Bacteriology at UW-Madison
Most microbiologists distinguish two groups of antimicrobial agents
used in the treatment of infectious disease: antibiotics, which
are natural substances produced by certain groups of microorganisms,
and chemotherapeutic agents, which are chemically synthesized.
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. Furthermore, some
antimicrobial compounds, originally discovered as products of
microorganisms, can be synthesized entirely by chemical means. In the
medical and parmaceutical worlds, all these antimicrobial agents used
in the treatment of disease are referred to as antibiotics,
interpreting the word literally.
The modern era of antimicrobial chemotherapy began in 1929, with Fleming's discovery of the powerful bactericidal substance, penicillin, and Domagk's discovery in 1935 of synthetic chemicals (sulfonamides) with broad antimicrobial activity.
In the early 1940's, spurred partially by the need for antibacterial
agents in WW II, penicillin was isolated and purified and injected into
experimental animals, where it was found not only to cure infections
but also to possess incredibly low toxicity for the animals. This fact
ushered into being the age of antibiotic chemotherapy, and an intense
search for similar antimicrobial agents of low toxicity to animals that
might prove useful in the treatment of infectious disease. The rapid
isolation of streptomycin, chloramphenicol and tetracycline soon
followed, and by the 1950's, these and several other antibiotics were
in clinical usage.
From a patient point of view, the most important property of an antimicrobial agent is its selective toxicity, i.e., that the agent acts in some way that inhibits or kills bacterial pathogens but has little or no toxic effect on the patient.
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
-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.
Kinds of Antimicrobial Agents and their Primary Modes of Action
The table below is a summary of thetypes or classes of antibiotics and their properties including their biological source, spectrum and mode of action.
|Chemical class||Examples||Biological source||Spectrum (effective against)||Mode of action|
|Beta-lactams (penicillins and cephalosporins)||Penicillin G, Cephalothin||Penicillium notatum and Cephalosporium species||Gram-positive bacteria||Inhibits steps in cell wall (peptidoglycan) synthesis and murein assembly|
||Ampicillin, Amoxicillin||Gram-positive and Gram-negative bacteria||Inhibits steps in cell wall (peptidoglycan) synthesis and murein assembly|
|Clavulanic Acid||Augmentin is clavulanic acid plus Amoxicillin||Streptomyces clavuligerus||Gram-positive and Gram-negative bacteria||Inhibitor of bacterial beta-lactamases|
|Monobactams||Aztreonam||Chromobacterium violaceum||Gram-positive and Gram-negative bacteria||Inhibits steps in cell wall (peptidoglycan) synthesis and murein assembly|
|Carboxypenems||Imipenem||Streptomyces cattleya||Gram-positive and Gram-negative bacteria||Inhibits steps in cell wall (peptidoglycan) synthesis and murein assembly|
|Aminoglycosides||Streptomycin||Streptomyces griseus||Gram-positive and Gram-negative bacteria||Inhibits translation (protein synthesis)|
|Gentamicin||Micromonospora species||Gram-positive and Gram-negative bacteria esp. Pseudomonas||Inhibits translation (protein synthesis)|
|Glycopeptides||Vancomycin||Amycolatopsis orientalis (formerly designated Nocardia orientalis)||Gram-positive bacteria, esp. Staphylococcus aureus||Inhibits steps in murein (peptidoglycan) biosynthesis and assembly|
|Lincomycins||Clindamycin||Streptomyces lincolnensis||Gram-positive and Gram-negative bacteria esp. anaerobic Bacteroides||Inhibits translation (protein synthesis)|
||Streptomyces erythreus||Gram-positive bacteria, Gram-negative bacteria not enterics, Neisseria, Legionella, Mycoplasma||Inhibit translation (protein synthesis)|
|Polypeptides||Polymyxin||Bacillus polymyxa||Gram-negative bacteria||Damages cytoplasmic membranes|
|Bacitracin||Bacillus subtilis||Gram-positive bacteria||Inhibits steps in murein (peptidoglycan) biosynthesis and assembly|
|Polyenes||Amphotericin||Streptomyces nodosus||Fungi (Histoplasma)
