ANTIBIÓTICOS - Profª Leila Brito Queiroz

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ANTIBIÓTICOS 
DROGAS ANTIMICROBIANAS E ANTIBIOGRAMA 
Profa. Ms Leila Queiroz 
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HISTÓRIA DA QUIMIOTERAPIA 
n  ALEXANDER FLEMING (1881-1955) 
288 CHAPTER 10 Controlling Microbial Growth in the Body: Antimicrobial Drugs
Chemicals that affect physiology in any manner, such as caffeine,
alcohol, and tobacco, are called drugs. Drugs that act against dis-
eases are called chemotherapeutic agents. Examples include insulin,
anticancer drugs, and drugs for treating infections—called
antimicrobial agents (antimicrobials), the subject of this chapter.
In the pages that follow we’ll examine the mechanisms by
which antimicrobial agents act, the factors that must be considered
in the use of antimicrobials, and several issues surrounding resis-
tance to antimicrobial agents among microorganisms. First, how-
ever, we begin with a brief history of antimicrobial chemotherapy.
The History of 
Antimicrobial Agents
Learning Objectives
✓ Describe the contributions of Paul Ehrlich, Alexander Fleming,
and Gerhard Domagk in the development of antimicrobials.
✓ Explain how semisynthetic and synthetic antimicrobials differ
from antibiotics.
The little girl lay struggling to breathe as her parents stood
mutely by, willing the doctor to do something—anything—to
relieve the symptoms that had so quickly consumed their four-
year-old daughter’s vitality. Sadly, there was little the doctor
could do. The thick “pseudomembrane” of diphtheria, com-
posed of bacteria, mucus, blood-clotting factors, and white
blood cells, adhered tenaciously to her pharynx, tonsils, and
vocal cords. He knew that trying to remove it could rip open
the underlying mucous membrane, resulting in bleeding, possi-
bly additional infections, and death. In 1902, there was little
medical science could offer for the treatment of diphtheria; all
physicians could do was wait and hope.
At the beginning of the 20th century, much of medicine in-
volved diagnosing illness, describing its expected course, and
telling family members either how long a patient might be sick
or when they might expect her to die. Even though physicians
and scientists had recently accepted the germ theory of disease
and knew the causes of many diseases, very little could be done
to inhibit pathogens, including Corynebacterium diphtheriae (ko˘-
rı¯¿ne¯-bak-te¯r¿e¯-u˘m dif-thi¿re¯-ı¯), and alter the course of infections.
In fact, one-third of children born in the early 1900s died from
infectious diseases before the age of five.
It was at this time that Paul Ehrlich (1854–1915), a visionary
German scientist, proposed the term chemotherapy to describe the
use of chemicals that would selectively kill pathogens while hav-
ing little or no effect on a patient. He wrote of “magic bullets”
that would bind to receptors on germs to bring about their death
while ignoring host cells, which lacked the receptor molecules.
Staphylococcus
aureus
(bacterium)
Penicillium
chrysogenum
(fungus)
Zone where
bacterial growth
is inhibited
Figure 10.1 Antibiotic effect of the mold Penicillium
chrysogenum. Alexander Fleming observed that this mold secretes
penicillin, which inhibits the growth of bacteria, as is apparent with
Staphylococcus aureus growing on this blood agar plate.
➤
Antibiotic Overkill
A young woman was taking antibiotic pills for a urinary infection.
Several days into her course of medication, she began to
experience peculiar symptoms. At first they were hardly
noticeable. Very quickly, however, they worsened and became
embarrassing and unbearable.
She noticed a white coating on her tongue, bad breath, and
an awful taste in her mouth. Despite persistent brushing and
mouthwash applications, she was unable to completely remove
the film. Furthermore, she had excessive vaginal discharges
consisting of a cheeselike white substance. When she began to
have vaginal itching, she finally decided it was time to seek help.
Reluctantly she revisited her personal physician and
described the symptoms. Her doctor explained the symptoms
and provided additional prescriptions to alleviate her distress.
