Bacteria Flashcards
Describe the differences between prokaryotes and eukaryotes
The diversity of life can be classified within three domains:
bacteria, archaea (prokaryotes); and eukarya (eukaryotes).
Differences between eukaryotes and prokaryotes:
Prokaryotes lack a membrane-bound nucleus, and membrane bound organelles ^[some recent studies suggest the presence of organelle like structures]. Eukaryotes have both. Prokaryotes are unicellular, and reproduce by asexual reproduction.
Common features of prokaryotes and eukaryotes (which can be unicellular or multicellular, and can reproduce by either asexual or sexual reproduction) include a cytoskeleton, a plasma membrane, cell wall, ribosomes, chromosomes and cell division
> note: these will differ in either composition, size and structure, two of these, or all three
In general, eukaryotes differ from prokaryotes in terms of their size and complexity. Among the prokaryotes, the differences are usually related to metabolism of nutrients, e.g. nitrogen utilisation and methanogenesis, as well as toleration for extreme environmental conditions in the case of archaea.
The basic biological processes in both eukaryotes and prokaryotes are similar— this is because all living things must produce offspring like themselves for their species to survive. Note similar does not necessarily mean identical.
Define bacteria
Bacteria are very small, single-cell organisms which occur as cocci, bacilli and spirilla or spirochetes
These three basic forms are often stuck together in long strings, in pairs or found in clusters that look like little squares, or random grape-like clusters.
Explain why bacteria are important
- Bacteria play a pivotal role in disease e.g. infection, and contributing to AMR
- Bacteria play a pivotal role in health
> - cover us in invisible body armour e.g. normal flora ^[#microbiology]
> - educate immune system to fight against pathogens and protect body
> - digest food
> - make vitamins - Bacteria are also fundamental in industry
> - agriculture: nitrogen fixation, nitrogen cycling, converting cellulose to animal proteins
> - manufacture of food: e.g. fermented and preserved foods
> - production of antibiotics and other drugs
> - production of probiotics
> - biotechnology: production of vaccines, and GMOs
> - insecticides
> - enzymes
> - fuels
> - solvent
> - model systems
Describe the process by which bacteria grow and replicate
Bacteria undergo binary fission (asexual reproduction), a simple type of cell division ^[see [Cell Biology Lecture 2]].
- The bacterial cell elongates as new material is synthesised
- It replicates (or duplicates) its chromosome and separates newly formed DNA molecules so one chromosome is in each each half of the cell
- A partition or septum is formed at the midcell, dividing parent cell into two progeny cells
- which each have their own chromosome and a complement of other cellular constituents ^[note that this process is similar to mitosis except that there is no division of a nuclear membrane]
Growth of bacteria in culture broth (where nutrient concentrations decline and concentrations of waste increases) can be plotted as the logarithm of number of viable cells versus incubation time.
- While phases relate to ‘life in the lab’ bacteria encounter conditions in their natural environment that mimic what occurs in a broth culture ^[closed system?]
- Lag phase
- occurs when bacteria are introduced into a fresh culture
- No immediate increase in cell numbers (because they are synthesising new components)
- metabolically active, synthesising enzymes, producing energy/ATP - Exponential phase
- bacteria reproducing at maximal rate, doubling in number at regular intervals i.e. 2 $^n$ - Stationary phase
- occurs when nutrients are depleted
- growth plateaus
- total number of viable cells remain constant due to- balance between cell division and cell death
- population ceases to divide but remains metabolically active
- Death phase
- Occurs when nutrient deprivation and build up of toxic waste causes irreparable harm to cells
- Number of viable bacteria declines exponentially at a constant rate i.e. 2 $^n$
Physical factors that influence (the efficiency of) bacterial growth:
- oxygen concentration
- oxygen concentration determines growth, and site of infection
- obligate aerobes require oxygen to grow
- obligate anaerobes are killed in oxygen. Aerobes are a lot more efficient and they divide more rapidly, and produce more progency ^[[biochem]] [[Metabolism Lecture 3 ]]
- facultative anaerobes can grow in presence or absence of oxygen
- note that facultative anaerobes are usually most infectious e.g. E. coli
- aerotolerant anaerobe: ignores the presence of oxygen e.g.
