• Sulfonamides

  Sulfonamides: Audio Overview

  • Sulfonamides (sulfa drugs) are a class of synthetic antimicrobial agents that marked the beginning of modern antibiotic therapy in the 1930s.¹ 

    • These were the first effective systemic antibacterial drugs, derived from the red dye prontosil (sulfanilamide), and these agents represented as dramatic improvement in treatment of infections prior to the availability of penicillins.

      • Sulfonamides are analogues of p-aminobenzoic acid (PABA), a substrate bacteria require for folic acid synthesis.¹  

        • By mimicking PABA, sulfonamides inhibit folate synthesis in microbes.²

          •  Although their usage has declined due to widespread resistance and the availability of other antibiotics, sulfonamides – particularly in combination with trimethoprim – remain important agents for certain infections.

        Sulfonamide Structures²

        Generic tertiary sulfonamide	Sulfamethazine (SMZ)	Sulfadiazine (SDZ)

   

  • All antimicrobial sulfonamides are synthetic derivatives of the parent compound, sulfanilamide.

    • Sulfonamides are chemically defined by a specific organosulfur functional group with the general structure R-S(=O)₂ , where a sulfonyl group (O=S=O) is directly linked to an amine group (-NH₂).

      • This core structure is relatively rigid, a property that contributes to the crystalline nature of these compounds.

        • Pharmacological and physical characteristics of individual sulfonamide agents are determined by the various chemical substitutions made on this fundamental nucleus.³ 

    • Distinguishing between antimicrobial sulfonamides and non-antibiotic drugs that also contain a sulfonamide functional group, such as certain diuretics (e.g., hydrochlorothiazide), hypoglycemic agents (e.g., sulfonylureas), and anti-inflammatory drugs (e.g., celecoxib) is important.

      • Key structural feature implicated in many of the severe hypersensitivity reactions to sulfonamide antibiotics is the arylamine group at the N4 position of the benzene ring.

        • This group is absent in most non-antibiotic sulfonamides, meaning that a history of allergy to a sulfonamide antibiotic does not typically predict an allergic reaction to a non-antibiotic sulfonamide.³

  • Sulfonamide Mechanism of Action

    • Attribution: Figure 1 from reference 4: Mechanism of action of sulfonamides. Hassanein M Sulfonamides: far from obsolete. Int J Contemp Pediatr. 2019 November;6(6): 2740-2745. https://www.researchgate.net/publication/336704352_Sulfonamides_far_from_obsolete

     

    • Sulfonamides are bacteriostatic agents that inhibit bacterial growth by interfering with folic acid synthesis.

      • The bacteriostatic nature implies that adequate host immune function is important for clearing the infection after sulfonamides suppress bacterial proliferation.

    • As structural analogues of PABA, these agents and competitively inhibit dihydropteroate synthase (DHPS), which normally converts PABA into dihydrofolic acid.^5,1 

      • This enzyme catalyzes the condensation of 6-hydroxymethyl-7,8-dihydropterin pyrophosphate with para-aminobenzoic acid (PABA) to form dihydropteroate, an essential precursor for bacterial DNA synthesis.

        • As structural analogs of PABA, sulfonamides compete for the enzyme's active site with higher affinity than the natural substrate, effectively blocking bacterial folate production.7

      • By blocking this first step in folate production, sulfonamides prevent bacteria from synthesizing folic acid cofactors needed for DNA and RNA building blocks, thereby halting cell growth and division. ²  Human cells are not harmed by sulfonamides as folic acid synthesis inhibitors because mammals do not synthesize folic acid from PABA. Therefore, the selective toxicity stems from a fundamental difference in folate metabolism: bacteria must synthesize folate de novo, while humans obtain it through dietary sources.⁶

        • This biochemical distinction allows sulfonamides to kill bacteria without significantly affecting human cells, providing an excellent therapeutic index when used appropriately.

    • To summarize then:

      • 1. Bacteria that cannot utilize preformed folate must synthesize it de novo from PABA.

