BUG

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BUG SUPERBUG(with apologies to George Bernard
Shaw)

• DR PETER VICKERS
• UNIVERSITY OF HERTFORDSHIRE

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THE SCALE OF THE PROBLEM

• The 1990s brought a world-wide resurgence of
  bacterial and viral diseases - an important
  factor of this being antibiotic resistance genes
  in virtually all major bacterial pathogens
• US National Institute of Health estimated in 2000
  that 50 000 tons of antibiotics will be used
  every year throughout the world on humans,
  animals and plants.
• It makes the possibility of a mass mutation of
  microbes (bugs) and thus new diseases, inevitable.

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ENTEROCOCCI

• In the late 1980s, certain species and strains of
  enterococci had evolved to become immune to all
  known antibiotics, including vancomycin.
• By 1994, some mutant enterococci were not just
  immune to vancomycin, but actually fed off it.
• One strain of Enterococcus faecalis had actually
  become dependent on vancomycin for growth.

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ANTIBIOTICS ARE ABUSED

• As far back as 1983, a WHO report estimated that
  doctors used antibiotics irrationally or
  inappropriately on anything between 40 and 66
  of all occasions.
• By 1994, between 60 - 70 000 patients died from
  nosocomial infections, 50 of which are caused by
  drug-resistant micro-organisms
• Things have not improved - think of MRSA,
  resistant TB, E coli 0147 - the list goes on.
• Are we running out of viable antibiotics?

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BACTERIA ANTIBIOTICS (1)

• When you take antibiotics, the drug kills vast
  numbers of bacteria in the gut and elsewhere on
  and in the body - but countless millions remain.
• The bacteria that survive include species
  unaffected by the drug, or else drug-resistant
  variants of a vulnerable species.
• After a number of courses of antibiotics, these
  surviving bacteria are liable to multiply and to
  fill the space created by the drug.

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BACTERIA ANTIBIOTICS (2)

• What antibiotics do is to exert selective
  pressure on bacteria.
• This distorts and accelerates their evolution, so
  that previously vulnerable species become drug
  resistant.
• In a population of many millions of bacteria,
  there will be some chance mutants - maybe one in
  a million - that just happen to be invulnerable
  to a drug that kills all the others.
• These once insignificant mutants survive,
  multiply and colonise the space left by the
  destruction of other bacteria in that species
  that were previously vulnerable to the drug.

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BACTERIA ANTIBIOTICS (3)

• However, the creation of drug-resistant bacteria
  is not just a matter of chance mutation.
• Although microscopic, bacteria are very complex
  and adaptable living organisms.
• Their genetic organisation includes structures
  that probably evolved in order to resist
  naturally occurring chemicals contained in rival
  living things (think of fungi and penicillin),
  just as plants, insects, birds, animals and
  humans have evolved ways to resist predators.

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BACTERIA ANTIBIOTICS (4)

• These structures, which are rings of genetic
  material contained within the bacterial cell
  wall, but not the nucleus, are PLASMIDS.
• The codes for bacterial resistance to one or more
  antibiotics are contained within the plasmids.
• Under selective pressure from drugs, these
  plasmids can be transferred within bacteria from
  the same or different species.
• Therefore, bacteria are able to transfer multiple
  drug resistance within and between species.

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GENETIC RECOMBINATION

• In order for a genetic mutation - for example,
  drug resistance - to be passed from one bacterium
  to another, there needs to be some form of
  genetic recombination.
• This is not something exclusive to bacteria - we
  do the same thing at the beginning of meiosis.
• There are 3 ways in which bacteria are able to
  transfer genetic information
• 1. Transformation
• 2. Conjugation
• 3. Transduction

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TRANSFORMATION (1)

• During this process, genes are transferred from
  one bacterium to another as naked DNA in
  solution
• Some bacteria, perhaps after death, release their
  DNA into the environment (our bodies).
• Other bacteria can then encounter the DNA and,
  depending on the particular species and growth
  conditions, take up fragments of the DNA into
  their cytoplasm and then combine that DNA into
  their own DNA
• A cell having this new combination of genes is a
  kind of hybrid - or recombinant cell.

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TRANSFORMATION (2)

• All descendants of this recombinant cell will be
  identical to it.
• Transformation occurs naturally among very few
  types of bacteria, including bacilli,
  haemophilus, neisseria and certain strains of
  streptococcus and staphylococcus, because even
  though only a small portion of DNA is
  transferred, it is still a very large molecule to
  pass through the cell wall.
• So, this works best when the donor and recipient
  cells are very closely related.

