a small amount of
cytoplasm, cell membrane,
peptidoglycan, very little water
– a thick, keratin-like coat -dipicolinic
acid
Selective toxicity of antimicrobials
• Antimicrobials used with aim to target
structures and functions of bacteria not
present in Eukaryote human cells
– Cell Wall
– Ribosome
– Metabolic pathways
Selective toxicity of antimicrobials
• Penicillin target the Cell wall not present in
Eukaryote cells
• Tetracyclin, Macrolides target 30S & 50S
subunits of ribosome
• Co-trimoxazole attacks Folic acid synthesis
pathway unique to Bacteria
• Quinolones inhibit Topoisomerase IV enzyme
of bacteria
MCQs GEN BACTERIOLOGY
You have isolated a cell with a peptidoglycan cell
wall. What other structure can you safely
assume the cell has?
A. a mitochondrion
B. a Nuclear membrane
C. a plasma membrane
D. a nucleus
Ans: c
MCQs GEN BACTERIOLOGY
A young boy is suffering from acute pharyngitis. You perform a
throat swab and isolate a Gram positive cocci β- hemolytic
colonies on blood agar. This bacterium protects itself from
osmotic lysis by which of following structures?
A. Cell membrane
B. Peptidoglycan layer
C. Hydrolytic enzymes
D. Ribosomes
E. DNA
Ans: B
MCQs GEN BACTERIOLOGY
All of the following are found in the cell walls of
Gram positive bacteria except:
A. Lipid A
B. Peptidoglycan
C. N-acetylglucosamine
D. Lipoteichoic acid
E. Teichoic acid
Ans: A