      • 2. Sulfonamides, as structural analogues of PABA, act as competitive inhibitors of the bacterial enzyme dihydropteroate synthase (DHPS).⁸

      • 3. By occupying the active site of DHPS, sulfonamides block the incorporation of PABA into dihydropteroic acid.

        • This action prevents the synthesis of dihydrofolic acid and, subsequently, its active form, tetrahydrofolic acid (THF).

      • 4. THF is a vital one-carbon donor required for the synthesis of essential building blocks, including purines (for RNA and DNA), thymidine (for DNA), and certain amino acids.

        • THF depletion halts bacterial DNA replication and protein synthesis, leading to the cessation of growth.⁸

  • Sulfonamides and dihydrofolate reductase (DHFR) inhibitors, e.g. trimethoprim

    • Sulfonamides are often used in combination with dihydrofolate reductase (DHFR) inhibitors such as trimethoprim (for bacterial infections) or pyrimethamine (for protozoal infections) to produce a synergistic, sequential blockade of folic acid synthesis.

      • By inhibiting successive steps in the pathway (sulfonamides block dihydropteroate synthase and trimethoprim or pyrimethamine block dihydrofolate reductase – see figure above), the drugs together prevent the conversion of PABA to tetrahydrofolate, which is the active folate form required for synthesizing nucleic acids.⁹

      • Inhibition associated with this two-drug combination leads to a maximal antibacterial effect that is often bactericidal, even though each agent alone is bacteriostatic.¹⁹ 

        • The classic combination is trimethoprim-sulfamethoxazole (TMP-SMX), also known as co-trimoxazole, usually formulated in a 1:5 ratio (TMP:SMX). TMP-SMX exhibits true antimicrobial synergism against a broad range of organisms and has become a staple therapy for many infections.¹⁰

        • Trimethoprim selectively inhibits bacterial DHFR with far greater affinity (by a factor of ~50,000 to 100,000) than the human enzyme, accounting for its efficacy and relative safety in humans.

        • When paired with a sulfonamide, the combination not only increases potency but also reduces the likelihood of resistance development: bacteria must acquire mutations in two separate enzymes/pathways to overcome the blockade.¹¹

    • Clinically, TMP-SMX is a broad-spectrum, bactericidal combination used for both treatment and prophylaxis of various infections (detailed under Therapeutic Uses).

      • Another example of synergism is sulfadiazine + pyrimethamine, which is the first-line therapy for toxoplasmosis, leveraging the same sequential folate inhibition principle in protozoa.

        • This combination (plus leucovorin rescue) is highly effective at treating Toxoplasma encephalitis.¹²

          • Toxoplasma gondii : the protozoan parasite infecting warm-blooded animals including humans and is the causative agent of toxoplasmosis

            "The only known definitive hosts for Toxoplasma gondii are members of family Felidae (domestic cats and their relatives). "Unsporulated oocysts are shed in the cat’s feces (1). "Although oocysts are usually only shed for 1–3 weeks, large numbers may be shed. "Oocysts take 1–5 days to sporulate in the environment and become infective. "Intermediate hosts in nature (including birds and rodents) become infected after ingesting soil, water or plant material contaminated with oocysts (2). "Oocysts transform into tachyzoites shortly after ingestion. These tachyzoites localize in neural and muscle tissue and develop into tissue cyst bradyzoites (3). "Cats become infected after consuming intermediate hosts harboring tissue cysts (4). Cats may also become infected directly by ingestion of sporulated oocysts. Animals bred for human consumption and wild game may also become infected with tissue cysts after ingestion of sporulated oocysts in the environment (5). Humans can become infected by any of several routes: Eating undercooked meat of animals harboring tissue cysts (6). Consuming food or water contaminated with cat feces or by contaminated environmental samples (such as fecal-contaminated soil or changing the litter box of a pet cat) (7). Blood transfusion or organ transplantation (8). Transplacentally from mother to fetus (9). "In the human host, the parasites form tissue cysts, most commonly in skeletal muscle, myocardium, brain, and eyes; these cysts may remain throughout the life of the host. Diagnosis is usually achieved by serology, although tissue cysts may be observed in stained biopsy specimens (10) . Diagnosis of congenital infections can be achieved by detecting T. gondii DNA in amniotic fluid using molecular methods such as PCR (11)." Toxoplasma gondii cyst brain tissue (hematoxylin and eosin-stained) Toxoplasma gondii cyst (hematoxylin and eosin-stained) Attribution: Toxoplasmosis. DPDx - Laboratory Identification of Parasites of Public Health Concern. CDC. Parasite biology. Toxoplasma gondii. https://www.cdc.gov/dpdx/toxoplasmosis/index.html

