Lecture 7- antimicrobials Flashcards
who was Alexander flemming?
Flemming discovered the first antibiotic, penicillin, in 1928 at St Mary’s hospital in London
Penicillin was isolated from the mould Penicillium notatum
In a 1945 interview with The New York Times, Alexander Fleming, who won a Nobel Prize that year for his discovery of pencillin, warned that misuse of the drug could result in selection for resistant bacteria
what are the scary estimates of disease?
700 000 annual deaths currently attributable to infections by drug- resistant pathogens
Likely to increase, if unchecked, to 10 million by 2050
running up a bill of US$100 trillion in terms of lost global production between now and then
This, however, is based on an assumption that resistance will reach 100%
O’Neill J. Tackling drug-resistant infections globally: Final report and recommendations.
The review on antimicrobial resistance; London: HM Government and the Wellcome Trust 2016
how does antibiotic resistance occur?
Bacteria mutate at a constant rate from mistakes being made during cell replication
These mistakes can be good, bad or neutral
If they are good we say that they confer a fitness advantage and the phenotype gives the cell an advantage to survive in that particular environment
If they are neutral they have no effect on the bacterium
However if they are bad the cell is likely to die and that particular mutation will not survive in future generations
The particular environment the cells are in provide a SELECTIVE PRESSURE and SELECT the phenotypes that survive the best under those conditions
what are the mechanisms that help a bacterium survive drugs?
Permeability changes in the bacterial cell wall which restricts antimicrobial access to target sites
Active efflux of the antibiotic from the microbial cell
Enzymatic modification of the antibiotic
Degradation of the antimicrobial agent
Acquisition of alternative metabolic pathways
to those inhibited by the drug
Modification of antibiotic targets
Overproduction of the target enzyme
what are aminoglycosides?
Discovery
In the early 1940s the first aminoglycoside discovered was streptomycin in Streptomyces griseus
In the 1960s gentamicin was recovered from the actinomycete Micromonospora purpurea
Mechanism
Aminoglycosides inhibit protein synthesis and/or alter the integrity of bacterial cell membranes
Resistance mechanism
The major encountered aminoglycoside resistance mechanism is the modification of the antibiotic by sp. enzymes.
These proteins are classified into three major classes according to the type of modification:
AAC (acetyltransferases),
ANT (nucleotidyltransferases or adenyltransferases),
APH (phosphotransferases)
Gentamicin is the most commonly used aminoglycoside, but amikacin may be particularly effective againstresistantorganisms. Aminoglycosides are used in the treatment ofsevere infectionsof the abdomen andurinarytract, as well as bacteremia andendocarditis
what is β-Lactam?
Discovery
the first antibiotic discovered was a _-lactam, penicillin in 1928
all _-lactam antibiotics have a _-lactam nucleus in their molecular structure
The _-lactam antibiotic family includes penicillins and derivatives, cephalosporins,
carbapenems, monobactams, and _-lactam inhibitors
Mechanism
The _-lactam antibiotics work by inhibiting the cell wall synthesis by binding to so-called penicillin-binding proteins (PBPs) in bacteria and interfering with the structural cross linking of peptidoglycans and as such preventing terminal transpeptidation in the bacterial cell wall.
This weakens the cell wall of the bacterium and finally results in cytolysis or death due to osmotic pressure
Resistance mechanisms
The most common and important mechanism through which bacteria can become resistant against β-lactams is by expressing β-lactamases, for example extended-spectrum β- lactamases (ESBLs), plasmid-mediated AmpC enzymes, and carbapenem-hydrolyzing β- lactamases
what is Chloramphenicol?
Discovery
In 1947, the first chloramphenicol was isolated from Streptomyces venezuelae
Chloramphenicol became available for clinical use in 1948 and was soon accepted as the prototype broad-spectrum antibiotic, active against virtually all gram-positive and many gram-negative bacteria including rickettsia.
Mechanism
Chloramphenicol is a highly specific and potent inhibitor of protein synthesis through its affinity for the peptidyltransferase of the 50S ribosomal subunit.
The relative simplicity of the structure of chloramphenicol prompted the development of economic routes to its chemical synthesis, making it the first totally synthetic antibiotic of pharmaceutical importance.
Resistance Mechanism
The first and still most frequently encountered mechanism of bacterial resistance to chloramphenicol is enzymatic inactivation by acetylation of the drug via different types of chloramphenicol acetyltransferases.
