Introduction to Bacteriology Flashcards
MMID Lectures 2-4
How do prokaryotes differ from eukaryotes?
Prokaryotes have closed, circular DNA, no histones, and no introns. Can contain up to 34% imported DNA.
Name the main shapes used to classify bacteria.
- Rods (bacilli): e.g., E. coli
- Spheres (Cocci): e.g., Staphylococcus aureus
- Spirals
- Threads/Filaments: e.g., Treponema pallidum.
What is the main structural difference between Gram-positive and Gram-negative bacteria?
- Gram-positive: thick peptidoglycan layer and one membrane.
- Gram-negative: thin peptidoglycan layer between two membranes
What is peptidoglycan made of?
- Carbohydrate part: NAG and NAM linked by β-1,4 glycosidic bonds.
- Peptide part: amino acids in L- and D-stereochemistry attached to NAM
How does peptidoglycan polymerisation differ between Gram-positive and Gram-negative bacteria?
- Gram-negative: fewer peptide cross-links, looser organisation.
- Gram-positive: more peptide cross-links for structural rigidity.
What is teichoic acid, what type of bacteria is it found in and what are its functions?
Polymer of alcohol molecules joined by phosphate groups, found in Gram-Positive Bacteria, Functions: structural integrity, contributes to surface composition, role in inflammatory response.
Describe the function of porins in Gram-negative bacteria.
Control entry and exit of molecules (e.g., antibiotics), Size limit: molecules >700 Daltons cannot pass into cell.
What are the three components of lipopolysaccharides (LPS) in Gram-negative bacteria?
- Lipid A: pro-inflammatory, sits in the outer membrane.
- Core oligosaccharide: bridge to oligosaccharide chain.
- Oligosaccharide chain: highly variable, aids in identification.
What are the functions of bacterial capsules and slime layers?
Protect bacteria and prevent dehydration, aids immune evasion by hiding recognition molecules.
What generates the energy for flagellar motion in bacteria?
Proton gradients across membranes.
Describe the polar flagellar arrangement.
Single flagellum at one end.
Describe the amphitrichous flagellar arrangement.
Two flagella, one at each end.
Describe the lopotrichous flagellar arrangement.
Bunch of flagella at one end.
Describe the peritrichous flagellar arrangement.
Flagella around the entire cell like a halo.
What are the roles of pili (fimbriae) in bacteria?
Adhesion to surfaces, subsequent retraction allows movement toward a surface.
Describe the composition of pili (fimbriae) in bacteria.
Protein filaments with specialised adhesion subunits at the tip.
What is the signficance of peptidoglycan containing both L and D steroisomers of amino acids when proteins only contain the L form?
Aids function of peptidoglycan to provide mechanical strength: D amino acids will not be broken down by proteolytic enzymes.
What is special about DAP (amino acid found in peptidoglycan)?
It is not one of the 20 amino acids that makes up proteins.
In which type of bacteria does its peptidoglycan contain an interpeptide chain attached to its DAP?
Gram-Positive peptidoglycan.
What is the order of the 5 amino acids attached to NAM in peptidoglycan, starting from the NAM component?
L-alanine, D-glutamate, DAG, D-alanine, D-alanine.
4 reasons
Why is it important to identify pathogens?
- Predict symptoms.
- Suggest treatment.
- Identify the source of infection for better transmission management.
- Predict when infections might occur.
What factors need to be considered when using bacterial culture conditions to identify bacteria?
Bacterial response at different temperature, different levels of oxygen availability, different carbon sources (e.g., glucose, lactose), interaction with indicators, blood, bile salts, or antibiotics.
What are the four main types of culture media?
- Rich media grows everything.
- Selective media promotes growth of specific species.
- Differential media distinguishes species based on metabolic processes (e.g., colour changes).
- Enrichment media supports bacteria with specific growth requirements.
Give an example of enrichment media and its use.
Blood agar supports bacteria causing bloodstream infections and distinguishes Gram-positive bacteria via haemolytic activity.
What does the catalase test differentiate between?
Staphylococcus (catalase-positive, causes skin infections), Streptococcus (catalase-negative, causes respiratory infections).
What does the coagulase test identify?
Different strains of Staphlococcus bacteria: Staphylococcus aureus (positive: clumping observed), Staphylococcus epidermidis (negative).
What is the IMViC test used for?
Discriminating between coliform species by testing for tryptophanase activity, acid production, acetoin production, and citrate utilisation.
What are the three main types of antigens used in serotyping gut bacteria?
- O-antigens: sugar side chains of Gram-negative LPS.
- K-antigens: capsular monomers.
- H-antigens: flagellar proteins.
What is CHROMagar, and when is it effective?
A medium that identifies bacteria (e.g., UTI pathogens) by producing distinct colours, works well with high bacterial concentrations such as in urine.
Why does the BactAlert blood culture system take time to identify bacteria?
Requires bacterial growth to detect CO₂ production and identify bacteria, needs 36–72 hours for results and subsequent identification.
How does PCR help in bacterial identification?
- RAPD-PCR uses primers (random sequences) to create genetic fingerprints for tracking transmission. Sequences are unknown so will not enable species identification.