1. What happened to the young woman in this situation?
2. How had her body’s defenses been violated?
3. How can she avoid a repeat of this situation?
CLINICAL APPLICATIONS
Ehrlich’s search for antimicrobial agents resulted in the dis-
covery of one arsenic compound that killed trypanosome para-
sites and another that worked against the bacterial agent of
syphilis. A few years later, in 1928, the British bacteriologist
Alexander Fleming (1881–1955) reported the antibacterial action
of penicillin released from Penicillium (pen-i-sil¿e¯-u˘m) mold,
which creates a zone where bacteria don’t grow (Figure 10.1).
Though arsenic compounds and penicillin were discovered
first, they were not the first antimicrobials in widespread use:
Ehrlich’s arsenic compounds are toxic to humans, and peni-
cillin was not available in large enough quantities to be useful
until the late 1940s. Instead, sulfanilamide, discovered in 1932 by
the German chemist Gerhard Domagk (1895–1964), was the first
practical antimicrobial agent efficacious in treating a wide array
of bacterial infections.
Selman Waksman (1888–1973) discovered other microor-
ganisms that are sources of useful antimicrobials, most notably
species of soil-dwelling bacteria in the genus Streptomyces
(strep-to¯-mı¯¿se¯z). Waksman coined the term antibiotics to
+CARACTERÍSTICAS DOS 
ANTIMICROBIANOS 
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CARACTERÍSTICAS DOS 
ANTIMICROBIANOS 
n  ORIGEM 
ü  Microbiana (Antibióticos): produtos microbianos ou 
derivados – substâncias do metabolismo secundário; 
ü  Semi-sintética: Um antibiótico natural modificado por um 
grupo químico; 
ü  Sintético: Produzidos laboratorialmente ou de origem vegetal 
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CARACTERÍSTICA DOA 
ANTIMICROBIANOS 
n  QUANTO À AÇÃO 
ü  Bacteriostática; 
ü  Bactericida. 
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MECANISMOS DE AÇÃO 
n  TOXICIDADE SELETIVA (Paul Ehrlich) 
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MECANISMO DE AÇÃO 
n  ESPECTRO DE AÇÃO 
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MECANISMO DE AÇÃO 
n  DESCONHECIMENTO DO AGENTE PATOGÊNICO/
INFECÇÕES MISTAS 
DROGAS DE AMPLO ESPECTRO 
DESTRUIÇÃO DA MICROBIOTA NORMAL 
PROLIFERAÇÃO DA MICROBIOTA 
SUPLEMENTAR 
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MECANISMO DE AÇÃO 290 CHAPTER 10 Controlling Microbial Growth in the Body: Antimicrobial Drugs
agent must be more toxic to a pathogen than to the pathogen’s
host. Selective toxicity is possible because of differences in
structure or metabolism between the pathogen and its host.
Typically, the more differences, the easier it is to discover or cre-
ate an effective antimicrobial agent.
Because there are many differences between the structure
and metabolism of pathogenic bacteria and their eukaryotic
hosts, antibacterial drugs constitute the greatest number and di-
versity of antimicrobial agents. Fewer antifungal, antiprotozoan,
and anthelmintic drugs are available because fungi, protozoa,
and helminths—like their animal and human hosts—are eukary-
otic and thus share many common features. The number of effec-
tive antiviral drugs is also limited, despite major differences in
structure, because viruses utilize their host cells’ enzymes and ri-
bosomes to metabolize and replicate. Therefore, drugs that are ef-
fective against viral replication are likely toxic to the host as well.
Although they can have a variety of effects on pathogens,
antimicrobial drugs can be categorized into several general
groups according to their mechanisms of action (Figure 10.2):
• Drugs that inhibit cell wall synthesis. These drugs are
selectively toxic to certain fungal or bacterial cells, which
have cell walls, but not to animals, which lack cell walls.
• Drugs that inhibit protein synthesis (translation) by target-
ing the differences between prokaryotic and eukaryotic
ribosomes.