Chloramphenicol acetyltransferase (CAT), catalyses the acetyl-CoA dependent acetylation of the antibiotic
In E. coli, chromosomal mutations of the mar locus can result in resistance by reduced drug uptake.
A mobile gene cassette can carry a chloramphenical efflux system that provides resistance in some Gram positive organisms.
what are glycopeptides?
Discovery
In the late 1950s, the first glycopeptide, vancomycin was introduced in a clinical setting.
Vancomycin resistant Enterococi are a major public health burden today
Mechanism
the molecular target of these glycopeptide antibiotics is the d-alanyl–d-alanine (d-Ala–d-Ala) terminus of the cell wall peptidoglycan precursor.
After the glycopeptides are bound to their target, they inhibit the subsequent transglycosylation reaction by steric hindrance
Resistance Mechanism
for almost 30 years following its introduction, resistance to this glycopeptide was reported only rarely and appeared to have little clinical significance.
vancomycin resistance resulted from the production of modified peptidoglycan precursors ending in d-Ala–d-Lac (VanA, VanB, and VanD) or d-Ala–d-Ser (VanC, VanE, and VanG), to which glycopeptides exhibit low binding affinities
what is tetracycline?
Discovery
The first tetracycline antibiotic was characterized in 1948 as chlortetracycline from Streptomyces aureofaciens
Mechanism
they inhibit the growth of microbes by entering the bacterial cell, interacting with the ribosomes, and consequently blocking protein synthesis
Resistance mechanism
The resistance mechanisms for the tetracycline class of antibiotics fall into several categories; energy-dependent efflux pumps, ribosomal protection proteins (RPPs) that protect the binding site or cause another modification reducing binding, or via enzymatic inactivation
what are macrolides?
Discovery
The first macrolide, erythromycin A, was discovered in the early 1950s
The main structural component of this molecule is a large lactone ring to which amino and/or neutral sugars are attached by glycosidic bonds
Mechanism
inhibit protein synthesis by binding to the 50S ribosomal subunit of bacteria
Resistance mechanism
The most common mechanism of macrolide resistance is due to the presence of rRNA methylases, encoded by the erm genes.
These enzymes methylate the adenine residue(s) resulting in Macrolide resistance.
how is antibiotic resistance measured?
by minimum inhibitory concentration
the lowest concentration of a chemical which prevents visible growth of a bacterium
The MIC of a chemical is determined by preparing solutions of the chemical in vitro at increasing concentrations, incubating the solutions with the separate batches of cultured bacteria, and measuring the results using agar dilution or broth microdilution.
Results have been graded into susceptible (often called sensitive), intermediate, or resistant to a particular antibiotic by using a cut off point.
Cut off points are agreed upon values, published in guidelines of a reference body, such as the U.S. Clinical and Laboratory Standards Institute (CLSI), the British Society for Antimicrobial Chemotherapy (BSAC) or the European Committee on Antimicrobial Susceptibility Testing (EUCAST)
how can we use genome sequencing to monitor antibiotic resistance?
The cost of whole genome sequencing(WGS) for bacteria has decreased significantly to ~$60 an isolate
This means that clinical isolates for bacterial species are being sequenced for epidemiological and biological purposes worldwide
Known antibiotic resistance genes can be recognised from genome sequencing to provide a resistance profile for the strain
This allows us to build up surveillance of the spread of resistance within that species
what is the use of antibiotics in livestock?
global agricultural consumption probably exceeds that of humans.
estimates of the amount of antibiotics consumed in livestock production conservatively place this at 63 151 tonnes in 2010.
many antibiotics are used in animal production, in sub-therapeutic doses and with long exposure periods, these production systems create ideal conditions for bacteria to fix genes that confer resistance
does the use of antibiotics in animals contribute to antibiotic resistance?
These genes can subsequently be transmitted to human-adapted pathogens or to human gut microbiota via people, contaminated food or the environment.
They also provide ideal conditions for the amplification of genes that may have arisen in people or the environment.
HOWEVER
If human exposure to antibiotic-resistant commensal bacteria from food animals could be limited or prevented, how much difference would it make to the impact of these bacteria (and resistance-encoding accessory elements)
the rate of input of resistant bacteria from animal sources is small relative to the amplification achieved by the human use of antibiotics and the transmission of resistant strains among humans.
if extensive animal use precedes extensive human use of drugs, the animal use may well shorten the time before resistance becomes problematic in the human flora.
once evidence of the medical impact of antimicrobial use is apparent, regulation of
the animal use of those drug classes would have little or no effect.
