Download Alteration of the target site

With appropriate examples, describe the different types of
mechanism that make bacteria resistant to antibiotics.
When bacteria become resistant to antibiotics it is easy to forget that even the
most resilient of these can be inhibited or killed by a sufficiently high dose of
antibiotic. Unfortunately, patients can’t always tolerate this critical
concentration. There is huge variability in antibiotic susceptibility across
different bacteria. For example, most streptococchi pneumoniae are inhibited by
0.01mg/l of benzylpenicillin, whereas with E.Coli around 5000 times more is
needed which would be dangerous to the body.
There are four main mechanisms of antibiotic resistance: antibiotic inactivation
or modification; alteration of target site; alteration of metabolic pathway; and
reduced antibiotic accumulation.
Antibiotic Modification
Of the four mechanisms the most well known is the antibiotic modification
where there is no change to the target site but instead the antibiotic is prevented
from reaching the target site. The prime example of this is beta lactamases which
enzymatically cleave the beta lactam ring on the antibiotic rendering it inactive.
Some beta lactamases are quite general whereas others are more specific (ie.
staphylococchus aureus penicillinase) though most will have some effect on
penicillins and cephalosporins. Beta lacamases are characteristic of both gram
positive and gram negative bacteria though only some show significant inhibition
to beta lactamase inhibitors such as clavulanic acid.
Reduced antibiotic accumulation
When bacteria display this type of resistance they protect their target site (which
is, again, unaltered) by either preventing the antibiotic from entering the cell or
pumping it back out faster than it can flow in resulting in only very small
concentrations of antibiotic in the cell and hence very little effect on the target
site. In gram negative bacteria, beta lactam antibiotics gain access to the cell via a
water-filled porin protein channel. Pseudomonas aeruginiosa are resistant to
imipenem antibiotics because they lack the specific D2 porin channel through
which the imipenem gains access to the cell. Some antibiotics (ie tetracyclines)
can enter certain resistant bacteria but are pumped out again by an energy
dependent transport pump hence preventing the accumulation.
Alteration of the target site
Some bacteria achieve resistance through changing the shape of the target site
therefore reducing the affinity of the site for the substrate. Enterococci are a
good example of this as they are inherently resistant to cephalosporins because
the enzyme (known as penicillin binding protein) responsible for the production
of peptidoglycans (and therefore the cell wall) has low affinity for them and
hence don’t bind strongly enough to inhibit the cell wall production. A good
example of acquired resistance is streptococcus pneumonia which is usually
highly susceptible to penicillin and cephalosporins. These Strep. Pneumoniae can
encorporate DNA from other bacteria which cause the expression of a different
penicillin binding protein and hence the cephalosporins no longer bind with
enough affinity to inhibit the proteins.
Alteration of the metabolic pathway
The final mechanism of bacterial resistance involves the production of a different
enzyme which is not susceptible to the antibiotic which performs the same
function as the protein that is susceptible. The most famous example of this is
MRSA (methicillin resistant staphylococchus aureus) which expresses an
alternative penicillin binding protein (PBP2a) as well as the normal PBP. The
PBP2a is not inhibited by the antibiotics usually used against staph. aureus such as
flucloxacillin and hence the cell continues peptidoglycan production and
therefore its cell wall is structurally sound. One of the major antibiotics used
against MRSA now is vancomycin. However, when vancomycin resistant
enterococci were discovered, it meant that potentially MRSA could adopt the
vancomycin resistant
gene cluster and
become vancomycin
resistant itself. This is
also an example of the
alternative target site
mechanism as the
resistant protein
produced has a
slightly different site
which the vancomycin
cannot bind to and
hence cell wall
synthesis continues.
Where do ‘new’ resistance mechanisms come from when
previously susceptible bacteria become resistant to
antibiotics?
Antibiotic resistance in bacteria can be intrinsic or acquired. Intrinsic resistance
is when a bacteria has the gene in its genome which makes it resistant – ie.
vancomycin resistant Escherichia coli. Acquired resistance can come about due
to mutation or by acquisition of foreign DNA and is when a bacterium that was
previously sensitive to an antibiotic is no longer sensitive.
Whether in the presence of antibiotics or not, mutations are always spontaneous.
Often these mutations are fatal to the bacterium. However, occasionally
mutations can produce genes coding for advantageous phenotypes such as
alternative penicillin binding proteins. The susceptible cells are rapidly culled in
the presence of an antibiotic leaving on the newly resistant one (or few) which
can then thrive thus a resistance subpopulation is produced. It is essentially
massively accelerated “natural” selection. These mutations can actually then be
passed onto non-resistance bacteria via horizontal gene transmission whereby
plasmids (small self-replicating circular pieces of DNA) can be passed from one
bacterium to another when they get close enough to each other. These plasmids
can carry many resistant genes each which can then be incorporated into the
previously susceptible bacterium’s genome by a transposon (jumping gene). It is
likely that the spread of the mecA gene in MRSA was caused by this
transposition.
Another important process is transformation. When bacteria are infected by
bacteriophages and die they release their DNA which can be taken up by other
bacteria thus transferring any useful genes to the new bacteria. This is most
commonly seen in staphylococci and is probably the main method for the spread
of penicillin resistance in staph pneumonia (mosaic PBP genes are resistant to
many types of penicillin).