• Drugs that disrupt unique components of the cytoplasmicmembrane.
• Drugs that inhibit general metabolic pathways not used by
humans.
• Drugs that inhibit nucleic acid synthesis.
• Drugs that block a pathogen’s recognition of or attachment
to its host.
In the following sections we examine these mechanisms in turn.
ANIMATIONS: Chemotherapeutic Agents: Modes of Action
Inhibition of Cell Wall Synthesis
Learning Objective
✓ Describe the actions and give examples of drugs that affect
the cell walls of bacteria and fungi.
A cell wall protects a cell from the effects of osmotic pressure.
Both pathogenic bacteria and fungi have cell walls, which
Inhibition of
protein synthesis
Aminoglycosides
Tetracyclines
Chloramphenicol
Macrolides
Disruption of
cytoplasmic membrane
Polymyxins
Polyenes (antifungal)
Inhibition of general
metabolic pathway
Sulfonamides
Trimethoprim
Dapsone
Inhibition of DNA
or RNA synthesis
Actinomycin
Nucleotide
 analogs
Quinolones
Rifampin
Inhibition of cell
wall synthesis
Penicillins
Cephalosporins
Vancomycin
Bacitracin
Isoniazid
Ethambutol
Echinocandins
(antifungal)Inhibition of pathogen’s
attachment to, or
recognition of, host
Arildone
Pleconaril
Human
cell membrane
Figure 10.2 Mechanisms of action of microbial
drugs. Also listed are representative drugs for each type
of action.
➤
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INIBIÇÃO DA SÍNTESE DA PAREDE 
CELULAR 
66 CHAPTER 3 Cell Structure and Function
(staphylococci), or cuboidal packets (sarcinae, sar¿si-nı¯) (Figure
3.12) depending on the planes of cell division. Rod-shaped
cells, called bacilli (ba˘-sil¿ı¯), typically appear singly or in chains.
Bacterial cell walls are composed of peptidoglycan, a
complex polysaccharide. Peptidoglycan in turn is composed of 
two types of regularly alternating sugar molecules, called 
N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM),
which are structurally similar to glucose (Figure 3.13). Millions
of NAG and NAM molecules are covalently linked in chains 
in which NAG alternates with NAM. These chains are the
“glycan” portions of peptidoglycan.
Chains of NAG and NAM are attached to other chains by
crossbridges of four amino acids (tetrapeptides). Figure 3.14 il-
lustrates one possible configuration. These peptide crossbridges
are the “peptido” portion of peptidoglycan. Depending on the
bacterium, tetrapeptide bridges are either covalently bonded to
one another or are held together by short connecting chains of
other amino acids as shown in Figure 3.14. Peptidoglycan cov-
ers the entire surface of a cell, which must insert millions of
new subunits if it is to grow and divide.
Scientists describe two basic types of bacterial cell walls as
Gram-positive cell walls or Gram-negative cell walls. They distin-
guish Gram-positive and Gram-negative cells by the use of the
Gram staining procedure (described in Chapter 4), which was
invented long before the structure and chemical nature of bacte-
rial cell walls were known.
Gram-Positive Bacterial Cell Walls
Learning Objective
✓ Compare and contrast the cell walls of acid-fast bacteria with
typical Gram-positive cell walls.
Gram-positive bacterial cell walls have a relatively thick layer
of peptidoglycan that also contains unique chemicals called
teichoic (tı¯-ko¯¿ik)4 acids. Some teichoic acids are covalently
linked to lipids, forming lipoteichoic acids that anchor the pepti-
doglycan to the cytoplasmic membrane (Figure 3.15a). Teichoic
acids have negative electrical charges, which help give the sur-
face of a Gram-positive bacterium a negative charge and may
play a role in the passage of ions through the wall. The thick cell
wall of a Gram-positive bacterium retains the crystal violet dye
used in the Gram staining procedure, so the stained cells ap-
pear purple under magnification.
