4/ CHAPTER 1 leading to the loss of porins can reduce antibiotic pene- Ribosomal resistance to gentamicin, tobramycin, and tration and lead to antibiotic resistance amikacin is less common because these aminoglyco PRODUCTION OF EFFLUX PUMPS ides have several binding sites on the bacterial ribo- some and require multiple bacterial Transposons have been found that encode for an their binding is blocked el tetracycline out of bacteria. Active efflux of antibi CONCLUSIONS has been observed in m bacteria,and this mechanism is used to resist Bacteria can readily transfer antibiotic resistance acrolide, and fluoroquinolone Bacteria have multiple mechanisms to destroy antibiotic treatment. S. aureus, S. epidermidis, otics, lower the antibiotic concentration, and interfere Pyogenes, group B streptococci, and S. pneumoniae with antibiotic binding. Under the selective pressures of also can utilize energy-dependent efflux pumps to prolonged antibiotic treatment, the qu stion Is not resist antibiotics whether, but when resistant bacteria will take over Modification of the Antibiotic Target ALTERATIONS OF CELL WALL PRECURSORS ANTI-INFECTIVE AGENT DOSING Vancomycin and teicoplanin binding requires that D- The characteristics that need to be considered when alanine-D-alanine be at the end of the peptidoglycan cell administering antibiotics include absorption(when deal- strains of Enterococcus faecium and Enterococcus faecalis os with oral antibiotics), volume of distribution, metab- wall precursors of gram-positive bacteria. Resistant m and excretion. These factors determine the dose of contain the vanA plasmid, which encodes a protein that each drug and the time interval of administration. To synthesizes D-alanine-D-lactate instead of D-alanine-D- effectively clear a bacterial infection, serum levels of the alanine at the end of the peptidoglycan precursor Loss antibiotic need to be maintained above the minimul of the terminal D-alanine markedly reduces vancomy inhibitory concentration(MIC) for a significant period. and teicoplanin binding, allowing the mutant bac- For each pathogen, the MIC is determined by serially terium to survive and grow in the presence of these diluting the antibiotic into liquid medium containing 101 bacteria per milliliter. Inoculated tubes are incubated overnight until broth without added antibiotic ha CHANGES IN TARGET ENZYMES become cloudy or turbid as a result of bacterial growth ial cell wall. Penicillin-resistant S. pneumoniae demon- clear-constitutes the MIC(Figure 1.2). Automated strate decreased numbers of PBPs or PBPs that bind analyzers can now quickly determine, for individual penicillin with lower affinity, or both. Decreased peni- pathogens, the MICs for multiple antibiotics, and these cillin binding reduces the ability of the antibiotic to kill data serve to guide the physician's choice of antibiotics the targeted bacteria The mean bactericidal concentration(MBC) is deter The basis for antibiotic resistance in MRSA is pro- mined by taking each clear tube and inoculating a plate duction of a low affinity PBP encoded by the meca of solid medium with the solution. Plates are then incu gene.Mutations in the target enzymes dihydropteroate bated to allow colonies to form. The lowest concentra- amide and trimethoprim resistance respectively. Single is, no colonies on solid medium-represents me MB c synthetase and dihydrofolate reductase cause sulfon- tion of antibiotic that blocks all growth of bacteria--th amino-acid mutations that alter dNa gyrase function Successful cure of an infection depends on multiple can result in resistance to fluoroquinolones host factors in addition to serum antibiotic concentration ALTERATIONS IN RIBOSOMAL BINDING SITE However, investigators have attempted to predict succe ful treatment by plotting serum antibiotic levels against Tetracyclines, macrolides, lincosamides, and amino- time. Three parameters can be assessed(Figure 1.3):time glycosides all act by binding to and disrupting the above the MIC (T>MIC), ratio of the peak antibiotic con of individual antibiotics later in this chapter). A num- area under the curve(AUC)to the MIC (AUCMC% nction of bacterial ribosomes(see the descriptions centration to the MIC(Cmax/MIC), and the ratio ber of resistance genes encode for enzymes that Cure rates for B-lactam antibiotics are maximized demethylate adenine residues on bacterial ribosomal maintaining serum levels above the MIC for >50% of RNA, inhibiting antibiotic binding to the ribosome. the time. Peak antibiotic concentrations are of lessleading to the loss of porins can reduce antibiotic pene￾tration and lead to antibiotic resistance. PRODUCTION OF EFFLUX PUMPS Transposons have been found that encode for an energy-dependent pump that can actively pump tetracycline