1 introduction bacterial cells
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Medical Microbiology: Introduction
Dr. Md. Abdullah Yusuf
Assistant Professor, Dept. of Microbiology
National Institute of Neurosciences & Hospital
Dhaka, Bangladesh
Email: ayusuf75@yahoo.com
Learning Objectives
Introduction and history of Microbiology• Important events of history• Koch’s postulates• Branches of Medical Microbiology• Role of Microbes in environment & medical
science
Important events of history
• Dutch Biologist “Anton Van Leeuwenhock” discovered “animicules” in a drop of water by his simple microscope in 1674
• Otto Muller (Danish biologist) organized bacteria into genus and species in1774
• In 1840, Friedrich Henle (German pathologist) proposed germ theory of disease
Conti…• Robert Koch
– Father of medical Microbiology• Luis Pasteur (Father of Modern Microbiology)
– identified causative agent of anthrax, rabies, plague, cholera and tuberculosis
• Discovery of penicillin– 1928 (Alexander Flemming)
• Discovery of sulphanilamide– 1935 (Gerhard Domagk’s)
• Discovery of streptomycin– 1928 (Selman Waksman)
Koch’s postulates1. The causative (etiological) agent must be
present in all affected organisms– but absent in healthy individuals
1. The agent must be capable of being isolated and cultured in pure form
2. When the cultured agent is introduced to a healthy organism,
– the same disease must occur1. The same causative agent must be isolated
again from the affected host
Limitation of Koch’s Postulate
• Genetic Diseases• Auto-immune Diseases• Non-culturable microbial diseases
Branches of Medical Microbiology
• Bacteriology– General– Systemic
• Virology– General
– Systemic
• Parasitology– Protogoology
– Helminthiology
• Mycology• Immunology
Difference between Virus And Bactria
• Structure• Nucleic Acid• Type of Nucleus• Ribosomes• Nature of Outer Surface• Motility• Method of Replication
Eukaryotes and Prokaryotes
• Properties• Differences• examples
What is bacterium?
• Prokaryotic cells– Pro means primitive– Karyote means nuceus
• Multiply by binary fission– 1divide to 2
• Devoid of chlorophil• ** single cell, living being, microscopic entity but
fantastic in structural organization, reproduction, metabolism and function
Eukaryotes• Eu means True (normal)• Karyote means nucleus• Much larger than most prokaryotes• Have subcellular, membrane-bound organelles• Includes all “higher" plants and animals• Microbiology includes
– Fungi– Protozoa– Algae– Multicellular Organisms (helminthes)
Fungus
• Eukaryotic organism • cell walls contains Chitin • Molds are multicellular
– masses of mycelia– composed of filaments called hyphae
• Yeasts are unicellular
Protozoa
• Eukaryotes• No Cell Wall• Surround and absorb food• May be motile via
– Pseudopods– cilia, or – flagella
Algae
• Eukaryotes• Cellulose cell walls• Use photosynthesis for energy• Produce molecular oxygen and organic
compounds
Virus
• Acellular• Consist of DNA or RNA• Core is surrounded by a protein coat• Coat may be enclosed in a lipid envelope• Viruses are replicated only when they are in a
living host cell• Living
Multicellular- helminthes
• Eukaryote• Multicellular animals• Parasitic flatworms and round worms are called
helminths• Microscopic stages in life cycles
Characteristics of Prokaryotes
Nuclear level
• Absence of – nuclear membrane– Nucleolie– Nucleoplasm– paired multiple chromosome– centriole
Conti..
Cytoplasmic level• Absence of membrane bound organelles
– Mitochondria– endoplasmic reticulum– golgi complex– Microtubules– Microfilaments
Conti..
