Structure and function of pathogenic bacteria Flashcards
a) only bacteria that is a parasite
b) cell size
c) genes
d) organelles present?
e) motility
f) cell division
g) sexual reproduction?
h) genome
i) mRNA
j) replication regulation
a) Chlamydia (sexually transmitted disease, infectious blindness
b) 0.5-3µm x 0.5µm
c) less than 100 - over 9000
d) no
e) several mechanisms (rotating flagella, extending/ retracting pili)
f) binary fission
g) no meiosis, horizontal genetic transfer (HGT)
h) haploid, single and circular, plasmids, bacteriophage
i) polycistronic (co-linear genes), unstable
j) transcription initiation
a) bacteria that produce endospores
b) what does Gram’s stain differentiate
c) bacteria that don’t Gram stain
a) bacteria that occur in soil produce dormant endospores to survive. Clostridium tetani (tetanus), C. perfringens (gas gangrene, food poisoning), C. botulinum (botulism food poisoning), Bacillus anthracis (anthrax)
b) Most pathogenic bacteria fall into two groups. Iodin crystal violet is washed out of Gram-negative cells, but not Gram-positive. Reflects the substantial differences in cell envelopes (Gram-positive have a thicker PG wall, Gram-negative has an extra outer membrane)
c) Mycobacteria (Mycobacterium tuberculosis - TB) are acid-fast and difficult to stain because of their waxy lipid coat. Chlamydia and Mycoplasma (non-gonococcal urethritis) lack a substantial cell wall
Example of bacteria
a) Gram-positive cocci: i) arranged as grape-like clusters, ii) arranged in pairs - diplococci, or chains
b) Gram-positive rods: i) curved or club-shaped, ii) spore forming)
c) Gram-negative cocci
d) Gram-negative rods
e) Gram-negative spiral
a) i) Staphylococcus aureus, Staph. epidermidis, ii) Streptococci mutans, Strep. pneumoniae, Strep. pyogenes, Enterococcus faecalis
b) i) Cornebacterium diphtheriae, Vibrio cholerae ii) Clostridium species
c) Neisseria species (gonorrhoeae, meningitidis, pharyngis)
d) Escherichia coli, Salmonella species, Pseudomonas aeruginosa, Bacteroides fragilis
e) Treponema species (syphilis), Helicobacter pylori (gastric ulcers)
Bacterial cell wall overview
Essential for structural stability. Made of peptidoglycan (huge macromolecule of alternating N-acetylglucosamine (NAG) and N- acetilmuramic acid (NAM), crosslinked by short oligopeptides, as well as forming vertical crosslinks to connect PG sheets into a thick layer. Gram-positive PG is 150-500 angstroms thick (1 x10⁻¹⁰ m). Gram-negative is thinner.
Bacterial membranes
a) general
b) features of Gram-negative bacteria
a) Lipid bilayer that acts as an osmotic barrier, site of signal reception, transport of nutrients and respiration. In Gram-negative bacteria is called the inner, cytoplasmic membrane
b) Periplasm between the two membranes in gram-negative bacteria contains hydrolytic enzymes (proteases, lipases, phosphatases, β-lactamases) and components of tansport systems. Outer membrane contains porins and other channels central to import and export.
Outer membrane outer leaflet contains lipopolysaccharide (LPS) - important in disease. The O-antigen of LPS is highly varable, generating different antigens and hence serotypes (important defence against host attack). Lipid A component is the endotoxin that is released from dying bacteria (a major PAMP triggering a wide-ranging immune response)
a) Bacterial capsules
b) Bacterial secretion (export) of proteins
a) many pathogenic bacteria also have a polysaccharide capsule surrounding them (eg respiratory pathogens Streptococcus pneumoniae and Haemophilus influenzae). Capsules prevent drying and protect against host complement and macrophage defence
b) proteins important in host interaction are typically exported and/or assembled on the bacterial surface (toxins, adhesins, flagella). In gram-positve and gram-negative, proteins cross the cell (inner) membrane using an N-terminal secretion signal and the standard secretion pathway (Sec). Gram-negative have additional export pathways to cross the periplasm and outer membrane.
Bacterial growth
a) mechanism
b) aerobic/anaerobic metabolism for growth
a) Use binary fission - genome duplication, segregation, cell septation, division. Lab cultures (E. coli) can double every 20-30 mins, in an infection can take hours. Bacteria in stationary phase aren’t inactive, they undergo radical changes in physiology and gene expression to enable survival and reduce antibiotic susceptibility.
b) most pathogenic bacteria are facultative anaerobes (Salmonella, Staphylococcus) that grow with or without oxygen (although less well). When growing aerobically, use glycolysis, citric acid cycle and e⁻ transport chain for oxi. p. to generate ATP from glucose
Some bacteria are strict anaerobes (Clostridium, Bacteroides) and generate ATP using fermentation and specialised e⁻ transport chains with alternative e⁻ acceptors (nitrate, sulphate, ferric iron) instead of oxygen - which is toxic to many anaerobes. All use organic material for carbon and energy
Bacterial motility
Pathogenic bacteria typically motile by means of flagella. Long flagella filament is rotated like a propeller by a motor anchored in the cell (inner) membrane.
Bacteria sense chemical signals and move towards them (nutrients) or away from them (toxic chemicals) by chemotaxis. Chemical receptors at the nose of the cell sense and transmit information, via a phosphorelay, to the flagellar motor switch. The switch controls the direction in which the helical propeller rotates: counter-clockwise (CCW) to swim, clockwise (CW) to tumble and randomly change direction.
a) how do bacteria sense their environment
b) how do bacteria adapt quickly to their environment
c) what is quorum sensing
a) large surface area:volume ratio ensures rapid knowledge of environmental change, sensed by signal transduction systems called histidine-aspartate phosphorelay (HAP). Signals in infection include temperature, pH, aas, osmolarity, stress. Some signalling pathway response regulators are transcription factors, which may co-regulate expression of many operons (regulon)
b) must be able to switch between genes depending on available food, or famine. Determined by changes of contiguous genes (operons), which is tightly coupled to translation and rapid mRNA turnover
c) special signal where some pathogens switch on virulence genes when bacterial population reaches a specific density. Recognise quorum by secreting a small signal molecule and sensing its concentration in the local environment
Bacterial adaptabitily and genetic change
a) mutations
b) DNA rearrangements
a) As well as immediate physiological adaptation, bacterial genes evolve. Genetic change driven by mutations in large populations of rapidly replication cells, resulting in variation in cell surface structures (Salmonella, E.coli have hundreds of O (LPS) and K (capsular polysaccharide) antigen serotypes)
b) caused by random capture and insertion of DNA elements called insertion sequences (IS). These are a few hundred bp long and enter genome using own recombination system. Also, transposons, which have picked up beneficial genes encoding antibiotic/heavy metal resistance, viulence factors (toxins) etc
Transposons can be incorporated into main genome of carried by plasmids. These extra-chromosomal genetic elements allow bacteria further flexibility and have enabled rapid spread of antibiotic resistance and virulence genes
How do bacteria acquire and transfer DNA
Three ways. Transduction by bacterial viruses (bacteriophage). Transformation - uptake of DNA from dead (lysed) bacteria. Conjugation - direct transfer requiring contact between teo bacteria facilitiated by specialised sex pili