out of bacteria. Active efflux of antibiotics has been observed in many enteric gram-negative bacteria, and this mechanism is used to resist tetracycline, macrolide, and fluoroquinolone antibiotic treatment. S. aureus, S. epidermidis, S. pyogenes, group B streptococci, and S. pneumoniae also can utilize energy-dependent efflux pumps to resist antibiotics. Modification of the Antibiotic Target ALTERATIONS OF CELL WALL PRECURSORS Alteration of cell wall precursors is the basis for VRE. Vancomycin and teicoplanin binding requires that D￾alanine-D-alanine be at the end of the peptidoglycan cell wall precursors of gram-positive bacteria. Resistant strains of Enterococcus faecium and Enterococcus faecalis contain the vanA plasmid, which encodes a protein that synthesizes D-alanine-D-lactate instead of D-alanine-D￾alanine at the end of the peptidoglycan precursor. Loss of the terminal D-alanine markedly reduces vancomycin and teicoplanin binding, allowing the mutant bac￾terium to survive and grow in the presence of these antibiotics. CHANGES IN TARGET ENZYMES Penicillins and cephalosporins bind to specific proteins called penicillin-binding proteins (PBPs) in the bacter￾ial cell wall. Penicillin-resistant S. pneumoniae demon￾strate decreased numbers of PBPs or PBPs that bind penicillin with lower affinity, or both. Decreased peni￾cillin binding reduces the ability of the antibiotic to kill the targeted bacteria. The basis for antibiotic resistance in MRSA is pro￾duction of a low affinity PBP encoded by the mecA gene. Mutations in the target enzymes dihydropteroate synthetase and dihydrofolate reductase cause sulfon￾amide and trimethoprim resistance respectively. Single amino-acid mutations that alter DNA gyrase function can result in resistance to fluoroquinolones. ALTERATIONS IN RIBOSOMAL BINDING SITE Tetracyclines, macrolides, lincosamides, and amino￾glycosides all act by binding to and disrupting the function of bacterial ribosomes (see the descriptions of individual antibiotics later in this chapter). A num￾ber of resistance genes encode for enzymes that demethylate adenine residues on bacterial ribosomal RNA, inhibiting antibiotic binding to the ribosome. Ribosomal resistance to gentamicin, tobramycin, and amikacin is less common because these aminoglyco￾sides have several binding sites on the bacterial ribo￾some and require multiple bacterial mutations before their binding is blocked. CONCLUSIONS Bacteria can readily transfer antibiotic resistance genes. Bacteria have multiple mechanisms to destroy antibi￾otics, lower the antibiotic concentration, and interfere with antibiotic binding. Under the selective pressures of prolonged antibiotic treatment, the question is not whether, but when resistant bacteria will take over. ■ ANTI-INFECTIVE AGENT DOSING The characteristics that need to be considered when administering antibiotics include absorption (when deal￾ing with oral antibiotics), volume of distribution, metab￾olism, and excretion. These factors determine the dose of each drug and the time interval of administration. To effectively clear a bacterial infection, serum levels of the antibiotic need to be maintained above the minimum inhibitory concentration (MIC) for a significant period. For each pathogen, the MIC is determined by serially diluting the antibiotic into liquid medium containing 104 bacteria per milliliter. Inoculated tubes are incubated overnight until broth without added antibiotic has become cloudy or turbid as a result of bacterial growth. The lowest concentration of antibiotic that prevents active bacterial growth—that is, the liquid media remains clear—constitutes the MIC (Figure 1.2). Automated analyzers can now quickly determine, for individual pathogens, the MICs for multiple antibiotics, and these data serve to guide the physician’s choice of antibiotics. The mean bactericidal concentration (MBC) is deter￾mined by taking each clear tube and inoculating a plate of solid medium with the solution. Plates are then incu￾bated to allow colonies to form. The lowest concentra￾tion of antibiotic that blocks all growth of bacteria—that is, no colonies on solid medium—represents the MBC. Successful cure of an infection depends on multiple host factors in addition to serum antibiotic concentration. However, investigators have attempted to predict success￾ful treatment by plotting serum antibiotic levels against time. Three parameters can be assessed (Figure 1.3): time above the MIC (T>MIC), ratio of the peak antibiotic con￾centration to the MIC (Cmax/MIC), and the ratio of the area under the curve (AUC) to the MIC (AUC/MIC). Cure rates for
-lactam antibiotics are maximized by maintaining serum levels above the MIC for >50% of the time. Peak antibiotic concentrations are of less 4 / CHAPTER 1