||Inactivate membranes containing sterols|
|Nystatin||Streptomyces noursei||Fungi (Candida)||Inactivate membranes containing sterols|
|Rifamycins||Rifampicin||Streptomyces mediterranei||Gram-positive and Gram-negative bacteria, Mycobacterium tuberculosis||Inhibits transcription (bacterial RNA polymerase)|
|Tetracyclines||Tetracycline||Streptomyces species||Gram-positive and Gram-negative bacteria, Rickettsias||Inhibit translation (protein synthesis)|
|Semisynthetic tetracycline||Doxycycline||Gram-positive and Gram-negative bacteria, Rickettsias Ehrlichia, Borrelia||Inhibit translation (protein synthesis)|
|Chloramphenicol||Chloramphenicol||Streptomyces venezuelae||Gram-positive and Gram-negative bacteria||Inhibits translation (protein synthesis)|
||Mainly Gram-negative bacteria||Inhibits DNA
||Gram-negative and some
Gram-positive bacteria (Bacillus
||Inhibits DNA replication
|Growth factor analogs||Sulfanilamide, Gantrisin,
||Gram-positive and Gram-negative bacteria||Inhibits folic acid metabolism
||Mycobacterium tuberculosis||Inhibits mycolic acid synthesis;
analog of pyridoxine (Vit B6)
Examination of the foregoing table reveals that there are a
of fundamental ways that antibacterial antibiotics work as therapeutic
agents. Recall that the target of an antibiotic should be unique to the
bacterium and not found, or not accessible to the antibiotic, in the
patient. These are the most important targets in bacteria that have
been exploited so far.
Antimicrobial Agents Used in the Treatment of Infectious Disease
Cell wall synthesis inhibitors
Cell wall synthesis inhibitors generally inhibit some step in the synthesis of bacterial peptidoglycan. They exert their selective toxicity against bacteria because humans cells lack cell walls.
Beta lactam antibiotics. Chemically, these antibiotics
4-membered beta lactam ring. They are the products of two genera of
fungi, Penicillium and Cephalosporium, and are
correspondingly represented by the penicillins and cephalosporins.
Chemical structures of some beta-lactam antibiotics. Clockwise: penicillin, cephalosporin, monobactam, carbapenem. Note the characteristic structure of the beta lactam ring.
The beta lactam antibiotics are stereochemically related to D-alanyl-D-alanine, which is a substrate for the last step in peptidoglycan synthesis, the final cross-linking between between peptide side chains. Penicillins bind to and inhibit the carboxypeptidase and transpeptidase enzymes that are required for this step in peptidoglycan biosynthesis. Beta lactam antibiotics are bactericidal and require that cells be actively growing in order to exert their toxicity.
Different beta lactams differ in their spectrum of activity and their effect on Gram-negative rods, as well as their toxicity, stability in the human body, rate of clearance from blood, whether they can be taken orally, ability to cross the blood-brain barrier, and susceptibility to bacterial beta-lactamases.
Natural penicillins, such as penicillin G or penicillin
V (benzyl penicillin), are produced by fermentation of Penicillium
They are effective against streptococci, gonococci and
staphylococci, except where resistance has developed. They are
considered narrow spectrum since they are not effective against
(Benzylpenicillin) is typically given by parenteral administration
because it is unstable in the acid of the stomach. However, this
achieves higher tissue concentrations than orally-administered
penicillins and this increases its antibacterial potential.
"PenG" may be used in treatment of bacterial endocarditis, gonorrhea,
syphilis, meningitis, and pneumonia.
Semisynthetic penicillins first appeared in 1959. A mold produces the main part of the molecule (6-aminopenicillanic acid), which can be modified chemically by the addition of side chains. Many of these compounds have been developed to have distinct benefits or advantages over penicillin G, such as increased spectrum of activity (effectiveness against Gram-negative rods), resistance to penicillinase, effectiveness when administered orally, etc.; amoxicillin and ampicillin have broadened spectra against Gram-negative bacteria and are effective orally; methicillin is penicillinase-resistant.