- temperature
- psychrophiles grow optimally at < 15C
- mesophiles grow optimally at temperatures between 20-45C. These tend to be human pathogens
- thermophiles grow optimally at temperatures above 6C
- hyperthermophiles are very rare
- pH
- several bacteria grow optimally in a range of neutral pH values (5-8)
- alkalophiles grow optimally at more alkaline extremes (9-11)
- acidophiles grow optimally at more acidic extremes (2-4)
Define the cell wall and its functions
The cell wall is a layer that lies just outside the plasma membrane i.e. it is essentially everything beyond the plasma membrane. It has several functions including:
- maintenance of cell strength
- protection of cell from osmotic lysis and toxic substances
- contributing to pathogenicity, helping bacteria adhere to and invade host cells
- can elicit an immune response or confer resistance to phagocytosis
- serve as an antibiotic target
- assist in bacterial classification
Gram-positive and Gram-negative bacteria have different cell walls
Describe the basis of Gram staining
Gram-positive and Gram-negative bacteria have different cell walls
The Gram stain is a differential stain. One group of bacteria i.e. the Gram positive bacteria, retain the stain, while the other i.e. Gram negative, do not.
Gram-positive retain Gram stain in their thick peptidoglycan. Gram negative bacteria do not retain Gram stain as the layer of peptidoglycan are not thick enough to retain the stain.
This is useful for therapy $^(c/b)$
Basis of the Gram stain
The basis of the gram stain lies in teh dundamental structural differences in the cell wall of these two groups of bacteria.
Rough procedure:
- crystal violet and iodine are added
- together they form a precipitate in peptodoglycan
- ethanol is applied as a wash step, will not wash out of Gram positive bacteria
- a counter stain is added which stains Gram negative bacteria
Peptidoglycan- is a common feature of both Gram-positive and Gram-negative bacteria
- it is a mesh-like structure composed of long chains of alternating sugar derivatives (NAG, NAM) linked together by four differnet amino acids ( 3o f which are not found in proteins)
- note direction of various crosslinks (up down and across)
- this increases strength, flexibikity, enables stretching and contracting in respinse to changes in osmotic pressure
- provides much of the strength and rigidity possessed by bacterial cell walls, but still elastic and porous
Describe the features of Gram positive bacteria
LTAs
- Gram positive bacteria have LTAs anchore d to plasma membrane and extending into the peptidoglycan
- LTAs are long anionic polymers of varied and species-specific function:
- they can be thought of as a framewoek
- they infleunce the rigidity and porosity of the clel wall
- they protect the cell from harmful substances e.g. antibiotics and host defense molecules
- assist in the binding of pathogenic bacteria to host cells i.e. are adherence factors
WTAs
- Gram positive bacteria have long anionic polymers called WTAs
- WTAs differ to LTAs in that they are anchored to the peptidoglycan, iniitiating from peptidoglycan and extending onto the cell surface
- WTAs perform similar function ot LLRAs and together form a significant fraction (50%) of the cell wall mass in producing organisms ie. adherence factors and providing structure and rigidity
Describe the features of Gram negative bacteria
The outer membrane
- Gram negative bacteria have an outer membrane as part of the cell wall which lies outside the peptidoglycan layer
- it is a lipid layer but differs to the plasma membrane: PLS are confined to inner leaflet, while outer leaflet is composed of glycolipids principally LPS
- Outer membrane proteins carry out several function including uptake of nutrients , efflux of metabolic waste, adhesion to host cells, translocation of effectors and helps to maintain integrity and barrier function
LPS
- a molecule that comprises both lipids and carbohydrates
- it consists of three parts: Lipid A (endotoxin, contributes to septic shock). core polysaccharide (which projects from the surface), and O antigen side chains
- The O antigen side chain differs in composition between bacterial strains which aids in classification