Suggestion that regulators should have little concern about the use of drugs in animals for which resistant commensals are already problematic in humans
Will all bacteria eventually become untreatable?
Antibiotic natural products are ancient and so is resistance
Nevertheless, despite long histories of antibiotic production and exposure, environmental bacteria are not resistant to all known antibiotics.
This means that there are barriers to the acquisition of complete resistance
There are fitness costs to carrying resistance genes
This also provides a SELECTIVE PRESSURE
A recent study found that Methicillin resistant Staphylococcus aureus
is in decline in Boston
why arent we finding new antibiotics?
A lot of the products being approved are second, third or fourth generation antibiotics, meaning they are just follow-up compounds, without a novel mechanism of action.
The biggest barrier for companies is the regulatory burden: costs of trials are so high and society is not willing to pay the high price for antibiotics
In recent years, new antibiotics have struggled to reach the market due to difficulties in demonstrating efficacy or they have had unacceptable side effects.
In addition, in order to gain a licence, new antibacterials have to demonstrate they are not inferior to existing drugs where the comparator is available generically.
Drug companies have focused on areas that maximize profits- especially for chronic diseases where patients need drugs for years on end.
how can we discover new antibiotics?
A multichannel device, the iChip, was used to isolate and grow uncultured bacteria.
A sample of soil is diluted so that one bacterial cell is delivered to a given channel.
The device is covered with two semi-permeable membranes and placed back in the soil. Diffusion of nutrients and growth factors through the chambers enables growth of uncultured bacteria in their natural environment.
Extracts from 10,000 isolates obtained by growth in iChips were screened for antimicrobial activity on plates overlaid withS. aureus.
what is teixobactin?
An extract from a new species of β-proteobacteria provisionally namedEleftheria terrae showed good activity.
Based on 16S rDNA andin silicoDNA/DNA hybridization, this organism belongs to a new genus related toAquabacteria
The compound was isolated and named Teixobactin
Unable to generate resistant mutants and showed new form of activity; binds cell wall precursors
what are alternatives to antibiotics?
Prophylaxis
Vaccines
Bacterial vaccines are available for Vibrio cholerae, Steptococcus pneumoniae, Neisseria meningitidis,
Mycobacterium tuberculosis, Haemophilus influenzae, Salmonella typhi, Bacillus anthracis, Bordatella
pertussis, Clostridium tetani, Corynebacterium diptheriae and Coxiella burnetii
Probiotics, prebiotics, synbiotics, and competitive exclusion
Bacteria taken as supplements to try to maintain or improve commensal gut bacterial, very limited evidence of success of this
Alternative therapies
Phage therapy: therapeutic use of bacteriophages (very specific bacterial viruses) to treat pathogenic bacterial infections
Endo- or exolysins: hydrolytic enzymes produced by bacteriophages in order to cleave the bacterial cell walls
Bacteriocins: proteinaceous or peptidic toxins produced by bacteria to inhibit the growth of similar or closely related bacterial strain(s)
Predatory bacteria: some species of bacteria kill and then consume other microorganisms
what is horizontal gene transfer?
plasmid conjugation, recombination and bacteriophage
what are examples on the literature / outbreaks?
Pneumonia (life threatening)
Streptococcus pneumoniae is the leading causes of pneumonia
Highly recombining respiratory pathogen than can also cause sepsis and meningitis
Vaccine and antibiotic selective pressures have produced several successful penicillin(β-Lactam) resistant clones
Several serotypes have seen increasing rates of multidrug resistance and expansion in the population
Gonorrhea (non life threatening but can cause infertility)
Highly recombining sexually transmitted pathogen
Gonorrhea has now developed nearly ubiquitous resistance to all first line antibiotics previously used to treat it
It has now demonstrated its capacity to develop resistance to the extended- spectrum cephalosporins(β-Lactam), the last remaining option for first-line treatment of gonorrhea.
Gonnorrhea is the closest bacteria to the emergence of an untreatable clone
Can use WGS to reconstruct sexual transmission networks and monitor the spread of resistance through epidemiology
Genomic epidemiology of Neisseria gonorrhoeae with reduced susceptibility to cefixime in the USA: a retrospective observational study