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CONJUGATION (1)

• Conjugation is mediated by plasmids.
• These are sub-cellular organisms (rings of
  nucleic acid) that live and multiply only within
  the bacterial cell wall.
• They are always beneficial to the bacterium and
  can be vital to bacterial health and survival.
• They are like mini-chromosomes, but are not
  usually essential for cell growth.
• Plasmids carry information that is not contained
  in bacterial DNA.

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CONJUGATION (2)

• This information includes genes that protect the
  bacterial host against poisons in the environment
  - including antibiotics.
• Conjugation requires that there be direct
  cell-to-cell contact
• The conjugating cells must generally be of
  opposite mating types - donor cells must carry
  the plasmid and recipient cells usually do not.
• The bacterial chromosomes themselves do not cross
  from cell to cell, only the plasmid.

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CONJUGATION (3)

• Since most plasmids occur in more than one copy
  in any cell, the original carrier retains its
  plasmid, and the recipient gets a copy.
• The plasmid carries genes that code for the
  synthesis of sex pilli - projections from the
  donors cell surface that contact the recipient
  and help to bring 2 cells into direct contact.
• During conjugation, the plasmid is replicated
  through transfer of a single-stranded DNA copy to
  the recipient, where the complementary strand is
  replicated.

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CONJUGATION (4)

• In E. coli, the F factor is the first plasmid
  observed to transfer between cells.
• Donors carrying F factor (F cells) transfer the
  plasmid to recipients (F- cells), which become F
  cells as a result.

bacterial chromosome
Replication transfer of F factor
plasmid
F cell
F- cell
F cell
F cell
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CONJUGATION (5)

• Once within the recipient cell, donor DNA can
  combine with the recipients DNA, as occurs in
  transformation.
• By the process of conjugation between an these
  cells, a cell can acquire new versions of
  chromosomal genes (just as in transformation).

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TRANSDUCTION (1)

• In this process, bacterial DNA is transferred
  from the donor cell to the recipient cell inside
  a virus that infects bacteria - a bacteriophage
  (phage).
• During the process of infection, the phage
  attaches to the bacterial cell wall and injects
  its DNA into the bacterium.

Infecting phage
Bacterial DNA
Phage DNA
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TRANSDUCTION (2)

• The phage DNA acts as a template for the
  production of new phage DNA, and also directs the
  production of phage protein coats.
• During phage development inside the infected
  bacterium, the bacterial chromosome breaks apart,
  and at least some fragments of the bacterial
  chromosome happen to be packaged inside phage
  protein coats.

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TRANSDUCTION (3)

• These resulting phage particles thus carry
  bacterial DNA instead of phage DNA.

Phage DNA
Bacterial DNA
lysis
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TRANSDUCTION (4)

• When the released phage particles later infect a
  new population of bacteria, bacterial genes will
  be transferred to the newly infected cells.

Bacterial DNA
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TRANSDUCTION (5)

• So, transduction of cell DNA by a phage virus can
  lead to recombination between the DNA of the
  first host bacterium and the DNA of the second
  host bacterium.
• This method of generalised transduction is
  typical of phages such as P1 of E. coli and phage
  P2 of Salmonella.
• All genes contained within a bacterium infected
  by a generalised transducing phage are equally
  likely to be packaged in a phage coat and
  transferred.

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CAUSES OF DRUG RESISTANCE (1)

• Overuse
• Not completing courses of antibiotics
• Over-the-counter sales in developing countries
• Factory farming - used on animals found in meat
  and milk
• Agricultural waste (which carries antibiotics
  faeces, etc.) polluting earth, rivers

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CAUSES OF DRUG RESISTANCE (2)

• Antibiotics getting into the food chain
• Over-crowded environments, e.g. hospitals,
  factory farms, developing world
• Poor social and physical infrastructures
• Irresponsible attitudes to antibiotics
• High costs of developing antibiotics

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WHAT TO DO? (1)

• Need for a balance between benefit and risk?
• Publish an annual list of essential antibiotics?
• Keep a reserve list of antibiotics - only to be
  used in an emergency (such as vancomycin at the
  moment)?
• Make invasive drug-resistant bacterial infection
  notifiable?
• Ensure that antibiotics are only available on
  prescription?

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WHAT TO DO? (2)

• Encourage drug companies to market drugs
  consistently world-wide?
• Create an independent drug-regulating agency?
• Establish national and/or international groups to
  audit antibiotics and antibiotic use?
• Publicise information about drug-resistant
  infections?
• Give microbiology/immunology a higher profile in
  medical and nursing education?

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WHAT TO DO? (3)

• Protect yourself against infection (good
  nutrition, etc.) - make the individual
  responsible for self?
• Patients to keep a note of drugs taken, what for,
  and for how long - a drug diary?
• Educate the general population to work with
  doctors and not demand antibiotics for
  everything?
• Cut down on travel?
• Develop new antibiotics - gene therapy?

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Finally something to think about