      • Similarly, sulfadoxine + pyrimethamine (Fansidar) has been used for Plasmodium falciparum malaria treatment and intermittent prophylaxis (though resistance has limited its use).¹³

  • Antimicrobial Spectrum of Activity

    • Sulfonamides alone have a broad in vitro spectrum, inhibiting many Gram-positive and Gram-negative bacteria, as well as certain protozoa (e.g. Plasmodium, Toxoplasma) and fungi like Pneumocystis jirovecii.^14,19  

    • Classical sulfonamides (e.g. sulfisoxazole, sulfamethoxazole) historically showed activity against streptococci, staphylococci, Haemophilus influenzae, Escherichia coli and other Enterobacterales, Nocardia, Chlamydia trachomatis, Cyclospora/Isospora, and others.^19,14 

    • In combination with trimethoprim (TMP-SMX), the spectrum is enhanced and often bactericidal, covering organisms such as:

      • Staphylococcus aureus, including many community-acquired MRSA strains (TMP-SMX is one of the oral agents active against MRSA skin infections).¹⁵

      • Streptococcus pneumoniae with the caveat that resistance is common as many strains are now resistant.^16,17

        • Group A Streptococcus (S. pyogenes) is not reliably killed by sulfonamides¹⁸.

          • TMP-SMX is not recommended group A streptococcal pharyngitis, (a.k.a. strep throat)as it fails to eradicate the bacteria or prevent rheumatic fever complications

      • Enterobacterales (e.g. E. coli, Klebsiella, Enterobacter, Proteus, Shigella, Salmonella): variable activity with many resistant.¹⁹ 

        • TMP-SMX is active against many E. coli (hence its use in UTIs), Shigella, and some Salmonella, but resistance in these families is now widespread.²⁰

      • Haemophilus influenzae may be susceptible. In one report the rate of susceptibility was 36.4%.²¹

      • Neisseria species – many strains of N. meningitidis and N. gonorrhoeae were sulfonamide-susceptible in the past, but resistance is now high, limiting use. (Sulfonamides are no longer recommended for gonorrhea, and meningococcal prophylaxis with sulfa has been replaced by other agents due to resistance.)^22,23  

      • Anaerobic bacteria are not inhibited by sulfonamides to a useful extent.¹⁹ 

      • Pseudomonas aeruginosa is intrinsically resistant.¹⁹

      • Enterococci are also generally resistant because they can utilize exogenous folate and bypass the sulfonamide mechanism.¹⁹ 

      • Treponema pallidum (syphilis) is not treated by sulfonamides.¹⁹ 

        • In summary, the in vitro spectrum is broad, but actual clinical utility is narrower today due to resistance.

  • Bacterial Resistance Mechanisms

    • Resistance to sulfonamides is widespread and has markedly limited their stand-alone use.

      • After extensive of sulfonamide use in both humans and agriculture, many bacterial strains have acquired one or more resistance mechanisms.

      • Key sulfonamide resistance mechanisms include:

        • Altered target enzyme: Bacteria can acquire mutations in the gene for dihydropteroate synthase (folP gene), producing an enzyme with lower affinity for sulfonamides.

          • This common mechanism (e.g. seen in E. coli and other Enterobacterales) may be plasmid-mediated.

