Bacteria Flashcards
Describe other important classification factors for bacteria
Growth temperature Ability to form heat spores Motility Cell shape Ability to use various carbon and nitrogen sources Special nutritional requirements
How does the gram staining technique work?
Stains the cell wall of gram +ve bacteria
Application of crystal violet➡️ application of iodine➡️ alcohol wash➡️ application of counter stain (safranin)
What are the requirements for the growth of bacteria?
Growth for bacteria is increase in cell number therefore they need the elements of their composition
Require energy to synthesis of the anhydride bonds that links the macromolecules
Via fermentation of sugars of respiration (Chemical reductant of an oxidant)
How do medically relevant pathogens acquire carbon?
Using an inorganic substrate as a reductant and CO2 as a carbon source (hydrogen/thio-sulphate)
Use organic carbon in an assimilable form eg glucose
How do medically relevant bacteria acquire nitrogen?
They assimilate nitrate and nitrite reductively by conversion to ammonia
Describe the sources for ‘other’ bacteria requirements eg. Sulphur, phosphorus etc.
Most bacteria can use sulphate
Phosphate is assimilated as free inorganic phosphate
Iron uptake facilitated by production of siderophores that chelate iron
How can different media be used to determine bacterial type?
Blood agar- bacteria that lyse blood
Mannitol salts agar- bacteria that ferment mannitol are yellow
MacConkey agar- bacteria that ferment lactose are pink
Give examples on how you would classify bacteria
Gram -ve or +ve
Genome sequencing
Describe the basic mechanism of transcription in bacteria
RNA polymerase bind to gene promoter by the sigma factor which then dissociates and rebinds once transcription is completed and transcribed genes into RNA
Transcription regulators can promote it repress expression of a gene- often dimers with a helix-turn-helix motif that allows the protein to sit in the grooves of the DNA
How can gene expression change?
Programmed mutations Phase variation Epigenetics Changes to sigma factors Changes in the activity of regulatory proteins
Describe the function programmed mutations
Phase variation genes can be turned on/off randomly at high frequency- usually code for immunogenic proteins so there is variation in large populations
Can be achieved by genomic recombination, strand slippage, methylation
Phase variation is a passive activity it is not a response to anything
How is phase variation achieved by genomic recombination?
Phase variation can be achieved via genomic rearrangement- eg. E. coli change the expression of fimA by inverting the promoter using integrases FimE and B that recognise a pattern of repeats (requires Lrp, IHF, H-NS)
Eg. Salmonella can express two different types of flagella, the expression of one also transcribes the repressor of the other, the the promoter of that gene is inverted by Hin that recognises his sequences then transcription of both genes is stopped to the other flagella is transcribed
How is phase variation achieved by strand slippage?
Many phase variable genes are associated with short sequence repeats (SSR) the copying of which by DNA or RNA polymerase is prone to errors due to strand slippage
SSRs can be added or removed depending on is the mismatch occurs on the lagging or leading strand the number of these repeats can lead to altered expression or a different of the gene
The same effect can be achieved by slippage in one base pair
How can phase variation be achieved by methylation?
In E. coli after DNA has been replicated it is methylated by dam methylase- recognises GATC sites, it does this to identify the parent strand for proofreading DNA synthesis
Methylation can also affect expression of genes eg. E. coli can express pap (a pilus that helps it binds to bladder epithelia) expression depends on if Lrp binds before or after methylation of certain GATC sites, before activates transcription, after represses transcription
What is a regulon?
All the genes in a regulon are controlled by the same regulatory mechanism- they are turned on/off together
Eg. Multiple antibiotic resistance (mar) regulon in E. coli - over 60 genes controlled by MarA, when activated it results in many responses including decreased poring expression and increased efflux pump expression
How do bacteria effectively respond to stress?
Common way: two-component systems
One sensor protein, a protein kinase, with a partner which, once phosphorylated, effects a change once the sensor has been activated, alters transcription of genes
Give an example of a transcriptional regulator that responds to stress inside s bacterium
Mer operon
MerR binds to DNA, recruits RNAP and initiates transcription of the target gene when Hg is present
MerD can bind the promoter and repress transcription in the absence of Hg
Give some examples of sigma factors
Sigma70- RpoD- housekeeping Sigma54- RpoN- nitrogen limitation Sigma38- RpoS- starvation phase Sigma32- RpoH- heat shock E. coli 0157.H7 carries two deadly shiga toxins associated with bacteriophages- the transcription of which is triggered under stress- turns the phage genes on and accidentally triggers their transcription - Abx can cause patient death.
What is antigenic variation
Antigenic variation is the ability to rapidly induce changes to surface proteins to adapt to surroundings, whilst remaining structurally functional
Describe the production of beta-lactamase
AmpC gene found in n enterobacteriaceae
Requires permease, AmpG, amidase, AmpD, and local regulator AmpR
What causes complementary interactions between host and bacteria cells?
Adhesins
- Flagella, agents of motility
- Fimbriae/pili- can be rigid or flexible- may give a certain amount of elasticity
- Gram -ve outer membrane proteins (Omps) promote more intimate attachment, may promote invasion
- Gram +ve cell wall proteins- MSCRAMMS- Microbial Surface Components Recognising Adhesive Matrix Molecules
Describe skin infections
Skin is a natural barrier to infection, commensald prevent colonisation, fatty acids keep pH low, 5.5
Antibacterial products produces by sebum
Hair follicles and nipples can be infected and through bites- flea regurgitation of plague bacteria, mosquito saliva
Describe the respiratory tract and infections
Large particles filtered by nasal hairs, small particles reach lungs and only very small particles can reach alveoli
Mucous blanket traps bacteria and macrophages patrol the alveoli
Eg, Mycoplasma pneumoniae has cell surface projections to attach to neuraminic acid receptors on cell surface
Describe the oropharynx and disease
Saliva- constant wash and antibacterial substances
Bacteria need strong attachment
Some attach to mucosal surface, others colonise the gingival crevice eg. Actinobacillus actinomycetemcomitans
Most attach to the enamel and then attach to each other eg. Streptococcus mutans
Describe the gastrointestinal tract and infection
Few bacteria in stomach
Many in the small intestine
Very large amount in large intestine
Helicobacter pylori in stomach produces urease, has a sheathed flagellum and outer membrane adhesins
Describe the urogenetal tract and infection
Women more prone than men
Cystitis in the bladder
Pyelonephritis in kidney
Flushing of urinary tract prevents bacteria establishing
Must have strong adhesive factors- E. coli- type1 Fimbriae and P-Fimbriae
What can MSCRAMMS bind to?
Collagen Elastin Fibronectin Fibrinogen Laminin Thrombospondin Vitronectin
What are the factors that effect whether a microbe can cause disease?
Host factors- genetics, vaccination status, health, nutrition, co-infections
Pathogen factors- strain (virulence factors) infectious dose
Describe routes of entry across surface barriers
Arthropod vectors eg. Lyme disease- ticks
Plague- fleas
Typhus- louse
Wounds/punctures/trauma eg. Staph aureus can invade and spread to surrounding tissues or enter the bloodstream➡️ infective endocarditis, abscesses, vertebral osteomyelitis, septic arthritis and meningitis
Active invasion- via adhesins and complementary receptor-ligand interactions- determine tropism and pathogen location
