insight review articlesNATURE|VOL 406|17 AUGUST 2000|www.nature.com 775Antibiotics can kill bacteria (bacteriocidal) orsometimes just nullify growth (bacteriostatic).Most antibiotics in human use asantibacterials are natural products, elaboratedby one species of microbe (bacteria or fungi)as chemical weapons, often in times of crowding, to kill offother microbes in the neighbouring microenvironment.Over the past 60–70 years most antibiotics have beendiscovered by screening of soil samples for such naturalproducts that kill bacteria, including known pathogens,first on culture plates and then in animal infections. These include penicillins and cephalosporins from fungiand a host of antibiotics from different strains of thefilamentous bacterium Streptomyces, such as streptomycin,erythromycin, tetracycline and vancomycin. Semisyntheticmodifications have produced second- and third-generation b-lactams of both the penicillin andcephalosporin classes whereas total synthesis has createdthe second-generation erythromycins — clarithromycinand azithromycin. As of the end of 1999, only thefluoroquinolones (for example, ciprofloxacin) represent atotally synthetic, significant class of antibiotic.Targets for the main classes of antibacterial drugsTo understand how antibiotics work and, concomitantly,why they stop being effective requires a brief look at the targets for the main classes of these antibacterial drugs. Assummarized in Box 1, there are three proven targets for themain antibacterial drugs: (1) bacterial cell-wall biosynthe-sis; (2) bacterial protein synthesis; and (3) bacterial DNAreplication and repair.Cell-wall biosynthesisThe layer of the bacterial cell wall that confers strength is thepeptidoglycan, a meshwork of strands of peptide and glycanthat can be covalently crosslinked (Fig. 1a). The larger thefraction of adjacent peptide strands that are connected inamide linkage by action of a family of transpeptidases, thehigher the mechanical strength to osmotic lysis. Transglyco-sylases act on the glycan strands to extend the sugar chains by incorporation of new peptidoglycan units from N-acetylglucosamine-b-1,4-N-acetylmuramyl-pen-tapeptide-pyrophosphoryl-undecaprenol (lipid II).Bifunctional enzymes containing both transpeptidase andtransglycosylase domains are the target sites for the killing ofbacteria by the b-lactam-containing penicillins andcephalosporins, which act as pseudosubstrates and acylatethe active sites of the transpeptidases (also termed penicillin-binding proteins or PBPs)5(Fig. 1b). The ring-opened, penicilloylated transpeptidases deacylate veryslowly, and so occupy the enzyme active sites, preventingnormal crosslinking of peptide chains in the peptidoglycanlayer and leaving it mechanically weak and susceptible tolysis on changes in osmotic pressure.In addition to penicillins and cephalosporins, the van-comycin family of glycopeptide antibiotics also target thepeptidoglycan layer in the cell-wall assembly. But ratherthan targeting the enzymes involved in peptide crosslinking,vancomycin ties up the peptide substrate6and thereby prevents it from reacting with either the transpeptidases orthe transglycosylases. The net effect is the same: failure tomake peptidoglycan crosslinks leads to a weaker wall thatpredisposes the treated bacteria to a killing lysis of the cell-wall layer. The cup-shaped undersurface of the vancomycinantibiotic makes five hydrogen bonds to the D-Ala-D-Aladipeptide terminus of each uncrosslinked peptidoglycanpentapeptide side chain (Fig. 1c), which accounts for thehigh affinity of the antibiotic for its target, both in partiallycrosslinked walls and in the lipid II intermediate. Because b-lactams and vancomycin work on adjacent steps — substrate and enzyme — they show synergy when used incombination.Protein synthesisThe RNA and protein machinery of the prokaryotic ribo-somes is sufficiently distinct from the analogous eukaryoticmachinery that there are many inhibitors of protein synthe-sis, targeting different steps in ribosome action, with selective antibacterial action. These include such importantantibiotics as the macrolides of the erythromycin class7, thetetracyclines8(which are products of the aromatic polyke-tide biosynthetic pathways) and the aminoglycosides9(ofwhich streptomycin was the founding member, supplantednow by later synthetic variants such as kanamycin) (Fig. 2a).Given the large number of molecular steps involved in initi-ation, elongation and termination of protein assembly bythe ribosome, it is not surprising that there would be manysteps of binding or catalysis that could be interdicted bythese and many other classes of protein-synthesis inhibitors.This multiplicity also indicates that protein synthesis willprovide a multifaceted target for new antibiotics and this isthe mechanism for the action of oxazolidinones10, one ofMolecular mechanisms that confer antibacterial drug resistanceChristopher WalshBiological Chemistry and Molecular Pharmacology Department, Harvard Medical School, Boston, Massachusetts 02115, USAAntibiotics — compounds that are literally ‘against life’ — are typically antibacterial drugs, interfering withsome structure or process that is essential to bacterial growth or survival without harm to the eukaryotic hostharbouring the infecting bacteria. We live in an era when antibiotic resistance has spread at an alarmingrate1–4and when dire predictions concerning the lack of effective antibacterial drugs occur with increasingfrequency. In this context it is apposite to ask a few simple questions about these life-saving molecules. Whatare antibiotics? Where do they come from? How do they work? Why do they stop being effective? How do wefind new antibiotics? And can we slow down the development of antibiotic-resistant superbugs?© 2000 Macmillan Magazines Ltdinsight review articles776 NATURE|VOL 406|17 AUGUST 2000|www.nature.comwhich has been approved in the United States in the first quarter of 2000.DNA replication and repairThe fluoroquinolones, such as ciprofloxacin (Fig. 2b), are syntheticantibiotic structures that kill bacteria by targeting the enzyme DNAgyrase11(Box 1), the enzyme responsible for uncoiling the inter-twined circles of double-stranded bacterial DNA that arise after eachround of DNA replication. DNA topoisomerases are classified as typeI or type II according to whether transient single-strand breaks (typeI) or transient double-strand breaks (type II) are made in the