Some additional chemicals are associated with the walls
of some Gram-positive bacteria. For example, species of
Mycobacterium (mı¯ ¿ko¯-bak-te¯r¿e¯-u˘m), which include the causative
(a) (b)
Figure 3.12 Bacterial shapes and arrangements.
(a) Spherical cocci may be in arrangements such as single, chains
(streptococci), clusters (staphylococci), and cuboidal packets. 
(b) Rod-shaped bacilli may also be single or in arrangements 
such as chains.
➤
(a) (b)
O
OHH
OH
H
HH
H H
OH
OH
H
H
CH2OH CH2OH
HO
H
OH
H
H
Glucose N-acetylglucosamine
NAG
CH3
NH
C O
HH
CH2OH
CH3
NH
C O
CH3
C O
O
HC
O OO
O O
N-acetylmuramic acid
NAM
Figure 3.13 Comparison of the structures of glucose, 
NAG, and NAM. (a) Glucose. (b) N-acetylglucosamine (NAG) and 
N-acetylmuramic acid (NAM) molecules linked as in peptidoglycan. Blue
shading indicates the differences between glucose and the other two
sugars. Orange boxes highlight the difference between NAG and NAM.
➤
NAM
N AM
NAM
NAM
NAM
NAM
NAM
NAM
NAM
N AG
NAG
NAM
NAM
NAM
NAM
NAG
NAG
N AM
NAM
NAG
NAG
NAM
NAM
N AG
NAG
NAM
Connecting chain
of amino acids
Tetrapeptide
(amino acid)
crossbridge
Sugar
backbone
Figure 3.14 Possible structure of peptidoglycan.
Peptidoglycan is composed of chains of NAG and NAM linked by
tetrapeptide crossbridges and, in some cases, as shown here, connecting
chains of amino acids to form a tough yet flexible structure. The amino
acids of the crossbridges differ among bacterial species.
➤
4From Greek teichos, meaning wall.
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INIBIÇÃO DA SÍNTESE DA PAREDE 
CELULAR 
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INIBIÇÃO DA SÍNTESE DA PAREDE 
CELULAR 
n  BETA-LACTÂMICOS 
ANEL BETA-LACTÂMICO 
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PENICILINA 
n  Grupo de 50 antibióticos; 
n  Podem ser naturais ou semissintéticas; 
n  PENICILINA G (PENICILINA NATURAL) 
Streptococcus, Staphylococcus e diversas Espiroquetas 
 
-  3 a 4 horas; 
-  Sensível a acidez 
estomacal 
PENICILINA BENZATINA – aumento 
do tempo no organismo (meses) 
PENICILINA V – estável na acidez 
estomacal 
Penicillium notatum 
PENICILINA PROCAÍNA – detectável 
até 24h no organismo 
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PENICILINA 
n  TRANSPEPTIDASE 
PENICILINA 
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CEFALOSPORINA 
n  Ação similar à da Penicilina: 
O mais utilizado 
QUATRO GERAÇÕE DE CEFALOSPORINA 
Diferem quanto ao espectro de 
ação e métodos de administração 
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CEFALOSPORINA 
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INIBIDORES DA SÍNTESE PROTÉICA 
Diferenças entre os Ribossomos: 
ü  Cloranfenicol 
ü  Eritromicina 
ü  Estreptomicina (aminoglicosídeos) 
ü  Tetraciclinas 
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DANO À MEMBRANA PLASMÁTICA 
n  INDUZEM MUDANÇAS NA PERMEBILIDADE DA MEMBRANA: 
- POLIENOS (anti-fúngico) 
- ANFOTERICINA B E NISTATINA 
AÇÃO ANTI-FÚNGICA 
294 CHAPTER 10 Controlling Microbial Growth in the Body: Antimicrobial Drugs
NH2
OH
O
H3C
CH3
CH3
CH3
HO
OOH
O
O
HO
HO
HO
HO
HO
HOOC OH
O
Amphotericin B
(a)
Pore
Phospholipid of fungal
cytoplasmic membrane Amphotericin B Ergosterol
(b)
OH
Figure 10.5 Disruption of the cytoplasmic membrane by 
the antifungal amphotericin B. (a) The structure of amphotericin B. 
