Unit 2: Bacteriology
66
Lecture 5 - The Principles of
antibacterial therapy
Definitions
Most microbiologists distinguish 2 groups of antibacterial
agents used in the treatment of infectious disease
: antibiotics, which are natural substances produced by
certain groups of microorganisms (fungi or bacteria),
and chemotherapeutic agents, which are chemically
synthesized substances. A hybrid substance is
a semisynthetic antibiotic, wherein a molecular version
produced by the microbe is subsequently modified by the
chemist to achieve desired properties. One feature of
antibactrial pharmaceuticals is “selective toxicity,” that
is, they act upon bacteria at very low concentration levels
without causing damage to the macroorganism. The most
important group of anti-infective agents is the antibiotics.
The term “antibiotic” is often used in medical contexts to
refer to all antibacterial pharmaceuticals, not just to
antibiotics in this narrower sense. the relations between an
anti-infective agent, the host organism & a bacterial
pathogen
Characteristics of Antibiotics
Antibiotics may have a cidal (killing) effect or a static
(inhibitory) effect on a range of microbes. The range of
bacteria or other microorganisms that is affected by a
certain antibiotic is expressed as its spectrum of action.
Antibiotics effective against procaryotes that kill or
inhibit a wide range of Gram-positive and Gram-negative
bacteria are said to be broad spectrum. If effective
mainly against Gram-positive or Gram-negative bacteria,
they are narrow spectrum. If effective against a single
organism or disease, they are referred to as limited
spectrum.
A clinically-useful antibiotic should have as many of
these characteristics as possible:-
It should have a wide spectrum of activity with the ability
to destroy or inhibit many different species of pathogenic
organisms.
It should be nontoxic to the host and without undesirable
side effects.
It should be nonallergenic to the host.
It should not eliminate the normal flora of the host.
It should be able to reach the part of the human body
where the infection is occurring.
It should be inexpensive and easy to produce.
It should be chemically-stable (have a long shelf-life).
Microbial resistance is uncommon and unlikely to develop.
Fundamental ways that antibacterial
antibiotics work as therapeutic agents:
The target of an antibiotic should be unique to the
bacterium and not found, or not accessible, to the patient.
These are the most important targets in bacteria that have
been exploited so far.
1) Attack bacterial cell wall synthesis
. Bacteria have
murein in their cell walls, not found in the host, and
murein (peptidoglycan) is essential to the viability of the
bacterium.
Beta lactam antibiotics represented by the
penicillins and cephalosporins, are example of these
antibiotics which contain a 4-membered beta lactam ring,
they are the products of two genera of fungi,
Penicillium and Cephalosporium.
2) Interfere with protein synthesis
. Attack is almost
always at the level of translation using 70S ribosomes in
the translation machinery.
The most important antibiotics
with this mode of action are
the tetracyclines, chloramphenicol, the macrolides (e.g.
erythromycin) and the aminoglycosides (e.g.
streptomycin).
3) Interference with nucleic acid synthesis
(RNA and
DNA), which exploits differences between RNA
polymerases and DNA replication strategies in bacteria
and eucaryotes.
Two nucleic acid synthesis inhibitors
which have selective activity against procaryotes and
some medical utility are the quinolones and rifamycins.
4) Inhibition of an essential metabolic pathway
that
exists in the bacterium but does not exist in the host. This
is usually brought about through the use of competitive
chemical analogs for bacterial enzymatic reactions.
The
sulfonamides (e.g. Gantrisin and Trimethoprim) are
inhibitors of the bacterial enzymes required for the
synthesis of tetrahydofolic acid (THF), the vitamin form
of folic acid essential for 1-carbon transfer reactions.
5) Membrane inhibition or disruption
doesn't work
too well because of the similarities between eucaryotic
and bacterial membranes. However, the outer membrane
of Gram-negative bacteria is a reasonable point of attack
and some membrane inhibitors are included in the
discussion below.
The only antibacterial antibiotics of
clinical importance that act by this mechanism are
the polymyxins, produced by Bacillus polymyxa .
Unit 2: Bacteriology
66
Spectrum of Action
Each anti-infective agent has a certain spectrum of action,
which is a range of bacterial species showing natural
sensitivity to the substance. Some antiinfective agents
have a narrow spectrum of action (e.g., vancomycin).
Most, however, have broad spectra like tetracyclines,
which affect all eubacteria.
Efficacy (syn. kinetics of action)
The efficacy of an anti-infective agent defines the way it
affects a bacterial population. Two basic effects are
differentiated: bacteriostasis, i.e., reversible inhibition of
growth, and irreversible bactericidal activity. Many
substances can develop both forms of efficacy depending
on their concentration, the type of organism, and the
growth phase. Many of these drugs also have a
postantibiotic effect (PAE) reflecting the damage inflicted
on a bacterial population. After the anti-infective agent is
no longer present, the bacterial cells not killed require a
recovery phase before they can reproduce again. The PAE
may last several hours.
