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Chemotherapeutic agents
survival, whereas
anaerobic
bacteria (e.g. those bacteria
associated with gangrene) do not use oxygen.
If culture and sensitivity testing is not possible,
either because the source of the infection is not iden-
tifiable or because the person is too sick to wait for
test results to determine the best treatment, clinicians
attempt to administer a drug with a broad spectrum of
activity against gram-positive or gram-negative bacteria
or against anaerobic bacteria. Antibiotics that interfere
with a biochemical reaction common to many organ-
isms are known as broad-spectrum antibiotics. These
drugs are often given at the beginning of treatment until
the exact organism and sensitivity can be established.
Because these antibiotics have such a wide range of
effects, they are frequently associated with adverse
effects. Human cells have many of the same properties
as bacterial cells and can be affected in much the same
way, so damage may occur to the human cells, as well as
to the bacterial cells.
Because there is no perfect antibiotic which is
without effect on the human host, clinicians try to
select an antibiotic with selective toxicity, or the ability
to strike foreign cells with little or no effect on human
cells. Certain antibiotics may be contraindicated in
some people because of known adverse effects; this
includes those people who are immunocompromised,
Bacterial
Flagellum
Cell wall
Plasma
membrane
Cytoplasm
Plasmid
Ribosomes
Nucleoid (circular
DNA)
Capsule
Pili
Tetracyclines
Fluroquinolones
Aztreonams
Penicillins
Carbapenems,
lincosamides and
ketolides
Sulfonamides
Antimycobacteria
FIGURE 9.2
Sites of cellular action of carbapenems, lincosamides, aztreonams, penicillins, sulfonamides, tetracyclines and antimycobacterials.
Carbapenems and lincosamides change protein function and prevent cell division or cause cell death. Aztreonam alters cell membranes
to allow leakage of intracellular substances and causes cell death. Penicillins prevent bacteria from building their cells during division.
Sulfonamides inhibit folic acid synthesis for RNA and DNA production. Tetracyclines inhibit protein synthesis, thereby preventing
reproduction. Antimycobacterial drugs affect mycobacteria in three ways: They (1) affect the mycotic coat of the bacteria, (2) alter DNA
and RNA and (3) prevent cell division.
Bacterial
Flagellum
Cell wall
Plasma
membrane
Cytoplasm
Plasmid
Ribosomes
Nucleoid (circular
DNA)
Capsule
Pili
Macrolides
Fluroquinolones
Aminoglycosides
Cephalosporins
FIGURE 9.1
Sites of cellular action of
aminoglycosides, cephalosporins,
fluoroquinolones and macrolides.
Aminoglycosides disrupt the cell
membrane. Cephalosporins cause
bacteria to build weak cell walls when
dividing. Fluoroquinolones interfere with
the DNA enzymes needed for growth
and reproduction. Macrolides change
protein function by binding to the cell
membrane to cause cell death or prevent
cell division.