- reduce membrane fluidity, helps maintain low permeability, protects bacteria from host defences
Compare and contrast Gram positive and negative bacteria
- outer membrane in Gram negative bacteria
- thicker peptidoglycan in Gram positive i.e. not absent in Gram negative, hence stains, but poorly
- LTA WTA in Gram positive
- ## LPS and outer membrane in Gram negative
Describe the process of serotyping and the factors used to define serotypes
Serotyping
a procedure involving antibodies i.e. serological
it is used to differentiate between strains of bacteria that have differences in the antigenic determinants expressed on bacterial cell surface
this is very important as bacteria evolve rapidly and so changes to antigenic determinants occur quickly
essentially any feature of the bacteria can work e.g. LPS, membrane proteins, fimbriae, toxins secreted, TAs, polysaccharide, capsules and flagella
the strains differentiated by antigenic differences are known as serotypes or serovars. these hold diagnostic value as they can dictate value i.e is the infection from a singular strain or mixed, and should one or multiple types of antibiotics used to treat
Describe the phases of bacterial growth
Growth of bacteria in culture broth (where nutrient concentrations decline and concentrations of waste increases) can be plotted as the logarithm of number of viable cells versus incubation time.
- While phases relate to ‘life in the lab’ bacteria encounter conditions in their natural environment that mimic what occurs in a broth culture ^[closed system?]
- Lag phase
- occurs when bacteria are introduced into a fresh culture
- No immediate increase in cell numbers (because they are synthesising new components)
- metabolically active, synthesising enzymes, producing energy/ATP - Exponential phase
- bacteria reproducing at maximal rate, doubling in number at regular intervals i.e. 2 $^n$ - Stationary phase
- occurs when nutrients are depleted
- growth plateaus
- total number of viable cells remain constant due to- balance between cell division and cell death
- population ceases to divide but remains metabolically active
- Death phase
- Occurs when nutrient deprivation and build up of toxic waste causes irreparable harm to cells
- Number of viable bacteria declines exponentially at a constant rate i.e. 2 $^n$
Describe physical factors that influence the efficiency of bacterial growth
- oxygen concentration
- oxygen concentration determines growth, and site of infection
- obligate aerobes require oxygen to grow
- obligate anaerobes are killed in oxygen. Aerobes are a lot more efficient and they divide more rapidly, and produce more progency ^[[biochem]] [[Metabolism Lecture 3 ]]
- facultative anaerobes can grow in presence or absence of oxygen
- note that facultative anaerobes are usually most infectious e.g. E. coli
- aerotolerant anaerobe: ignores the presence of oxygen e.g.
- temperature
- psychrophiles grow optimally at < 15C
- mesophiles grow optimally at temperatures between 20-45C. These tend to be human pathogens
- thermophiles grow optimally at temperatures above 6C
- hyperthermophiles are very rare
- pH
- several bacteria grow optimally in a range of neutral pH values (5-8)
- alkalophiles grow optimally at more alkaline extremes (9-11)
- acidophiles grow optimally at more acidic extremes (2-4)
Describe the components of bacteria
- cytoplasm: an aqueous compartment enclosed by the membrane where all celullar functions take place e.g. DNA replication, transcription, translation, energy production and removal of metabolic waste
- plasma membrane: a mechanical boundary in all bacteria composed of PLs; encompasses the cytoplasm
- the plasma membrane has a range of functions. Primarily it is a selectively permeable barrier allowing nutrients in and waste out. It is also crucially the site of oxidative phosphorylation***, and allows for detection of environmental cues for chemotaxis
- cell envelope: which comprises the plasma membrane and all surrounding layers external to it. This typically includes the plasma membrane, cell wall (comprised of peptidoglycan), outer membrane (sometimes), capsule (sometimes)