(b) The proposed action of amphotericin B. The drug binds to molecules
of ergosterol, which then congregate, forming a pore.
➤
Azoles, such as fluconazole, and allylamines, such as
terbinafine, are two other classes of antifungal drugs that disrupt
cytoplasmic membranes. They act by inhibiting the synthesis of
ergosterol; without ergosterol, the cell’s membrane does not re-
main intact, and the fungal cell dies. Azoles and allylamines are
generally harmless to humans, because human cells do not
manufacture ergosterol.
Most bacterial membranes lack sterols, so these bacteria are
naturally resistant to polyenes, azoles, and allylamines; how-
ever, there are other agents that disrupt bacterial membranes.
An example of these antibacterial agents is polymyxin, produced
by Bacillus polymyxa (ba-sil¿u˘s po-le¯-miks¿a). Polymyxin is effec-
tive against Gram-negative bacteria, particularly Pseudomonas
(soo-do¯-mo¯¿nas), but because it is toxic to human kidneys it is
usually reserved for use against external pathogens that are re-
sistant to other antibacterial drugs.
Pyrazinamide disrupts transport across the cytoplasmic
membrane of M. tuberculosis (too-ber-kyu¯-lo¯¿sis). The pathogen
uniquely activates and accumulates the drug. Unlike many
other antimicrobials, pyrazinamide is most effective against in-
tracellular, nonreplicating bacterial cells.
Some antiparasitic drugs also act against cytoplasmic mem-
branes. For example, praziquantel and ivermectin change the per-
meability of cell membranes of several types of parasitic worms.
Inhibition of Metabolic Pathways
As we discussed in Chapter 5, metabolism can be defined sim-
ply as the sum of all chemical reactions that take place within
an organism. Whereas most living things share certain meta-
bolic reactions—for example, glycolysis—other chemical reac-
tions are unique to certain organisms. Whenever differences
exist between the metabolic processes of a pathogen and its
host, antimetabolic agents can be effective.
Various kinds of antimetabolic agents are available, includ-
ing atovaquone, which interferes with electron transport in pro-
tozoa and fungi; heavy metals (such as arsenic, mercury, and
antimony), which inactivate enzymes; agents that disrupt tubu-
lin polymerization and glucose uptake by many protozoa and
parasitic worms; drugs that block the activation of viruses; and
metabolic antagonists such as sulfanilamide, the first commer-
cially available antimicrobial agent.
Sulfanilamide and similar compounds, collectively called
sulfonamides, act as antimetabolic drugs because they are
structural analogs of—that is, are chemically very similar to—
para-aminobenzoic acid (PABA; Figure 10.6a). PABA is crucial in
the synthesis of nucleotides required for DNA and RNA synthe-
sis. Many organisms, including some pathogens, enzymatically
convert PABA into dihydrofolic acid, and then dihydrofolic
acid into tetrahydrofolic acid (THF), a form of folic acid that is
used as a coenzyme in the synthesis of purine and pyrimidine
nucleotides (Figure 10.6b). As analogs of PABA, sulfonamides
compete with PABA molecules for the active site of the enzyme
involved in the production of dihydrofolic acid (Figure 10.6c).
This competition leads to a decrease in the production of THF,
and thus of DNA and RNA. The end result of sulfonamide com-
petition with PABA is the cessation of cell metabolism, which
leads to cell death.
Note that humans do not synthesize THF from PABA; in-
stead, we take simple folic acids found in our diets and convert
them into THF. As a result, human metabolism is unaffected by
sulfonamides.
Another antimetabolic agent, trimethoprim, also interferes
with nucleic acid synthesis. However, instead of binding to the
enzyme that converts PABA to dihydrofolic acid, trimethoprim
binds to the enzyme involved in the conversion of dihydrofolic
acid to THF, the second step in this metabolic pathway.