• Presence of – Ribosome– Plasmid
• No protoplasmic streamming in the form of Brownian movement and Tyndall phenomena
• No change in definite shape and formation of pseudopodia
Characteristics of prokaryotes (Structural)
Cell coverings (cell envelope)• Innermost layer
– bilaminar phospholipid + Lipoprotein – cytoplasmic membrane (fluid stage) without
any sterol• Two membrane in Gram negative bacteria• Condensation at some points forming
mesosome
Conti…
2) Outer to Cell Membrane– cell wall containing specialized unique
chemicals called peptidoglycan
3) Outer to Cell Wall– investing layer either capsule/glycocalyx/slime
layer
4) S layer– Single type protein lattice outer to CW
Characteristics of Prokaryotes
• No genetic exchange during chromosome replication
• Show secretory and excretory activities• Show chemical selectivity, motility and alteration
of life stage (spore)• Wide range of temperature and pH tolerance• Heterogenous nutritional requirements and host
specificity• Can produce self protective molecule bacteriocin• Become infected by particular virus• Can acquire extrachromosomal DNA (plasmid)
Difference Between Eukaryotes and Prokaryotes
• Nature of Nucleus• Mitotic Division• Presence of histones in DNA• Membrane bound Organelles• Size of Ribosomes• Peptidoglycan in Cell Wall
Bacterial cells
Bacterial cells
• essential structures – chemical composition– Organization– Functions
• non-essential structures– chemical composition– Organization– Functions
Structures of bacteria
• Essential Structures (maintain viability)– Cell Wall– Cell Membrane– Mesosome– Ribosome– Nucleoid– Periplasm
Structures of bacteria
• Non-essential Structures (viability is not affected in their absence)– Flagella– fimbria (pili)– Capsule– Glycocalyx– slime layer– Plasmid– spore
Cell wall
• Component– peptidoglycan (PG)
– Teichoic acid– teichorunic acid– Polysaccharides– LPS– Lipoproteins
• PG backbone: NAG and NAM• Tetrapeptide side chains attached to NAM• Interpeptide bridge
Conti…
• Function– osmotic protection– integrity of cellular structures– virulence factors
– Fully permeable to ions, aminoacids and sugars
– makes it rigid
– determines shape– Acts as antigen– Used as serological diagnosis
Peptidoglycan
Gram positive cell envelope
• Covalently bound to the thick peptidoglycan are – teichoic acid (their backbones are usually phosphorus
containing polymers of ribitol or glycerol) or – teichuronic acid (glucuronic acid- containing
polysaccharides
• negatively charged molecules concentrate metal ions from the surroundings
Conti…
• Teichoic acids can also direct autolytic enzymes to sites of peptidoglycan digestion (autolysis).
• This is needed to insert sections of cell wall for growth and division.
• Lipoteichoic acid is primarily associated with the cell membrane.
The Gram negative cell envelope• Covalently linked to the thin peptidoglycan is the
Braun lipoprotein which binds the outer membrane to the cell wall.
• Like other membranes it contains proteins and phospholipids.
• Unlike other membranes it contains lipopolysaccharide
Conti…
• Lipopolysaccharide – helps to provide a permeability barrier
• LPS consists of three regions– an outer O antigen– a middle core– an inner lipid A region
• core contains several sugars– lipid A contains β hydroxyfatty acids
(uncommon in nature)– The molecule displays endotoxin activity
Conti…
• Porins in the outer membrane form channels to allow passage of small hydrophilic nutrients (such as sugars) through the outer membrane.
Cell membrane
• Phospholipid bilayer– Except triple layer in Mycoplasma
• Interspersed with lipoprotein• Selectively permeable
Cell Membrane
• Functions– Transport– Synthesis– Excretion– Secretion– Metabolism– division by mesosome
Nucleoid
• Double stranded coiled helical DNA molecule – remain in a single chromosome
• no introns• no non-coding sequences• no regulatory sequence• no long terminal repeats (LTRs)• Operon present
Nucleoid
• Function– bears genes and genetic characters– maintain metabolism
Ribosome
• 70 s in sedimentation co-efficient• Aggregated• Function
– Protein Synthesis
Non-essential structures
• Flagellum/-a (long whip like)– Filamentous Protein Appendages– Composed of protein as flagellin unit– Arise from cytoplasmic membrane– Account for most bacterial motility– “Run and tumble”– Chemotaxis, phototaxis, aerotaxis, and
magnetotaxis– Antigenic structure
Fig 3.42
Non-essential structures
Pilus/-i• Thin hair like appendages• Composed of protein as pillin subunit• Arise from cytoplasmic membrane• Function
– Attachment– Conjugation– Acts as virulence factors– Antigenic