          • Some Neisseria spp. have even acquired new folP gene variants via horizontal transfer, raising concern for broader dissemination of resistance.^24,25

        • Overproduction of PABA (Para-amino benzoic acid): By synthesizing an excess of the natural substrate PABA, the bacteria can out-compete the drug at the enzyme site.²⁶ 

          • This mechanism has been noted in resistant Staphylococcus aureus and certain Neisseria gonorrhoeae strains, among others.²⁷

          • The high PABA levels make it more likely that the natural substrate will bind instead of the competing sulfonamide.

        • Decreased permeability or active efflux: Some bacteria reduce uptake of sulfonamide or increase efflux pumps to remove it, lowering the intracellular drug concentration.

          • This mechanism is less specific but contributes to resistance in various Gram-negative organisms.²⁸

            • "Location of drug efflux pumps and pathways of drug influx and efflux across the outer membrane in intermembrane in gram-negative bacteria"²⁸

              Attribution: corresponds to figure 1 from reference 28. MFP:  Membrane Fusion  Protein OMP:  Outer Membrane Protein Li X Plesiat P Nikaido H The Challenge of Efflux-Mediated Antibiotic Resistance in Gram-Negative Bacteria. Clinical Microbiology Reviews. Volume 28, Number 2. 2015. https://journals.asm.org/doi/10.1128/cmr.00117-14 (reference 28)

        • Alternative metabolic pathway: A less common mechanism is bacteria acquiring the ability to use exogenous folic acid or an alternative pathway for folate metabolism, rendering them less dependent on the PABA → folate synthesis route. (For example, enterococci inherently use preformed folate and thus are intrinsically resistant to sulfonamides.)⁴

        • Plasmid-mediated resistance genes: Plasmids can carry genes encoding a drug-resistant dihydropteroate synthase enzyme.

          • Such plasmids (e.g. the sul genes like sul1, sul2, sul3) are now prevalent in many E. coli and other Gram-negatives, often alongside other resistance genes.

            • These plasmid-borne enzymes function normally but do not bind sulfonamides effectively.

            • Plasmid transfer between bacteria (even across species) has accelerated the spread of sulfonamide resistance.²⁵

        • More about Resistance and Sulfonamides:

          • The prevalence of sulfonamide resistance, particularly among Gram-negative pathogens like E. coli, is a major clinical challenge.

            • Resistance rates frequently exceed the 20% threshold at which the Infectious Diseases Society of America (IDSA) recommends against the empiric use of TMP/SMX for uncomplicated urinary tract infections.

            • Surveillance data reveal significant global and regional variation.

              • For example, a 2020 meta-analysis in Iran reported a 62% resistance rate for E. coli to cotrimoxazole³¹, while 2023 data from Wales showed a 33.8% resistance rate in E. coli bloodstream isolates.³⁰  

              • The World Health Organization (WHO) noted that in 2020, approximately one in five urinary tract infections caused by E. coli globally demonstrated reduced susceptibility to cotrimoxazole.²⁹

          • A critical factor driving the persistence of high-level sulfonamide resistance is the phenomenon of genetic linkage and co-selection.

            • The sul genes are often located on the same mobile genetic elements (plasmids and integrons) that carry genes conferring resistance to other antibiotic classes, such as tetracyclines (tet genes) and aminoglycosides (strA/B for streptomycin). ³²

          • This genetic linkage means that the use of an unrelated antibiotic can inadvertently select for bacteria carrying the entire multi-drug resistance plasmid. In this scenario, sulfonamide resistance is essentially a "hitchhiker" that is maintained in the bacterial population even in the absence of direct selection pressure from sulfonamides themselves.

            • This dynamic helps explain why resistance rates have remained stubbornly high for decades, even in regions where sulfonamide consumption has declined ³³ , and it underscores the complexity of combating antimicrobial resistance, which requires a holistic view of linked genetic determinants rather than a narrow focus on individual drug-pathogen interactions.

July, 2025