Although nontoxic, penicillins occasionally cause death when administered to persons who are allergic to them. In the U.S. there are 300 - 500 deaths annually due to penicillin allergy. In allergic individuals the beta lactam molecule attaches to a serum protein and initiates an IgE-mediated inflammatory response.
Cephalosporins are beta lactam antibiotics with a similar
mode of action to penicillins. They are produced by species of
Cephalosporium molds. The have
a low toxicity and a somewhat broader spectrum
than natural penicillins. They are often used as penicillin
substitutes against Gram-negative bacteria and in surgical
prophylaxis. They are subject to degradation by some bacterial
beta-lactamases, but they tend to be resistant to beta-lactamases from S.
Cycloserine inhibits the early stages of murein synthesis
where D-alanyl-D-alanine is added to the growing peptide side chain.
The antibiotic resembles D-alanine in spatial structure, and it
competitively inhibits the racemase reaction that converts L-alanine to
D-alanine and the synthetase reaction that joins two D-alanine
molecules. The affinity of cycloserine for these enzymes is about a
hundred times greater than that of D-alanine. Cycloserine enters
bacterial cells by means of an active transport system for glycine and
can reach a relatively high intracellular concentration. This
concentrating effect, along with its high affinity for susceptible
enzymes, enables cycloserine to function as a very effective
antimicrobial agent. However, it is fairly toxic and has limited use as
a secondary drug for tuberculosis.
Cycloserine is an oral broad spectrum antibiotic effective against tuberculosis, by inhibiting cell wall synthesis of TB bacilli at the early stages of peptidoglycan synthesis. For the treatment against tuberculosis, it is classified as a second line drug.
Glycopeptides, such as the antibiotic vancomycin, inhibit both transglycosylation and transpeptidation reactions during peptidoglycan assembly. They bind to the muropeptide subunit as it is transferred out of the cell cytoplasm and inhibit subsequent polymerization reactions. Vancomycin is not effective against Gram-negative bacteria because it cannot penetrate their outer membrane. However, it has become important in clinical usage for treatment of infections by strains of Staphylococcus aureus that are resistant to virtually all other antibiotics (MRSA).
Vancomycin is a glycopeptide antibiotic used in the prophylaxis and treatment of infections caused by Gram-positive bacteria. It has traditionally been reserved as a drug of "last resort", used only after treatment with other antibiotics had failed, although the emergence of vancomycin-resistant organisms means that it is increasingly being displaced from this role by linezolid and the carbapenems.
Cell membrane inhibitors
These antibiotics disorganize the structure or inhibit the function
of bacterial membranes. The integrity of the cytoplasmic and outer
membranes is vital to bacteria, and compounds that disorganize the
membranes rapidly kill the cells. However, due to the similarities in
phospholipids in eubacterial and eucaryotic membranes, this action is
rarely specific enough to permit these compounds to be used
systemically. The only antibacterial antibiotics of clinical importance
that act by this mechanism are the polymyxins, produced
by Bacillus polymyxa. Polymyxin is effective mainly against
bacteria and is usually limited to topical usage. Polymyxins bind to
membrane phospholipids and thereby interfere with membrane function.
Polymyxin is occasionally given for urinary tract infections caused by Pseudomonas
strains that are gentamicin, carbenicillin and tobramycin resistant.
The balance between effectiveness and damage to the kidney and other
is dangerously close, and the drug should only be given under close
supervision in the hospital.
Protein synthesis inhibitors
Many therapeutically useful antibiotics owe their action to inhibition of some step in the complex process of protein synthesis. Their attack is always at one of the events occurring on the ribosome and never at the stage of amino acid activation or attachment to a particular tRNA. Most have an affinity or specificity for 70S (as opposed to 80S) ribosomes, and they achieve their selective toxicity in this manner. The most important antibiotics with this mode of action are the tetracyclines, chloramphenicol, the macrolides (e.g. erythromycin) and the aminoglycosides (e.g. streptomycin).
The aminoglycosides are products of Streptomyces species and are represented by streptomycin, kanamycin, tobramycin and gentamicin. These antibiotics exert their activity by binding to bacterial ribosomes and preventing the initiation of protein synthesis.