Some antiviral agents target the unique aspects of the me-
tabolism of viruses. After attachment to a host cell, viruses must
penetrate the cell’s membrane and be uncoated to release viral
genetic instructions and assume control of the cell’s metabolic
machinery. Some viruses of eukaryotes are uncoated as a result
-  FLUCONAZOL 
-  ITRACONAZOL 
Inibe a síntese do 
Ergosterol 
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INIBIDORES DA SÍNTESE DE ÁCIDOS 
NUCLÉICOS (DNA/RNA) 
n  RIFAMICINAS/RIFAMPICINAS 
n  QUINOLONAS E FLUOROQUINOLONAS 
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INIBIDORES COMPETITIVOS DA 
SÍNTESE DE METABÓLITOS 
n  SULFONAMIDAS 
- Drogas bacteriostáticas sintéticas 
O PABA é essencial para a síntese de ÀCIDO FÓLICO 
PARAMINOBENZÓICO 
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CONSIDERAÇÕES CLÍNICAS NA 
PRESCRIÇÃO DE ANTIBIÓTICOS 
n  Disponibilidade; 
n  Baixo custo; 
n  Estabilidade química; 
n  Facilmente administrável; 
n  Não tóxico e não alergênico; 
n  Toxicidade seletiva; 
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RESISTÊNCIA A DROGAS 
ANTIMICROBIANAS 
n  RESISTÊNCIA 
Fenômeno ESPERADO de seleção NATURAL 
Darwin (1809-1882) 
UTILIZAÇÃO DE ANTIMICROBIANOS 
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RESISTÊNCIA A DROGAS 
ANTIMICROBIANAS 
n  Capacidade ADQUIRIDA de um organismos em resistir a 
algum agente quimioterápico ao qual normalmente é 
susceptível; 
n  Alguns microrganismos são naturalmente resistentes a 
determinados antibióticos – Staphylococcus produtoes de 
PENICILINASE (β-lactamase). 
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MECANISMOS DE RESISTÊNCIA 
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AÇÃO PLASMIDIAL 
PILI SEXUAL 
CONJUGAÇÃO 
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TRANSFORMAÇÃO 
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TRANSDUÇÃO 
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REISTÊNCIA POR MATERIAL 
GENÉTICO 
TRANSFORMAÇÃO 
TRANSDUÇÃO 
CONJUGAÇÃO 
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USO INADEQUADO DE ANTIBIÓTICOS 
E COMO PREVENIR RESISTÊNCIA 
n  Uso indiscriminado em países em desenvolvimento – áreas 
rurais – utilização para dores de cabeça; 
-  Não obedecem dose e tempo – UTILIZAÇÃO NAS DOSE E 
TEMPO CORRETOS; 
-  Utilização de antibióticos para tratar infecções virais - 
SEMELHANÇA; 
-  Trocar de reavaliar o antibiótico tão logo a bactéria mostre 
sinais de resistência; 
-  Associar antibióticos quando necessário. 
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USO INADEQUADO DE ANTIBIÓTICOS 
E COMO PREVENIR RESISTÊNCIA 
n  Quando possível o agente infeccioso deve ser isolado do 
foco infeccioso e identificado; 
n  Solicitar Testes 
SOLICITAR ANTIBIOGRAMA 298 CHAPTER 10 Controlling Microbial Growth in the Body: Antimicrobial Drugs
Bacterial lawn Zone of inhibition
Figure 10.9 Zones of inhibition in a diffusion susceptibility
(Kirby-Bauer) test. In general, the larger the zone of inhibition around
disks, which are impregnated with an antimicrobial agent, the more
effective that antimicrobial is against the organismgrowing on the plate.
The organism is classified as either susceptible, intermediate, or resistant
to the antimicrobials tested, based on the sizes of the zones of inhibition.
If all of these antimicrobial agents diffuse at the same rate and are
equally safe and easily administered, which one would be the drug of
choice for killing this pathogen?
➤
unaffected by the antimicrobial. This results because the killing of
normal microbiota reduces microbial antagonism, the competition
between normal microbes and pathogens for nutrients and space.