Pharmacokinetics
Pharmacokinetics covers the principles of absorption,
distribution, and elimination of pharmacons by the
macroorganism. You could refere to standard textbooks of
pharmacology for details. The dosage and dosage interval
recommendations for antibacterial therapy take into
account the widely differing pharmacokinetic parameters
of the different anti-infective agents.
Side Effects
Treatment with anti-infective agents can cause side
effects, resulting either from noncompliance with
important therapeutic principles or specific patient
reactivity. On the whole, such side effects are of minor
significance.
Toxic effects
.
These effects arise from direct cell and
tissue damage in the macroorganism. Blood
concentrations of some substances must therefore be
monitored during therapy if there is a risk of cumulation
due to inefficient elimination (examples:
aminoglycosides, vancomycin).
Allergic reactions
.
For possible mechanisms (example:
penicillin allergy).
Biological side effects
.
Example: change in or
elimination of normal flora, interfering with its function
as a beneficial colonizer.
The Problem of Resistance
.
There are many features
of resistance of bacteria:-
o Clinical resistance. Resistance of bacteria to the
concentration of anti-infective agents maintained at the
infection site in the macroorganism.
o Natural resistance. Resistance characteristic of a
bacterial species, genus, or family.
o Acquired resistance. Strains of sensitive taxa can acquire
resistance by way of changes in their genetic material.
o Biochemical resistance. A biochemically detectable
resistance observed in strains of sensitive taxa. The
biochemical resistance often corresponds to the clinically
relevant resistance. Biochemically resistant strains
sometimes show low levels of resistance below the
clinically defined boundary separating resistant and
sensitive strains. Such strains may be medically
susceptible.
Significance
Problematic bacteria
. Strains with acquired resistance
are encountered frequently among Enterobacteriaceae,
pseudomonads, staphylococci, and enterococci. Specific
infection therapy directed at these pathogens is often
fraught with difficulties, which explains the label
problematic bacteria. They are responsible for most
nosocomial infections. Usually harmless in otherwise
healthy persons, they may cause life-threatening
infections in highly susceptible, so-called problematic
patients. Problematic bacteria are often characterized by
multiple resistances. Resistance to anti-infective agents is
observed less frequently in nonhospital bacteria.
Genetic variability
. The basic cause of the high
incidence of antibiotic resistance experienced with
problematic bacteria is the pronounced genetic variability
of these organisms induced by different mechanisms.
Most important are the mechanisms of horizontal transfer
of resistance determinants responsible for the efficient
distribution of resistance markers among these bacteria.
Selection
. The origin and distribution of resistant strains
is based to a significant extent on selection of resistance
variants. The more often anti-infective substances are
administered therapeutically, the greater the number of
strains that will develop acquired resistance. Each hospital
has a characteristic flora reflecting its prescription
practice. A physician must be familiar with the resistance
characteristics of this hospital flora so that the right anti-
infective agents for a “calculated antibiotic therapy” can
be selected even before the resistance test results are in.
Unit 2: Bacteriology
66
Such therapies take into account the frequency of
infections by certain bacterial species (pathogen
epidemiology) as well as current resistance levels among
these bacteria (resistance epidemiology).
Resistance Tests
Two standard test systems are used to determine the in-
vitro resistance levels of bacteria. The dilution series
tests, in which the minimum inhibitory concentration
(MIC) of an anti-infective agent required to inhibit
proliferation of a bacterial population is determined, and
the agar dilution test, in which the nutrient agar plates
containing antibiotic are inoculated (“spotted”) with the
test organisms. In the microbroth dilution test, the final
volume is usually 100 µl per microplate well. This test
type can also be automated. The final volume in a
macrobroth dilution test is 2 ml per tube.
Due to the complexity and time-consuming nature of
the above test types, routine laboratories often use the
agar diffusion test. This involves diffuse inoculation of
the nutrient agar plate with the test strain. Then disks of
filter paper containing the anti-infective agents are placed
on the agar. After the plates thus prepared are incubated,
the inhibition zones around the disks (i.e., whether or not
they develop and their size) provide information on the
resistance of the microorganisms tested. This is possible
because of the linear relation between the log2 MIC and
the diameter of the inhibition zones. To interpret the
results, the MICs or inhibition zones are brought into
relation with the substance concentrations present at a site
of infection at standard dosage levels. This calculation is
based on known averages for various pharmacokinetic
parameters (serum concentration, half-life) and
pharmacodynamics parameters (bactericidal activity or
not, postantibiotic effect, etc.). The interpretation also
takes into account clinical experience gained from therapy
of infections with pathogens of given suceptibility. Such
data are used to establish general guideline values
defining the boundary between susceptible and resistant
bacteria.The minimum bactericidal concentration
(MBC) is the smallest concentration of a substance
required to kill 99.9% of the cells in an inoculum. The
MBC is determined using quantitative subcultures from
the macroscopically unclouded tubes or (microplate)
wells of an MIC dilution series.