Capsule/Glycocalyx
• External surface layer composed of polysaccharides Except– Polypeptide in Bacillus– Hyaluronic acid for Streptococci
• Gel like• Forming either capsule (compact, complete and
tight investing) or slime layer (loose meshwork)• Functions
– prevent phagocytosis– Attachment of bacteria– Acts as Antigen
Plasmid
• Extra-chromosomal Double stranded Circular DNA
• Independent of replication• Types
– Transmissible– non-transmissible
• Function– Carries gene for its own replication
– Carries gene antibiotic resistance factors
– Carries gene Proteins & toxin
Endospores
• Highly resistant structures formed at adverse environment
• Composition– Bacterial DNA– small cytoplasm– cell membrane– Peptidoglycan– very little water– thick keratin like coat
Endospores
• Highly resistant to– heat, moisture, chemicals, radiation and
antibiotics• Special component
– calcium salt of Dipicolinic acid• No metabolic activities, remain dormant for
many years• Two events: sporulation and germination
The endospore
Steps of Sporulation
Classification of bacteria
• Basis of Classification• differences between Gram positive and Gram
negative bacteria• atypical characters of bacteria in relation to
staining
Basis of Classification
• Morphological Classification• Staining Properties• Thickness of Wall• According to Motility
Morphological Classification
Morphology of Bacteria• Cocci
– rounded or oval (Staphylococcus)• Bacilli
– elongated rod like (Esch.coli)• Vibrio (from vibration)
– coma shaped (Vibrio cholerae)• Spiral bacteria
– long slender curved body with wave like spiral (Spirillum minus, Helicobacter pylori)
Others Shapes
• Cocco-bacillus– longer than coccus, shorter than bacillus
(Bordetella, Brucella)• Filamentous
– long curved body like ribbon with branching (Actinomycetes, Nocardia)
Spirochaetes
Thickness of Wall• Rigid thick wall Bacteria
– Free-Living (Extracellular)– Non-Free Living (Obligate Intracellular)
• Flexible thin walled Bacteria– Treponema– Borrela– Leptospira
• Wall-less Bacteria– Mycoplasma– Ureaplasma
Rigid thick wall Bacteria
• Free-Living (Extracellular)– Gram Positive– Gram Negative– Acid-Fast
• Non-Free Living (Obligate Intracellular)– Rickettsia– Chlamydia
Staining Properties
• Gram Positive
• Gram Negative
• Acid-Fast Bacteria
Gram Positive Bacteria• Cocci
– Streptococcus– Staphylococcus
• Bacilli– Spore Forming
• Bacillus– Non-spore forming
• Clostridium• Corynebacterium• Listeria
Gram Negative Bacteria
• Cocci– Neisseria gonorrhoae– Neisseria meningitidis
• Bacilli– Enterobacteriaceae– Non-Enterobacteriaceae
According to Motility
• Motile– Flagellated Bacteria– Non-Flagellated Bacteria
• Non-Motile– All Cocci
Bacillus with peritrichous flagella
Morphological study of bacteria
• Staining• Motility test
StainingTheoretical approach
• Types
• Laboratory procedures
• findings of – Gram’s– Zeihl-neelsen (Z-N)– Albert’s staining
Types of Staining
• Simple Staining
• Differential Staining
• Special Staining
Types of Staining
Simple Staining
• using a single dye– methylene blue– Leishman
Types of Staining
Differential Staining
• using primary and counter dye
• performing a step of decolourisation
• to see differential characters of bacteria– Gram’s stain– Ziehl-Neelsen stain
– Albert stain
Types of Staining
Special Staining
• to see unique property of particular bacteria– Fluorescense stain
Gram’s staining
• Named according to inventor Christian Gram• Based on the property of bacterial cell envelope• stained at first by a primary dye & mordanted• Decolorized by acetone or alcohol• Counter stain is added• Gram positive bacteria
– can resist decolourisation and retain the primary dye
• Gram negative bacteria – can not resist decolourisation and take the counter
dye
Gram Positive Bacteria
• Thick layer of Peptidoglycan• Negatively charged teichoic acid on surface• Polysaccharide
Gram Negative Bacteria
• Cell wall much more complex• Thin peptidoglycan layer, filled and surrounded
with periplasm (protein rich gel-like fluid)
• Unique outer membrane on top– Bilayer, yet outer layer is LPS layer (lipid A
and O specific polysaccharide) – LPS acts as endotoxin (lipid A)
• Gram neg. bacteria are less sensitive to medications because outer membrane acts as additional barrier.
Fig 3.34
Atypical cell envelope
• Acid-fast and related bacteria – Mycobacteria– Nocardia
• Presence of Mycolic acids – long, branch chained fatty acids
Gram un-stainable cell wall
• Atypical peptidoglycan• Spirochaetes, Mycobacterium,
– High lipid content
– Tight fluid mosaic
Thank You
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