WEEK 2: INTRODUCTION TO BACTERIAL STRUCTURE AND CLASSIFICATION Flashcards
What is microbiology?
‘The study of living organisms which are too small to be viewed by the ‘unaided eye’ but visible with a microscope ‘
Outline organisms which can be studied in micro-biology.
Range from tiny viruses to larger bacteria, protozoa, algae, fungi & some parasites.
Microorganisms: WHY STUDY THEM?
Their pathogenicity, transmissibility & provocation of the immune system make them significant in medicine.
Give 5 characteristics of Pro-karyotes.
Prokaryotes:
Cell Structure: Prokaryotic cells are typically smaller and simpler in structure than eukaryotic cells. They lack a true nucleus and membrane-bound organelles.
- Nucleus: Prokaryotic cells do not have a well-defined nucleus.
Instead, their genetic material is typically a single, circular DNA molecule located in the nucleoid region of the cell.
-Double-stranded
–Single chromosome aggregated in cytoplasm
Haploid – single copy of each gene
-Most bacteria have autonomous smaller circles of DNA ‘plasmids.
Carry genes which confer special properties i.e., antibiotic resistance.
*Transcription coupled to translation in cytoplasm
Membrane-bound Organelles: Prokaryotes lack membrane-bound organelles such as mitochondria, endoplasmic reticulum, and the Golgi apparatus.
Ribosomes: They have smaller ribosomes (70S) compared to eukaryotic cells (80S).
Cell Wall: Many prokaryotes have a cell wall, which can be composed of different materials, including peptidoglycan in bacteria and pseudomurein in archaea.
Reproduction: Prokaryotes reproduce through binary fission, a process in which one cell divides into two genetically identical daughter cells.
Prokaryotes are represented by two major domains: Bacteria and Archaea. They are abundant and diverse and can be found in various environments, including soil, water, and the human body.
Many prokaryotes are unicellular, although some can form multicellular structures.
Prokaryotes are represented by two major domains.
Name them.
Where are they abundant?
Prokaryotes are represented by two major domains: Bacteria and Archaea.
They are abundant and diverse and can be found in various environments, including soil, water, and the human body.
Give 5 characteristics of eukaryotes.
Cell Structure: Eukaryotic cells are typically larger and more complex. They have a well-defined nucleus that contains the genetic material (DNA).
Nucleus: Eukaryotic cells have a true nucleus enclosed by a nuclear membrane, which separates the genetic material from the rest of the cell.
-Double stranded with associated proteins
-Packaged into linear chromosomes
-Diploid – 2 copies of each gene
-Several chromosome pairs e.g., baker’s yeast-16 pairs; human cells- 23 pairs
Key processes in distinct areas:
DNA replication; transcription-nucleus
Protein synthesis/translation-cytoplasm
Membrane-bound Organelles: Eukaryotes have various membrane-bound organelles, including the endoplasmic reticulum, Golgi apparatus, mitochondria, chloroplasts (in plants), and lysosomes, among others.
Ribosomes: They have larger ribosomes (80S) compared to prokaryotic cells (70S).
Cell Wall: While some eukaryotes have cell walls (e.g., plants and fungi), many do not, and the composition of their cell walls varies.
Reproduction: Eukaryotes can reproduce asexually and sexually. Asexual reproduction may involve processes like mitosis, while sexual reproduction involves the fusion of gametes.
Eukaryotes are a diverse group of organisms that include plants, animals, fungi, and various protists. They can be unicellular (e.g., yeast and some protists) or multicellular (e.g., humans, trees, and animals).
The distinction between prokaryotes and eukaryotes is a fundamental concept in biology and reflects the two primary cellular architectures found in the natural world.
Eukaryotes are a diverse group of organisms.
Give the 4 main kingdoms under eukaryotes.
Eukaryotes are a diverse group of organisms that include plants, animals, fungi, and various protists.
They can be unicellular (e.g., yeast and some protists) or multicellular (e.g., humans, trees, and animals).
The pro-karyotes has 3 main groups. Achaea and Bacteria.
What is the similarity between the 2 groups?
Single-celled
No nucleus membrane
No organelles (except ribosomes)
State the 4 characteristics of Archaea.
Archaea
1. Extremophiles
(Extreme: temperatures, pH, salinity, high hydrostatic & osmotic pressures)
- Lipids, cell walls & flagella different to Eubacteria
- Not associated with human disease
- Some have significant role in gastrointestinal tract of ruminant animals.
