Combating bacterial diseases Flashcards
Overview of how bacterial disease can be controlled
Reduce exposure by disinfection, treating water and sewage, pasteurising food and cooking it properly, limiting insect contact and avoiding significant contact with infected people/animals (quarantine, isolation)
Reducing susceptibility of individuals and populations by vaccination
Chemotherapy with antibiotics and other antibacterial agents
Vaccines against bacterial disease
a) principal diseases where vaccines used (and what type of vaccine)
b) new strategies being used
a) Diptheria, DPT (diptheria toxoid)
Tetanus, DPT (tetanus toxoid)
Whooping cough, DPT (killed Bordetella pertussis)
Pneumonia (polysaccharide from S. pneumoniae and H. influenzae
Meningitis (purified polysaccharide from Neisseria meningitidis, capsular polysaccharide of H. influenzae B coupled to tetanus toxoid)
Typhoid fever (live attenuated vaccine - Ty21A, Vi capsular polysaccharide vaccine)
Cholera (Killed whole cell or crude fraction of V. cholerae)
Tuberculosis (BCG, an attenuated strain of M. bovis)
b) involve genetically-engineered attenuated variants (to generate antibody and cell mediated responses) and subunit vaccines (toxin, adhesins, capsules) that raise antibody responses to block colonisation or toxin binding to receptors
Example of antibiotic use - Helicobacter pylori
a) how antibiotics were useful
b) how H. pylori causes disease
c) H. pylori and cancer
a) Causes 90% of gastric and duodenal ulcers, and is associated with gastritis. Patients could be treated with a relatively cheap short course of antibiotics, rather than expensive life-long daily ulcer medication
b) Colonises mucin layer near gastric mucosal cells in the antrum. Extremely motile by means of flagella, H. pylori binds cells using adhesins. Neutralises gastric acid by producing urease (splits urea to ammonia) which raises local pH to make gastric mucous less viscous, hence easier for H. pylori to swim towards epithelium
Causes intense mucosal inflammation and ulceration. Major damage occurs due to Helicobacter-induced IL-8 production by epithelial cells (attract PMNs) and destruction of epithelial cells by a pore-forming vaculolating cytotoxin VacA that H. pylori secretes.
Hexameric VacA toxin inserts into host cell membrane to form anion-selective channels that are endocytosed. VacA pores disturb ion balance in late endosomes and water flows in, swelling endosomes to form characteristic vacuoles. Progression of ulcer is accompanied by more inflammation, increasing tissue destruction
c) Correlation between H. pylori and gastric cancer. Possibly chronic inflammation exposes proliferating mucosal stem cells to dietary carcinogens and generates mutagenis ROS. The bacterial effector GacA (that is delivered into epithelial cells) interferes with signalling pathways, leading to increased cell proloferation
Antibiotic-associated diarrhoea and pseudomembraneous colitis
a) what is the cause
b) how does it occur
c) treatment
a) Clostridium difficile, a toxin producing anaerobic spore-forming bacterium that colonises the colon following antibiotic eradication of the normal human gut microflora (especially a problem in hospitals, and an important pathogen in horses)
b) causes diarrhoea and inflammation of the colonic mucosa, largely through action of two secreted toxins TcdA and TcdB. These glycosylate small GTPases in intracellular signalling pathways, resulting in subversion of actin cytoskeleton and disruption of tight junctions, so the epithelium becomes porous and results in cell destruction. Cell damage is also inditect due to inflammation and the formation of a pseudomembrane
c) Antibioticsa and rehydration therapy are primary treatments. However, refractory and recurrent infections are treated using faecal transplants. Donor faeces, screened for enteric pathogens, are introduced into the patient’s colon
Antibiotic resistance
a) how can it arise
b) mechanism of enzyme-mediated inactivation - β-lactamases
c) mechanism of enzyme modification of the antibiotic
a) Point mutations in chromosomal genes or acquisition of new genes by conjugation, transduction or transformation, and can be selected for by exposure to antibiotics
b) β-lactamases. Penicillin mimics the D-Ala-D-Ala peptidoglycan crosslink. It binds to the active site of transpeptidase, irreversibly inhibiting peotidoglycan crosslinking and causing cell lysis. β-lactamases cleave penicillins
c) many antibiotics inhibit translation by acting on the ribosome. Chloramphenicol binds the 50S subunit and inhibits peptidyl transferase activity. Aminoglycosides (gentamicin, neomycin) alter conformation of the 16S rRNA in the 30S subunit, where the ribosome reads mRNA and recruits aminoacyl-tRNA. Inhibit tRNA selection, polypeptide elongation. Some resistant bacteria produce acetyl transferases that acetylare chloramphenicol and aminoglycosides to prevent ribosome binding
Antibiotic resistance - alteration of target do that antibiotic no longer binds effectively (examples)
a) Tetracycline
b) Fluoroquinolones
c) Vancomycin
a) binds 30S ribosome subunit and blocks binding of aminoacyl tRNA. Some resistant bacteria produce ribosomal protection proteins (RPPs) that have structural similarity to translation wlongation factors. RPPs can dislodge tetracycline from the ribosome
b) (eg ciprofloxacin) blocks synthesis of neucleic acids by inhibiting gyrase and topoisomerase, enzymes that control DNA topology during replication and transcription. R-plasmid encoded quinolone resistance protein Qnr binds toporisomerase to physically block binding of the antibiotic
c) (a glycopeptide) binds D-Ala-D-Ala and blocks access to the transpeptidase, preventing crosslinking of peptidoglycan. Vancomycin-resistant Gram-positive pathogens produce different peptidoglycan biosynthetic enzymes that synthesise D-Ala-D-Lac alternative peptide crosslinks
Antibiotic resistance
a) metabolic by-pass (eg alternative enzymes for folate synthesis)
b) efflux pumps determine multidrug resistance (MDR)
a) Some antibiotics inhibit folate synthesis, an essential step in the synthesis of nucleotides (sulfonamides compete with pABA for binding to the enzyme dihydropteroate synthase, DHPS, and trimethoprim inhibits dihydrofolate reductase, DHFR). R plasmid-encoded DHPS and DHFR have a much lower binding affinity for the antibiotics than the normal bacterial enzymes
b) Gram-negative Pseudomonas and E. coli bacteria become less sensitive to many drugs. Efflux pumps in Gram-negatives are tripartite. Drugs bind to the inner membrane transporter, an ATPase or proton antiporter, and are ejected through a unique TolC exit duct spanning the periplasm and outer membrane.
How to counteract increasing drug resistance
Important to minimise selective pressure on resistance by intelligent antibiotic use in humans and animals. New antibacterials will hopefully be developed against new targets (eg lipid A synthesis, bacterial cell division, against resistance pumps). Some might also target pathogen-specific processes (adhesin assembly, although these would be narrower range than current antibiotics). In addition, combinations of antibiotics, combination therapies, and/or antibiotic potentiatiors (clavulanic acid) that inhibit resistance enzymes (β-lactamase) might also counteract resistance.
