Module 23 - Introductions to Bacteria Flashcards
Mention the four common ways to classify Bacteria.
- Genetic Composition
- Physical properties: shape, staining, etc
- Metabolic properties: relationship with O2, biochemical activity
- Pathogenicity: surface antigens. capacity to cause disease
Mention some characteristic of the bacteria genome.
- Bacterial genome smaller than eukaryotes
- It doesn’t have membrane-bound nucleus, but rather a nucleoid
- single chromosome (dsDNA) - comprises of genetic material and associated machinery
- variable in size
- Also contian plasmids
- dsDNA - separate from chromosome and replicate independently
- Majority of bacterial DNA are for functional genes (no introns)
What are core genome and pan-genome?
The core genome represents the genes present in all strains of a species. It typically includes housekeeping genes for cell envelope or regulatory functions.
The pan- or accessory genome refers to genes not present in all strains of a species.
Bacteria can evolve and acquire genetic diversity through __________ & ___________.
Mutations and horizontal gene transfer
What is horizontal gene transfer? Mention the known mechanisms that resulted in HGT.
It is the acquisition by an organism of genetic information by transfer from an organism that is not its parent and is typically a member of a different species. Known mechanisms include:
- plasmid
- transposons
- bacteriophage
- pathogenicity islands (a series of genes that includes one or more virulence determinants)
- integrons (genetic elements that allow efficient capture and expression of exogenous genes)
What is taxonomy?
Taxonomy is the classification of biological organisms based on common or shared characteristics. It is heavily informed by phylogenetics - measures of the evolutionary relationships between species.
________, __________, and ___________ classification defined the prokaryotic raxonomic systems until the late 20th century.
Morphological, biochemical, and metabolic
Molecular genetics is used by Carl Woese (1977) to establish that bacteria has a distinct lineage to _______.
Archaea
Mention a common characteristic of the bacteria/archaea genome. Explain how it can be used to classify them.
16S rRNA is the component of the 30S small subunit of a prokaryotic ribosome, which is highly conserved between different species of bacteria and archaea.
Due to this characteristic of the rRNA gene, it can be used as a molecular clock for mapping evolution as it consists of both highly conserved and hypervariable region.
- Nine hypervariable region (V1-V9): secondary structure of 30S ribosomal subunit
Highly conserved sequences between the V regions facilitates identification (via universal primers) by sequencing the same sections of the 16S sequence across the taxa.
The similarity of the variable regions can then be compared to determine the phylogeny/relatedness
What is the function of the 16S rRNA?
It serves a structural role by acting as a scaffold for the ribosomal proteins. It also interacts with 23S rRNA, aiding in the binding of the two ribosomal subunits (50S+30S).
As a part of the 30S subunit, the 3’ end of the 16S contains an anti-Shine-Dalgarno sequence, which helps the subunit binds to the mRNA sequence. It also binds to proteins S1 and S21 (involved in protein synthesis).
Mention the limitation of using 16S rRNA for phylogeny.
Limitations arise with closely related species, such as Enterobacteriaceae, that have up to 99% sequence similarity across the entire gene. This degree of similarity between the species of this genera makes it hard to pinpoint their evolutionary relationships.
Classification systems are also limited a define gene space, rather than the whole genome.
Mention the five basic shapes of Bacteria.
- Spherical (cocci)
- Rod (bacilli)
- Spiral (spirillia)
- Comma (vibrios)
- Corkscrew (spirochaetes)
Can occur as single cells, in pairs, chains or clusters
Cell shape is fixed due to ________ and ____________;
cell wall, peptidoglycan
Mention the different staining method for bacteria.
Explain the mechanism of Gram-staining.
It enables the separation of eubacteria into two distinct groups: gram-positive and gram-negative.
Gram-positive bacteria have more peptidoglycan and retain the crystal violet stain. Gram-negative bacteria have less peptidoglycan and do not retain the crystal violet stain when washed. It is then counter-stained by safranin
The steps are of the following:
- Fixation
- Crystal Violet Staining
- Iodine Treatment
- Decolorisation
- COunter stain with Safranin
6.
Mention the characteristics of gram-positive cells.
- Peptidoglycan is a major component of the cell wall
- Cell wall also contains teichoic acid and lipoteichoic acid
- Only one cell membrane, which is functionally similar to the inner membrane of Gram-negative organisms
Mention the characteristics of gram-negative cells.
- More complex than gram-positive cell wall
- Have an outer membrane, with protein channels (porin)
- Lipopolysaccharide (LPS) stabilises the outer membrane and acts as endotoxin (a PAMP: pathogen-associated molecular pattern)
Explain how acid-fast staining works.
Also called the Ziehl-Neelsen stain, it exploits the abundance of mycolic acids in the cell walls of acid-fast bacteria, which resist other staining methods. It is mainly used to identify mycobacteria.
- Stain with carbol fuchsin stains (all cells)
- De-stain with acid-alcohol (only non-acid-fast bacteria are destained)
- Counter-stain non-acid-fast bacteria with methylene blue (or malachite green)
Acid-fast bacteria will appear red because of the carbol fuchsin.
Explain the structure and function of capsules and the staining procedure to visualize it.
It is a polysaccharide-based material extending from the cell surface (glycocalyx). It contributes to virulence (differs in different bacterial species) by:
- facilitate adherence to host cells and surfaces
- protection against phagocytosis and immune surveillance
- protection against dehydration
Explain the structure and function of flagella and mention the staining method to visualise it.
It is a thin, long, hollow, helical filament, which enables locomotion and also often functions as a sensory organelle. it consists of a basal body, hook, and filament (composed of flagellin protein - H antigen)
It is visible by light microscopy with special stain (tannic acid and basic fuchsin).
Explain the four phases of bacterial growth.
- Lag Phase: bacteria adapt to growth condition, but not able to divide; they are remodelling their metabolism for the available growth conditions
- Log phase: exponential growth
- Stationary phase: depletion of a nutrient (growth factor) or formation of inhibitory waste products; mutations may occur
- Death phase: catastrophic nutrient depletion or evironmental/injurious factors
How does one test for hydrogen peroxide presence in bacterial metabolism?
By adding catalase, which catalyses the decomposition of hydrogen peroxide.
How does one detect bacterial lactose fermenters?
MacConkey agar is a selective and differential culture medium, which contains lactose and a pH indicator. It is used to selectively isolate gram-negative and enteric bacilli.
Lactose fermenter will produce acid and visualized as pink.
What are bacteriophages? Explain the concept of phage typing.
Bacteriophages are viruses that infect bacteria and archaea. Phage typing is a diagnostic approach that exploits the restricted host range of certain bacteriophages.