Describe Eubacteria under the following subtopics.
1. DNA
2. Cytoplasm
3. Ribosomes
4. Cell wall
- DNA:
-Double-stranded
-Tightly coiled into a region termed ‘nucleoid’ - no nuclear membrane
-Extrachromosomal DNA present as small circular DNA –’plasmids - Cytoplasm
Generally, no organelles except ribosomes (protein synthesis) - Ribosomes
Different structure in pro & eukaryotic cells i.e., 70S vs. 80S
(50S & 30S) in prokaryotes (60S & 40S) in eukaryotes - Cell wall
Comprises ‘peptidoglycan’ which surrounds the cytoplasmic membrane.
(Capsules, flagella & pili variable-important role in diagnosis & pathogenicity
In eukaryotes essential cellular processes occur in organelles e.g., mitochondria, endoplasmic reticulum But
Bacteria lack most organelles.
Where do most cellular functions occur?
In eukaryotes essential cellular processes occur in organelles e.g. mitochondria, endoplasmic reticulum
But
Bacteria lack most organelles, most cellular functions occur in the cytoplasmic membrane
Cytoplasmic membrane: a phospholipid bilayer (common to all living organisms)
Unlike eukaryotes, bacteria don’t have mitochondria. CM= site for energy production i.e., electron transport system (ETS) & ATP synthetase.
What powers these single-celled living organisms?
The ETS pumps protons across the CM creating a ‘proton gradient’ & ‘pH gradient’. Accumulation of protons (H+) externally vs. hydroxyl ions (OH-) internally causes a charge across the CM i.e. proton motive force-PMF.
External surface of CM ‘charged positive’ vs. inner surface is ‘charged negative’, like a battery (also outside of the CM is acidic vs. inside which is alkaline).
The PMF used in bacteria provides energy for various activities incl. rotation of the flagellum, active transport of certain solutes
PMF also used in making ATP by the ATPase which uses the protons when synthesizing ATP from ADP & phosphate
PMF is an area of research as a possible a “antimicrobial target.”
What is a cytoplasmic membrane?
CM phospholipids vary across bacteria
e.g., phosphatidylethanolamine, phosphatidyl glycerol, cardiolipin
Other macromolecules within the CM include:
Less common lipids e.g.
-Hopanoids (polycyclic lipids)
-Poly isoprenoid carriers
-Proteins
What are their functions?
It is a lipid bilayer.
CM phospholipids vary across bacteria.
e.g., phosphatidylethanolamine, phosphatidyl glycerol, cardiolipin
Other macromolecules within the CM include:
Less common lipids e.g.
-Hopanoids (polycyclic lipids): confer strength.
-Poly isoprenoid carriers involved in carriage of sugars for cell wall biogenesis
-Proteins i.e. involved in energy production; lipid biosynthesis; secretion & transport
State the functions of the cytoplasmic membrane.
- Permeability barrier: hydrophobic portion is a tight barrier to counteract diffusion of these substances (except H2O).
- Anchor for transmembrane proteins (e.g., transport proteins for solutes; nutrients; hydrophobic molecules).
- Site of generation energy for the cell.
State the 3 types of transport proteins found on the cytoplasmic membrane.
Types of transport proteins:
Uniporteri.e. unidirectional transport of across the CM e.g. K+ uniporter
Symporter/ or co-transporter i.e. two solutes transported in the same direction e.g. lactose permease, phosphate symporter
Antiporter/exchange diffusion i.e. one solute transported in direction & simultaneously 2nd solute is transported in opposite direction e.g. Na+-H+ antiporter
What does high concentration of solutes within cytoplasm causes?
High concentration of solutes within cytoplasm, causes high osmotic pressure.
What are the functions of a cell wall.
What is the cell wall of the eubacteria composed of?
In addition to the CM there is a rigid layer i.e., the cell wall (CW) which also deters possible bursting of the cell.
CW composed of peptidoglycan.
What the 4 functions of the cell wall peptidoglycan.
*Semi-permeable
*Anchors macromolecules e.g. teichoic acids, proteins & polysaccharides
*Cell growth, division & diffusion of molecules
*Cell-to-cell signaling.
State 2 components that make up linear glycan strands of peptidoglycan.
What kind of bond connects the NAM-NAG?
