C H A P T E R 8
Anti-infective agents
83
interfering with the host (see Box 8.2). The penicillins
work in this way.
• Some anti-infectives prevent the cells of the invading
organism from using substances essential to their
growth and development, leading to an inability to
divide and eventually to cell death. The sulfonamides,
the antimycobacterial drugs and trimethoprim-
sulfamethoxazole (a combination drug frequently used
to treat urinary tract infections) work in this way.
• Many anti-infectives interfere with the steps
involved in protein synthesis, a function necessary
to maintain the cell and allow for cell division. The
aminoglycosides, the macrolides and chloramphenicol
(see the section on adverse effects for information on
chloramphenicol) work in this way.
• Some anti-infectives interfere with DNA synthesis in
the cell, leading to inability to divide and cell death.
The fluoroquinolones work in this way.
• Other anti-infectives alter the permeability of the cell
membrane to allow essential cellular components
to leak out, causing cell death. Some antibiotics,
antifungals and antiprotozoal drugs work in this
manner.
Anti-infective activity
The anti-infectives used today vary in their
spectrum
of
activity; that is, they vary in their effectiveness against
invading organisms. Some anti-infectives are so selec-
tive in their action that they are effective against only
a few microorganisms with a very specific metabolic
pathway or enzyme. These drugs are said to have a
narrow spectrum of activity. Other drugs interfere with
biochemical reactions in many different kinds of micro-
organisms, making them useful in the treatment of a
wide variety of infections. Such drugs are said to have a
broad spectrum of activity.
Some anti-infectives are so active against the infec-
tive microorganisms that they actually cause the death of
the cells they affect. These drugs are said to be
bacteri
cidal
. Some anti-infectives are not as aggressive against
invading organisms; they interfere with the ability of the
cells to reproduce or divide. These drugs are said to be
bacteriostatic
. Several drugs are both bactericidal and
bacteriostatic, often depending on the concentration
of the drug that is present. Many of the adverse effects
noted with the use of anti-infectives are associated with
the aggressive properties of the drugs and their effect on
the cells of the host in addition to those of the pathogen.
Human immune response
The goal of anti-infective therapy is reduction of the
population of the invading organism to a point at which
the human immune response can take care of the infec-
tion. If a drug were aggressive enough to eliminate all
traces of any invading pathogen, it also might be toxic
to the host. The immune response (see Chapter 15)
involves a complex interaction among chemical
Bacterial
Flagellum
Cell wall
Plasma
membrane
Cytoplasm
Plasmid
Ribosomes
Nucleoid (circular
DNA)
Capsule
Pili
Blocking use of
needed nutrients
DNA synthesis
alteration
Membrane
disruption
RNA alteration
FIGURE 8.1
Anti-infectives can affect cells by disrupting the cell membrane interfering with DNA synthesis, altering RNA or blocking the use of
essential nutrients.
Teicoplanin (
Targocid
) is an antibiotic that
interferes with the cell wall synthesis of susceptible
staphylococcal bacteria. Adverse effects include
phlebitis, bronchospasm, pruritus and fever. Because
of the development of resistant strains and more
potent antibiotics, teicoplanin is only indicated for
serious infections that cannot be treated by other less
toxic drugs, such as osteomyelitis, bacteraemia and
septicaemia. It is given by intravenous or intramuscular
administration.
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BOX 8.2
Anti-infective mechanism: Interference
with cell wall synthesis