Microbial antagonism reinforces the body’s defense by limiting
the ability of pathogens to colonize the skin and mucous mem-
branes. Thus a woman using erythromycin to treat strep throat (a
bacterial disease) could develop vaginitis resulting from the ex-
cessive growth of Candida albicans (kan¿did-a˘ al¿bi-kanz), a yeast
that is unaffected by erythromycin and is freed from microbial an-
tagonism when the antibiotic kills normal bacteria in the vagina.
Efficacy
Learning Objective
✓ Compare and contrast Kirby-Bauer, Etest, MIC, and MBC
tests.
To effectively treat infectious diseases, physicians must know
which antimicrobial agent is most effective against a particular
pathogen. To ascertain the efficacy of antimicrobials, microbiol-
ogists conduct a variety of tests, including diffusion susceptibil-
ity tests, the minimum inhibitory concentration test, and the
minimum bactericidal concentration test.
Diffusion Susceptibility Test
Diffusion susceptibility tests, also known as Kirby-Bauer tests,
involve uniformly inoculating a Petri plate with a standardized
amount of the pathogen in question. Then small disks of paper
containing standard concentrations of the drugs to be tested are
firmly arranged on the surface of the plate. The plate is incubated,
and the bacteria grow and reproduce to form a “lawn” every-
where but the areas where effective antimicrobial drugs diffuse
through the agar. After incubation, the plates are examined for the
presence of a zone of inhibition—that is, a clear area where bac-
teria do not grow (Figure 10.9). A zone of inhibition is measured
as the diameter (to the closest millimeter) of the clear region.
If all drugs were equal, then the larger the zone of inhibi-
tion, the more effective that drug is; however, the size of a zone
depends on the rate of diffusion of the antimicrobial; for exam-
ple, drugs with lower molecular weights generally diffuse more
quickly than those with higher molecular weights. The size of a
zone of inhibition must be compared to a standard table for that
particular drug before accurate comparisons can be made. Dif-
fusion susceptibility tests enable scientists to classify pathogens
as susceptible, intermediate, or resistant to each drug.
CRITICAL THINKING
Sometimes it is not possible to conduct a susceptibility test,
because of either a lack of time or an inability to access the bacteria
(from an inner-ear infection, for instance). How could a physician
select an appropriate therapeutic agent in such cases?
Minimum Inhibitory Concentration (MIC) Test
Once scientists identify an effective antimicrobial agent, they
quantitatively express its potency as a minimum inhibitory
concentration (MIC). As the name suggests, the MIC is the
smallest amount of the drug that will inhibit growth and repro-
duction of the pathogen. The MIC can be determined via a broth
dilution test, in which a standardized amount of bacteria is
added to serial dilutions of antimicrobial agents in tubes or
wells containing broth. After incubation, turbidity (cloudiness)
indicates bacterial growth; lack of turbidity indicates that the
bacteria were either inhibited or killed by the antimicrobial
agent (Figure 10.10). Dilution tests can be conducted simultane-
ously in wells, and the entire process can be automated, with
turbidity measured by special scanners connected to computers.
Another test that determines minimum inhibitory concentra-
tion combines aspects of an MIC test and a diffusion susceptibil-
ity test. This test, called an Etest,3 involves placing a plastic strip
containing a gradient of the antimicrobial agent being tested on a
plate uniformly inoculated with the organism of interest (Figure
10.11). After incubation, an elliptical zone of inhibition indicates
antimicrobial activity, and the minimum inhibitory concentration
can be noted where the zone of inhibition intersects a scale
printed on the strip.
Minimum Bactericidal Concentration (MBC) Test
Similar to the MIC test is a minimum bactericidal concentra-
tion (MBC) test, though an MBC test determines the amount
of drug required to kill the microbe rather than just the
amount to inhibit it, as the MIC does. In an MBC test, samples
taken from clear MIC tubes (or alternatively, from zones of
Figure 10.9The drug ENO, a fluoroquinolone found in the uppermost
disk, is most effective.
3The name Etest has no specific origin.