Key stuff - Bacteria Flashcards
Bacterial cell clusters: names with examples.
diplococci - Neisseria meningitides
streptococci - Streptococcus pneumonia
clump of cocci - Staphylococcus aureus.
Gram-positive cell wall
Almost 90% peptidoglycan - thick layer.
Gram-negative cell wall
Consists of a thin layer of peptidoglycan surrounded by an outer membrane. Outer membrane composed of lipids, proteins, and lipopolysaccharide (LPS).
Peptidoglycan only found in Bacteria and makes up their cell wall. What is it’s structure?
Made up of G-M bonds, with chains linked via peptide bridges.
G = N-Acetylglucosamine
M = N-Acetylmuramic acid
Fimbriae (singular = fimbria) - Bacteria
short, thin, hair-like, proteinaceous appendages (up to 1,000/cell)
recognition and attachment to surfaces
Pili (s., pilus; sometimes called sex pili) - Bacteria
similar to fimbriae except longer, thicker, and less numerous (1-10/cell), required for mating
polar flagellum
flagellum at end of cell
monotrichous
one flagellum
amphitrichous
one flagellum at each end of cell
lophotrichous
cluster of flagella at one or both ends
peritrichous
spread over entire surface of cell
Plasmids
Usually small, closed circular DNA molecules
Exist and replicate independently of
chromosome
Not required for growth and reproduction
May carry genes that confer selective advantage (e.g., drug resistance)
Cellular inclusions
Granules of organic or inorganic material that are reserved for future use.
Cellular inclusions - Specialist bacteria with magnetosomes
Contain iron in the form of magnetite
Use is to orient cells in magnetic fields
Cellular inclusions - Gas vesicles
Used for buoyancy in some aquatic bacteria.
Eg. Cyanobacteria that perform photosynthesis and need sunlight.
Endosprores
made by some gram-positive bacteria.
Can survive for hundreds or even thousands of years
- produced under unfavourable conditions; perhaps when cells run out of nutrients.
Highly resistant to heat, drying, radiation, & chemicals very low water content.
Contain calcium dipicolinate – binds free water and helps dehydrate cell
Special proteins protect DNA
How to bacteria acquire iron.
not readily available so have proteins which strip out iron from blood.
Chemically defined media
exact chemical composition is known.
if you want specific strains/species.
Complex media
exact chemical composition not known.
Whole range of species will grow.
How do we grow in lab?
Solid culture meda : Nutrient agar plates.
inoculating loops physically drag cells across agar and this separates them so individual cells can be counted, essentially serial diluting.
microbiology growth.
increase in cell numbers
Bacterial generation time
Time needed for a population to double = doubling time= generation time.
different strains have different doubling times.
E.coli - 20 mins at 37degrees.
Exponential growth
growth with a constant doubling time.
Batch culture
culture grown in ‘closed system’
no additional nutrients added and no bacterial waste products removed during the culture period.
Typical growth curve for a bacterial population.
Lag phase: Time interval between inoculation and maximal division rate:
Cells adjust to new environment.
Log (exponential phase): Bacteria grow exponentially:
-Constant doubling time
-Growth rate is maximal
Stationary phase: Bacteria can no longer reproduce but are still alive
(e.g., no nutrients left or growth inhibited by bacterial products)
Death (decline) phase: Bacteria die.
Culturable bacteria
can be grown on media (liquid/solid).
Viable but nonculturable bacteria.
in a state of low metabolic activity and do not divide but are alive and have the ability to become cultural once resuscitated.
Or they can’t grow on conventional media (e.g Legionella pneumophila)
Total count:
Non-specific dye that stains all bacteria - (culturable, viable and VBNC and in many cases dead cells)
Viable count:
uses fluorescent activity dyes
counts all cells (cultural, viable and VBNC) with activity (e.g. enzymatic, active membranes etc).
Culturable count:
counts cells that can form colonies on solid media or increase turbidity in liquid media.
Direct: Microscopic Count
easy and fast
uses special microscope counting slide
does not differentiate between live and dead bacteria.
DAPI
general stain
Acridine orange
general stain, binds to cell walls.
Measuring bacterial growth using agar plates
Counting colony so is a culturable count, assuming each culturable cell will grow and divide to yield one colony.
Measuring bacterial growth via serial dilution.
To obtain appropriate colony numbers so individual cells or colonies can be counted.
Vibrio Cholerae
infects intestine and causes cholera.
In Africa villages reduced cholera by using their safe as a filter to pour water. into a collection vessel. Physical barrier as the bacteria are associated to zooplankton by attaching to chitin.
Can boil water to kill off the bacteria.
Indirect measurement of bacterial growth.
Spectophotometer for turbidity of bacterial growth - cells scatter light passing through the suspension.
Bacteriostatic antimicrobial agent.
Total and viable/culturable cell count stays the same but bacteria are stopped in tracks. Growth can continue once agent wears off.
Bacteriocidal antimicrobial agent.
Total cell count remains same but viable/culturable count decreases. Killed but still intact.
Bacteriolytic antimicrobial agents.
Both cell counts decrease. Cells killed and falls apart.
Heterotrophs
require organic molecules made by other organisms.