Streptomycin binds to 30S subunit of the bacterial ribosome,
specifically to the S12 protein which is involved in the initiation of
protein synthesis. Experimentally, streptomycin has been shown to
prevent the initiation of protein synthesis by blocking the binding of
initiator N-formylmethionine tRNA to the ribosome. It also prevents the
normal dissociation of ribosomes into their subunits, leaving them
mainly in their 70S form and preventing the formation of polysomes. The
overall effect of streptomycin seems to
be one of distorting the ribosome so that it no longer can carry out
normal functions. This evidently accounts for its antibacterial
but does not explain its bactericidal effects, which distinguishes
and other aminoglycosides from most other protein synthesis inhibitors.
Streptomycin is the first aminoglycoside antibiotic to be discovered, and was the first antibiotic to be used in treatment of tuberculosis. It was discovered in 1943, in the laboratory of Selman Waksman at Rutgers University. Waksman and his laboratory discovered several antibiotics, including actinomycin, streptomycin, and neomycin. Streptomycin is derived from the bacterium, Streptomyces griseus. Streptomycin stops bacterial growth by inhibiting protein synthesis. Specifically, it binds to the 16S rRNA of the bacterial ribosome, interfering with the binding of formyl-methionyl-tRNA to the 30S subunit. This prevents initiation of protein synthesis.
Kanamycin and tobramycin have been reported to bind to the ribosomal 30S subunit and to prevent it from joining to the 50S subunit during protein synthesis. They may have a bactericidal effect because this leads to cytoplasmic accumulation of dissociated 30S subunits, which is apparently lethal to the cells.
Aminoglycosides have been used against a wide variety of bacterial
infections caused by Gram-positive and Gram-negative bacteria.
Streptomycin has been used extensively as a primary drug in the
treatment of tuberculosis. Gentamicin is active against many
strains of Gram-positive and
Gram-negative bacteria, including some strains of Pseudomonas
aeruginosa. Kanamycin is active at low concentrations
against many Gram-positive
bacteria, including penicillin-resistant staphylococci. Gentamicin
and Tobramycin are mainstays for treatment of Pseudomonas infections.
An unfortunate side effect
of aminoglycosides has tended to restrict their usage: prolonged use is
known to impair kidney function and cause damage to the auditory nerves
leading to deafness.
The tetracyclines consist of eight related antibiotics which are all natural products of Streptomyces, although some can now be produced semisynthetically or synthetically. Tetracycline, chlortetracycline and doxycycline are the best known. The tetracyclines are broad-spectrum antibiotics with a wide range of activity against both Gram-positive and Gram-negative bacteria. Pseudomonas aeruginosa is less sensitive but is generally susceptible to tetracycline concentrations that are obtainable in the bladder. The tetracyclines act by blocking the binding of aminoacyl tRNA to the A site on the ribosome. Tetracyclines inhibit protein synthesis on isolated 70S or 80S (eucaryotic) ribosomes, and in both cases, their effect is on the small ribosomal subunit. However, most bacteria possess an active transport system for tetracycline that will allow intracellular accumulation of the antibiotic at concentrations 50 times as great as that in the medium. This greatly enhances its antibacterial effectiveness and accounts for its specificity of action, since an effective concentration cannot be accumulated in animal cells. Thus a blood level of tetracycline which is harmless to animal tissues can halt protein synthesis in invading bacteria.
The tetracyclines have a remarkably low toxicity and minimal side effects when taken by animals. The combination of their broad spectrum and low toxicity has led to their overuse and misuse by the medical community and the wide-spread development of resistance has reduced their effectiveness. Nonetheless, tetracyclines still have some important uses, such as the use of doxycycline in the treatment of Lyme disease.
Some newly discovered members of the tetracycline family (e.g.
chelocardin) have been shown to act by inserting into the bacterial
membrane, not by inhibiting protein synthesis.
The tetracycline core structure. The tetracyclines are a large family of antibiotics that were discovered as natural products of Streptomyces bacteria beginning in the late 1940s. Tetracycline sparked the development of many chemically altered antibiotics and in doing so has proved to be one of the most important discoveries made in the field of antibiotics. It is a classic "broad-spectrum antibiotic" used to treat infections caused by Gram-positive and Gram-negative bacteria and some protozoa.