Linear glycan strands composed of
-N-acetylglucosamine (NAG)
-N-acetylmuramic acid (NAM)
Rigidity of peptidoglycan conferred by cross linking in X & Y directions:
NAM-NAG connected by glycosidic covalent bonds
NAM-NAG chains cross-linked by amino acids i.e. L-alanine; D-glutamic acid; L-lysine; D-alanine (vary by no. & type across diff. bacterial species)
Further cross-linked by ‘glycine interbridges’
Structure & configuration of bacterial cell wall is vital for bacterial survival.
Breakdown in cell wall integrity is bactericidal.
Name a group of antibodies that target the bacterial cell wall.
Target for beta-lactam antibiotics
During bacteria cell wall synthesis, what is the function of:
*Transpeptidases-TP (Penicillin Binding Proteins-PBPs)
*Glycosyltransferases (GT)
During bacteria cell wall synthesis:
Transpeptidases-TP (Penicillin Binding Proteins-PBPs) catalyse cross-linking of glycan chains by removal of terminal D-alanine in the peptide chain
Glycosyltransferases (GT) exist as separate subunits or tightly associated with transpeptidases (e.g., PBP-2) catalyze formation of covalent bonds between NAM & NAG
Describe the MOA of Beta lactam antibiotics in the cell wall synthesis.
Note: D-Ala-D-Ala in the CW peptide structure is the substrate for transpeptidase/PBP
The core structure of beta-lactam antibiotics is the ‘-lactam ring’ an analogue of terminal D-Ala-D-Ala in peptidoglycan
-lactam antibiotics bind to active site of transpeptidase/PBP & inhibit cross linking of peptide chains in peptidoglycan.
Describe mechanism of antibody resistance in beta lactam antibodies.
Beta lactamases
Most common mechanism for drug resistance to β-lactam antibiotics
Break the -lactam ring by hydrolysis.
Porins
Gram -ve bacteria have porin channels within outer cell membrane. These inhibit passage of beta-lactams into periplasmic space where the cell wall is located.
Pumps
Gram -ve bacteria can express ABC transporters in the outer membrane that function as ‘efflux pumps.
Transport -lactam antibiotics out of the periplasmic space to the outer environment
Mutated PBPs e.g. PBP2a in MRSA
Mutation in binding site leads to lower affinity to β-lactam antibiotics.
Absence of a peptidoglycan e.g., mycobacteria
What are lipoteichoic acid (LTA)?
Lipoteichoic acid (LTA): a Gram +ve cell wall component that is anchored in the cytoplasmic membrane & extends through the CW. Teichoic acids covalently bound to CW.
State the functions of the LTAs.
LTAs recognized as antigens by immune cells: bind to Toll-like-receptor-2 (TLR-2) & trigger increased inflammation e.g., release of TNF-α, IL-1, IL-6
Aid in binding Ca2+ & Mg2+ for transport into cell
Cachectic (tumor necrosis factor-alpha/TNF-α):
a cytokine predominantly produced by macrophages. Upregulates the immune response: induces fever & facilitates increased production of other pro-inflammatory cytokines
IL-1
release triggered by immune system detection of LTAs & LPS (in CW of Gram-ve). Induces fever & acute inflammation i.e recruitment of WBCs by inducing vasodilation & diapedesis of WBCs, immune cells by activating endothelium of blood vessels.
IL-6
a cytokine produced by macrophages & monocytes, causes fever & facilitates in the production of several acute phase reactants.
State the 2 main components of the gram +ve cell wall.
Gram positive cell wall comprises:
Peptidoglycan: THICK/several sheets of peptidoglycan
Teichoic & lipoteichoic acids – virulence factors unique to Gram+ve bacteria cell wall
How is the cell wall of a gram -ve different from that of a gram +ve?
In contrast to Gram+ve bacteria, Gram-ve bacteria have an additional membrane as part of the CW: “outer membrane.”
In contrast to Gram+ve bacteria, Gram-ve bacteria have an additional membrane as part of the CW: “outer membrane.”
Describe the components of the outer membrane.
Asymmetric bilayer structure:
Inner leaflet – phospholipids
Outer leaflet: primarily LIPOPOLYSACCHARIDES with hydrophobic & hydrophilic properties
Proteins:
Transmembrane proteins e.g., ‘porins’ for diffusion of hydrophilic molecules
Structural proteins, receptor molecules for bacteriophages & other ligands