- MLST identifies strain-specific alleles by targeting essential genes.
- Rapid identification of MRSA (by showing presence of mecA gene).
What is the advantage of Whole Genome Sequencing (WGS) in outbreak detection?
Identifies similarities and differences between bacterial genomes, provides precise identification of transmission events.
What is a coliform?
a type of Gram-negative, rod-shaped bacterium.
When grown on blood agar, what is observed in α-haemolytic gram-positive bacteria?
Biliverdin is produced, observations are dark green cloudiness.
When grown on blood agar, what is observed in β-haemolytic Group A gram-positive bacteria?
Completely clear areas of blood agar.
When grown on blood agar, what is observed in β-haemolytic Group B gram-positive bacteria?
Slight orange haze in areas of partial clearing in the blood agar.
What is meant by the term ‘microbiota’ and what did it used to be called?
It is the community of commensal, mutualistic and pathogenic microorganisms that share our body space. Used to be called flora.
What is meant by the term ‘microbiome’?
It is the totality of organisms and their collective genetic material.
Why is it important to study microorganisms?
They are everywhere! Humans are constantly in contact with microorganisms. Even our genome contains viral genetic material. As humans, we are the hosts of many microorganisms.
What are the 3 main types of Host-Microbe Interactions?
Mutualism: Both host and microbes benefit from the symbiotic relationship (e.g. human gut microbiota) this is the most common interaction. Commensalism: Microbes benefit from the interaction but host does not (e.g. skin, nose and oral cavity microbiota).
Parasitism: Microbes benefits from interaction whilst harming the host. In parasitism, the microbe is referred to as pathogenic.
What is dysbiosis?
When alterations that generate an imbalance occur, pathogens may colonise, causing disease. Factors may include stress, production of toxins by pathogens, type of food ingested, medications (e.g. antibiotics), malfunction of organs. Can lead to infection of opportunistic pathogens.
How can we use sequencing to identify bacteria of the microbiome at the species level?
Based on the 16S rRNA gene which is composed of 9 hypervariable regions. Amplification using PCR and primers to target 16S rRNA gene allows us to compare to a bacterial species database to identify them at the species level.
Where in the human body is the microbiota most diverse?
The gut.
Which 5 biological processes in humans might the bacteria of the microbiota aid?
Extracting energy from food, producing essential vitamins, regulating our immune system, regulating glucose levels and metabolism, protecting us against pathogens.
What factors can affect the microbiota?
Obesity, age, malnutrition, environment, changes in diet can change microbiota composition. Studies transplanting microbiota of mice, show a relationship between obesity and microbiota. We don’t know if this is cause or effect though.
Approximately what percentage of the adult population suffer from irritable bowl syndrome (IBS)?
11%, they have an altered microbiome.
What does the gut-brain axis refer to?
Relationship between brain and gut, bloodstream and nerve system connects them. This is a form of bi-directional communication.
What are the key findings in relation to Gut-Brain axis research?
Serotonin, dopamine and other relevant neurological molecules are produced by some microbes living in out gut, such as Candida, E.coli. Animal Studies found associations between autism spectrum disorders and intestinal problems, faecal transplant from ASD individual causing ASD-like behaviours in previous neurotypical mice. Human studies found that stress increases gut permeability, allowing bacteria antigens to cross - neural mechanism unclear.
What are some examples of bacteriotherapy?
Probiotics, prebiotics, synbiotics and Faecal Micriobiota Transplantation
What are probiotics?
Live microorganisms which when administered in adequate amounts confer a health benefit on the host.
What are prebiotics?
Non-digestible food ingredient that beneficially affects the host by selectively stimulating the growth / activity of a limited number of bacteria in the colon.
What are synbiotics?
Food ingredients or dietary supplements combining probiotics and prebiotics in a form of synergism.
When are Faecal Microbiota Transplants used and what is the process of adminstration?
Used in patients with recurrent C. difficile infections as a last resort. 50g-300g faeces are donated from a healthy individual. Faecal material is washed in saline and administered in pills or as a faecal slurry (tube to stomach). Approximately 1-2 days after transplantation, recipient microbiota shifts dramatically to resemble healthy gut microbiome.
What is MALDI-TOF identification? When can it be used? How does it work?
- MALDI-TOF is used to identify bacterial species causing infection.
- Often requires cultures on plate but can use blood cultures directly from machines that purify bacterial proteins.
- Only works for single species identification (90% of the time this is the case for bloodstream infections).
- Ionisation of material, causing movement towards a magnetic field: size of molecule dictates speed of movement.
- Gives a series of peaks represent charge:mass of the mixture, indicative of a particular species.
- Spectrum compared to database containing sprectra of bacteria.
- Machine looks for closest match and gives a likelihood score for the match.
What are the main limitations of MALDI-TOF?
- Where there are multiple species causing infection, it is useless.
- Less accurate with less common species as there are less spectra of this species on the database.
- There is little clinical relevance in knowing the species. What matters is whether bacteria are resistant or susceptible to antibiotics. Often knowing the species would not identify this.