Chloramphenicol is a protein synthesis inhibitor that has a
spectrum of activity but it exerts a bacteriostatic effect. It is
effective against intracellular parasites such as the rickettsiae.
Unfortunately, aplastic anemia develops in a
small proportion (1/50,000)
of patients. Chloramphenicol was originally discovered and purified
the fermentation of a Streptomyces species, but currently it is
entirely by chemical synthesis. Chloramphenicol inhibits the bacterial
peptidyl transferase, thereby preventing the growth of the polypeptide
during protein synthesis.
Chemical structure of chloramphenicol
Chloramphenicol is entirely selective for 70S ribosomes and does not affect 80S ribosomes. Its unfortunate toxicity towards the small proportion of patients who receive it is in no way related to its effect on bacterial protein synthesis. However, since mitochondria originated from procaryotic cells and have 70S ribosomes, they are subject to inhibition by some of the protein synthesis inhibitors including chloramphenicol. This likely explains the toxicity of chloramphenicol. The eucaryotic cells most likely to be inhibited by chloramphenicol are those undergoing rapid multiplication, thereby rapidly synthesizing mitochondria. Such cells include the blood forming cells of the bone marrow, the inhibition of which could present as aplastic anemia. Chloramphenicol was once a highly prescribed antibiotic and a number of deaths from anemia occurred before its use was curtailed. Now it is seldom used in human medicine except in life-threatening situations (e.g. typhoid fever).
The macrolide family of antibiotics is characterized by
structures that contain large lactone rings linked through glycoside
bonds with amino sugars. The most important members of the group are erythromycin
and oleandomycin. Erythromycin is active against most
Gram-positive bacteria, Neisseria, Legionella and Haemophilus,
but not against the Enterobacteriaceae. Macrolides inhibit
bacterial protein synthesis by binding to the 50S ribosomal subunit.
Binding inhibits elongation of the protein by peptidyl transferase or
prevents translocation of the ribosome or both. Macrolides are
bacteriostatic for most bacteria but
are cidal for a few Gram-positive bacteria.
Chemical structure of a macrolide antibiotic, erythromycin.
Azithromycin, shown above, is a subclass of macrolide antibiotics. Azithromycin is one of the world's best-selling antibiotics. It is s derived from erythromycin, but it differs chemically from erythromycin in that a methyl-substituted nitrogen atom is incorporated into the lactone ring, thus making the lactone ring 15-membered. Azithromycin is used to treat certain bacterial infections, most often bacteria causing middle ear infections, tonsillitis, throat infections, laryngitis, bronchitis, pneumonia and sinusitis. It is also effective against certain sexually transmitted diseases, such as non-gonococcal urethritis and cervicitis.
Lincomycin and clindamycin are a miscellaneous group
synthesis inhibitors with activity similar to the macrolides. Lincomycin
has activity against Gram-positive bacteria and some Gram-negative
bacteria (Neisseria, H. influenzae). Clindamycin is a
derivative of lincomycin with the same range of antimicrobial
activity, but it is considered more effective. It is frequently used as
a penicillin substitute and is effective against Gram-negative
anaerobes (e.g. Bacteroides).
Clindamycin is a lincosamide antibiotic. It is usually used to treat infections with anaerobic bacteria but can also be used to treat some protozoal diseases, such as malaria. It is a common topical treatment for acne, and can be useful against some methicillin-resistant Staphylococcus aureus (MRSA) infections. The most severe common adverse effect of clindamycin is Clostridium difficile-associated diarrhea (the most frequent cause of pseudomembranous colitis). Although this side-effect occurs with almost all antibiotics, including beta-lactam antibiotics, it is classically linked to clindamycin use.
