Colistin is the most commonly used drug in the group of polymyxin antibiotics, isolated from Paenibacillus polymyxa. Its chemical structure consists of cationic polypeptides. Colistin binds to lipopolysaccharides on the bacterial outer cell membrane, which leads to membrane disruption and cell death. It is a bactericidal antibiotic active against gram-negative rods and most used as a last-line antibiotic to treat multidrug resistant organisms such as . Colistin must be given intravenously and is cleared by the kidneys. It shows high rates of nephrotoxicity and neurotoxicity.
Dapsone is a synthetic antibiotic related to sulfonamides with a diphenyl sulfone core. It inhibits bacterial growth by interfering with the folate synthesis pathway. This bacteriostatic antibiotic is used for treatment of leprosy caused by and for infections in immunocompromised patients. Dapsone also shows antiparasitic activity. It is well absorbed from the gastrointestinal tract, has good tissue penetration and undergoes extensive hepatic metabolization. Adverse drug effects are common and may be serious, like haemolytic anaemia.
Fosfomycin is a phosphonic antibiotic isolated from Streptomyces fradiae. It is a derivate of phosphonic acid with a highly reactive epoxide ring. Fosfomycin is a bactericidal antibiotic which binds to cellular enzymes crucial to peptidoglycan synthesis, damaging the bacterial cell wall. It is used as a single-dose drug to treat lower urinary tract infections caused by a wide range of bacteria including including multi-drug resistant organisms. However, resistances against Fosfomycin tend to develop quickly. This antibiotic is generally well-tolerated and cleared via the urine.
Nitrofurantoin is a synthetic nitrofuran antibiotic which has been in use for many decades. It consists of a furan ring with an attached nitro group. Bacterial enzymes interact with nitrofurantoin by reducing it to highly reactive metabolites, which disrupts multiple bacterial cellular processes (e.g., DNA and protein synthesis). Nitrofurantoin is a bacteriostatic antibiotic but may be bactericidal at higher concentrations. It is used in uncomplicated urinary tract infections and is active against and . Nitrofurantoin is well absorbed and mostly cleared by the kidneys. It is known to cause gastrointestinal adverse effects.
Aztreonam is a monobactam antibiotic isolated from Chromobacterium violaceum. It has a single beta-lactam ring at its core. Like other beta-lactam antibiotics, Aztreonam disrupts bacterial cell wall integrity by binding to penicillin-binding proteins inhibiting peptidoglycan synthesis. This bactericidal antibiotic is active against many gram-negative organisms, such as . It is not active against gram-positive and anaerobic bacteria. Aztreonam cannot be administered orally because of poor gastrointestinal absorption. It is mainly cleared by the kidneys. This drug is safe for penicillin-allergic patients as there is no relevant cross-reactivity.
Chloramphenicol is an old antibiotic originally isolated from Streptomyces venezuelae, which has since been produced synthetically. It consists of a benzene ring, a nitro group, and a side chain. This antibiotic inhibits bacterial protein biosynthesis by binding to the 23S ribosomal RNA of the 50S subunit, effectively blocking the formation of peptide bonds. Chloramphenicol is a bacteriostatic antibiotic active against a broad spectrum of gram-positive and gram-negative organisms including . However, due to well-known toxic side effects (i.e. bone marrow depression, aplastic anaemia), it is not a first-line antibiotic for any infection.