Agar Diffusion Test: This method, also known as the
“disk test,” is
used to test the resistance of a bacterial culture to
various anti-infective agents. The method provides a
basis for classification of a bacterial strain as
“susceptible,” “resistant,” or “intermediate”
according to the dimension of the inhibition zone.
Combination Therapy
Combination therapy is the term for concurrent
administration of two or more anti-infective agents. Some
galenic preparations combine two components in a fixed
ratio (example: cotrimoxazole). Normally, however, the
dividual substances in a combination therapy are
administered separately.
Several different objectives can be pursued with
combination therapy:
Broadening of the spectrum of action.
In mixed
infections with pathogens of varying resistance; in
calculated therapy of infections with unknown, or not yet
known, pathogenic flora and resistance characteristics.
Delay of resistance development.
In therapy of
tuberculosis; when using anti-infective agents against
which bacteria quickly develop resistance.
Potentiation of efficacy.
In severe infections requiring
bactericidal activity at the site of infection. Best-known
example: penicillin plus gentamicin in treatment of
endocarditis caused by enterococci or streptococci.
Combining the effects of anti-infective drugs can have
several different effects:
No difference. The combination is no more efficacious
than the more active of the two components alone.
Addition. Summation of the effects.
Synergism. Potentiation of the effects.
Antagonism
. The combination is less efficacious than
one of the twocomponents alone.
Rule of thumb
:
combinations of bacteriostatics with
substances that are bactericidal in the cell division phase
only often result in antagonism, e.g., penicillin plus
tetracycline in therapy of pneumococcal pneumonia.
Unit 2: Bacteriology
60
In-vitro investigations of the mechanism of action of a
combination when used against a pathogen usually
employ the so-called “checkerboard titration” technique,
in which the combinatory effects of substances A and B
are compared using a checkerboard-like pattern.
Chemoprophylaxis
One of the most controversial antibiotic uses is
prophylactic antibiosis. There are no clear-cut solutions
here. There are certain situations in which
chemoprophylaxis is clearly indicated and others in which
it is clearly contraindicated. The matter must be decided
on a case-by-case basis by weighing potential benefits
against potential harm (side effects, superinfections with
highly virulent and resistant pathogens, selection of
resistant bacteria).
Chemoprophylaxis is considered useful in, rheumatic
fever, pulmonary cystic fibrosis, recurring pyelonephritis,
following intensive contact with meningococci carriers,
before surgery involving massive bacterial contamination,
in heavily immunocompromised patients, in cardiac
surgery or in femoral amputations due to circulatory
problems. Chemoprophylaxis aimed at preventing a
postsurgical infection should begin a few hours before the
operation and never be continued for longer than 24–72
hours.
Immunomodulators
Despite the generally good efficacy of anti-infective
agents, therapeutic success cannot be guaranteed.
Complete elimination of bacterial pathogens also requires
a functioning immune defense system. In view of the fact
that the number of patients with severe
immunodeficiencies is on the rise, immunomodulators are
used as a supportive adjunct to specific antibiotic therapy
in such patients. Many of these “cytokines” produced by
the cells of the immune system can now be produced as
“recombinant proteins.” Myelopoietic growth factors have
now been successfully used in patients suffering from
neutropenia. Additional immunomodulators are also
available, e.g., interferon gamma (IFNc) and interleukin 2
(IL-2).
Summary:
Specific antibacterial therapy refers to treatment of
infections with antiinfective agents directed against the
infecting pathogen. The most important group of anti-
infective agents are the antibiotics, which are products of
fungi and bacteria (Streptomycetes). Anti-infective agents
are categorized as having a broad, narrow, or medium
spectrum of action. The efficacy, or effectiveness, of a
substance refers to its bactericidal or bacteriostatic effect.
Anti-infective agents have many different mechanisms of
action. Under the influence of sulfonamides and
trimethoprim, bacteria do not synthesize sufficient
amounts of tetrahydrofolic acid. All betalactam antibiotics
irreversibly block the biosynthesis of murein. Rifamycin
inhibits the DNA-dependent RNA polymerase
(transcription). Aminoglycosides, tetracyclines, and
macrolides block translation. All 4-quinolones damage
cellular DNA topology by inhibiting bacterial
topoisomerases. Due to their genetic variability, bacteria
may develop resistance to specific anti-infective agents.
The most important resistance mechanisms are:
inactivating enzymes, resistant target molecules, reduced
influx, increased efflux. Resistant strains (problematic
bacteria) occur frequently among hospital flora, mainly
Enterobacteriaceae, pseudomonads, staphylococci, and
enterococci. Laboratory resistance testing is required for
specific antibiotic therapy. Dilutions series tests are
quantitative resistance tests used to determine the
minimum inhibitory concentration (MIC). The disk test is
a semiquantitative test used to classify the test bacteria as
resistant or susceptible. In combination therapies it must
be remembered that the interactions of two or more
antibiotics can give rise to an antagonistic effect. Surgical
chemoprophylaxis must be administered as a short-term
antimicrobial treatment only.