Effects on Nucleic Acids
Some antibiotics and chemotherapeutic agents affect the synthesis of DNA or RNA, or can bind to DNA or RNA so that their messages cannot be read. Either case, of course, can block the growth of cells. The majority of these drugs are unselective, however, and affect animal cells and bacterial cells alike and therefore have no therapeutic application. Two nucleic acid synthesis inhibitors which have selective activity against procaryotes and some medical utility are the quinolones and rifamycins.Nalidixic acid is a synthetic chemotherapeutic agent that has activity mainly against Gram-negative bacteria. Nalidixic acid belongs to a group of compounds called quinolones. Nalidixic acid is a bactericidal agent that binds to the DNA gyrase enzyme (topoisomerase) which is essential for DNA replication and allows supercoils to be relaxed and reformed. Binding of the drug inhibits DNA gyrase activity.
Many of the synthetic chemotherapeutic agents (synthetic antibiotics) are competitive inhibitors of essential metabolites or growth factors that are needed in bacterial metabolism. Hence, these types of antimicrobial agents are sometimes referred to as anti-metabolites or growth factor analogs, since they are designed to specifically inhibit an essential metabolic pathway in the bacterial pathogen. At a chemical level, competitive inhibitors are structurally similar to a bacterial growth factor or metabolite, but they do not fulfill their metabolic function in the cell. Some are bacteriostatic and some are bactericidal. Their selective toxicity is based on the premise that the bacterial pathway does not occur in the host.
Sulfonamides were introduced as chemotherapeutic agents by Domagk in 1935, who showed that one of these compounds (prontosil) had the effect of curing mice with infections caused by beta-hemolytic streptococci. Chemical modifications of the compound sulfanilamide gave rise to compounds with even higher and broader antibacterial activity. The resulting sulfonamides have broadly similar antibacterial activity, but differ widely in their pharmacological actions. Bacteria which are almost always sensitive to the sulfonamides include Streptococcus pneumoniae, beta-hemolytic streptococci and E. coli. The sulfonamides have been extremely useful in the treatment of uncomplicated UTI caused by E. coli, and in the treatment of meningococcal meningitis (because they cross the blood-brain barrier).
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. Sulfonamides are structurally similar to
para aminobenzoic acid (PABA),
the substrate for the first enzyme in the THF pathway, and they
competitively inhibit that step. Trimethoprim is structurally similar
to dihydrofolate (DHF) and competitively inhibits the second step in
THF synthesis mediated by the DHF reductase. Animal cells do not
synthesize their own folic acid but obtain it in a preformed fashion as
a vitamin. Since animals do not
make folic acid, they are not affected by these drugs, which achieve
selective toxicity for bacteria on this basis.
The chemical structures of sulfanilamide and para-aminobenzoic acid (PABA). In bacteria, sulfanilamide acts as a competitive inhibitor of the enzyme dihydropteroate synthetase, DHPS, which catalyses the conversion of PABA to dihydropteroate, a key step in folate synthesis. Folate is necessary for the cell to synthesize nucleic acids (DNA and RNA), and in its absence, cells will be unable to divide. Hence, sulfanilamide and other sulfonamides exhibit a bacteriostatic rather than bactericidal effect.Three additional synthetic chemotherapeutic agents have been used in the treatment of tuberculosis: isoniazid (INH), para-aminosalicylic acid (PAS), and ethambutol. The usual strategy in the treatment of tuberculosis has been to administer a single antibiotic (historically streptomycin, but now, most commonly, rifampicin is given) in conjunction with INH and ethambutol. Since the tubercle bacillus rapidly develops resistance to the antibiotic, ethambutol and INH are given to prevent outgrowth of a resistant strain. It must also be pointed out that the tubercle bacillus rapidly develops resistance to ethambutol and INH if either drug is used alone. Ethambutol inhibits incorporation of mycolic acids into the mycobacterial cell wall. Isoniazid has been reported to inhibit mycolic acid synthesis in mycobacteria and since it is an analog of pyridoxine (Vitamin B6) it may inhibit pyridoxine-catalyzed reactions as well. Isoniazid is activated by a mycobacterial peroxidase enzyme and destroys several targets in the cell. PAS is an anti-folate, similar in activity to the sulfonamides. PAS was once a primary anti-tuberculosis drug, but now it is a secondary agent, having been largely replaced by ethambutol.