Autotrophs
CO2 is a principal carbon source.
Phototrophs
use light as energy source.
Chemotrophs
oxidise organic or inorganic compounds.
Obligate aerobes
Needs O2 for growth.
Obligate anaerobes
Cannot grow in presence of O2
Facultative anaerobes
Can grow with and without O2.
Aerotolerant anaerobes
Do not need O2, but tolerate it
Microaero-phillic
Need O2, but tolerate it only at a low concentration.
Psychrophiles
grow best below 15degrees
don’t grow above 20degrees
can grow below 0degrees
Mesophiles
grow best between 20-40degrees
many bacteria in our body are mesophiles.
Thermophiles
grow best between 45-80degrees
live in hot springs, compost heaps etc.
Hyperthermophiles
grow best above 80degrees
live in hot springs.
Acidophiles
grow best in acidic habitats
Alkaliphiles
grow best in alkaline habitats
Halophiles
grow in habitats with high salt concentration.
How many bacteria live in and on our body?
10^14
10^12 in gut helping with immune system.
Host
organism which supports growth of viruses, bacteria and parasites.
Pathogen
organism that causes disease, by impairing or interfering with the normal physiological activities of the host.
Pathogenicity
the ability to cause disease
Virulence
the degree or intensity of pathogenicity (determined by toxicity and invasiveness).
Infection
bacteria persist in host without necessarily causing tissue damage.
disease
overt damage to the host, parts of body cannot fulfil their normal functions.
Dr. William Farr
prominent supporter of the miasma theory. Convinced that cholera was transmitted by air.
Robert Koch (1843-1910)
Koch’s lab established germ theory of disease.
Formulated criteria for proving that a specific microorganism causes diseases: “Koch’s Postulates”
Developed simple methods for obtaining bacteria in pure culture
Filippo Pacini
a Italian physicist, first discovered cholera.
Koch’s postulates.
Key concepts:
a specific infectious disease is caused by a specific microbe.
determine aetiology (cause) of disease, first step in treatment and prevention.
Microbiologists used these steps to identify causes of emerging diseases.
Opportunistic pathogens
only cause serious disease when host defences are impaired.
Primary pathogens (Obligate)
Capable of causing disease in absence of immune defects.
Need to cause disease to survive.
colonisation
establishment of a stable population of bacteria in the host.
pathogen must be able to compete successfully for nutrients and surface attachment sites.
Association
involves non-specific forces. (e.g charge and hydrophobicity)
Adhesion
Involves specific bacterial adhesions and host receptors.
biofilm
clump of bacteria, can be mixed or monoculture.
Adhesins
Fimbriaae and pili
Capsules and slime layers
Flagella (in some species)
(lipo)trichroic acids (Gram-positives)
Host receptors
Blood group antigens
Extracellular matrix proteins e.g fibronectin, collagen.
Barriers to colonisation or infection.
Eyes: Lysozyme dissolves cell walls
Skin: physical barrier produces antimicrobials normal flora inhibits pathogens,
Normal flora competes with pathogen.
Mucus in lungs and trachea prevent colonisation.
Stomach pH inhibits or kills bacteria
Flushing of urinary tract prevents colonisation.
Lytic compounds that attack host tissue:
collagenase, phospholipase, haemolysins
Bacterial avoidance of phagocytosis:
bacteria produce structures preventing effective contact: capsules, special surface proteins.
survival inside phagocytic cells: often by very pathogenic bacteria: they can escape one of the most effective defences.
some can burst macrophages e.g legionella.
Avoidance of antibodies
Capsules (sometimes not immunogenic because they resemble host structures)
Antigenic variation: bacteria can switch between different types of a surface structure
e.g streptococcus pneumoniae can make over 50 capsule variants.
Sometimes degration of antibodies.
Capsules can prevent compliment activation.
Lipopolysaccharides (gram-negatives) sometimes hinder pore formation.
Nutrient acquisition Iron uptake.
Tissues:free iron levels are below that required to support bacterial growth.
Bacteria express high affinity iron uptake systems.
- Siderophores : bind iron with high affinity
- Direct binding of iron transport proteins. e.g transferrin.
Bacterial toxins
Exotoxins - act on specific targets (e.g protein synthesis)
Endotoxin (LPS): bound to cell
action is indirect: activates many host systems that cause damage.
Exotoxins
Made by Gram + and -ve
Protein
secreted by living bacteria
usually heat labile
highly immunogenic (very easily cause a immune response.)
potentially lethal
Endotoxin
Made by Gram-ve
LPS (liopolysaccharide)
Part of cell membrane,
released on cell lysis.
usually heat stable
weakly immunogenic
lethal at higher concentrations.
Tetanus toxin (exotoxin)
a neurotoxin, interferes with synapse function
Diphtheria toxin (exotoxin)
inhibits mammalian protein synthesis.
-Diphtheria has a mortality of up to 20% in very young/old.
Endotoxin structure
0-specific polysaccharide connected to core polysaccharide connected to lipid A.
-Lipid A part has endotoxin activity.
-Activates many host systems that cause damage, leading to fever; shock; blood coagulation; inflammation.
-Core polysaccharide is variable.
