Mechanism of Action

β-Lactam antibiotics are similar in that each contains a β-lactam ring in addition to other pharmacologically active side chains stemming from this central structure. Side chain manipulation is largely responsible for both spectrum of activity and stability against enzymatic degradation, pharmacokinetics, and adverse effects. β-Lactam antibiotics inhibit bacterial wall synthesis by binding to penicillin-binding proteins (PBPs). These PBPs are transpeptidases, carboxypeptidases, and endopeptidases involved in the structure and function of the cell wall.^1,2 The cell wall is made up of a peptidoglycan consisting of long polysaccharide chains of N-acetylglucosamine and N-acetylmuramic acid cross-linked by shorter peptide chains.³ There are three stages to peptidoglycan formation, including accumulation of peptidoglycan precursors in the cytoplasm, linkage of precursor products in a long polymer, followed by cross-linking by transpeptidation. β-Lactams inhibit this final transpeptidation step.

Transpeptidation cross-links adjacent sugar chains via their pentapeptides. Peptidoglycan transglycosylase and D-alanyl-D-alanine transpeptidase are responsible for this activity. β-Lactams inhibit D-alanyl-D-alanine transpeptidase activity by acetylation, forming stable esters with the open lactam ring attached to the enzyme’s active site. The propensity of D-alanyl-D-alanine trans- and carboxypeptidase to form stable bonds with β-lactams provides these enzymes with their collective name of penicillin-binding proteins (PBPs).³ PBPs lie on the outer side of the cytoplasmic membrane in gram-positive bacteria and are shielded only by the peptidoglycan and outer capsule. In gram-negative bacteria, most β-lactams must cross the outer membrane via porin channels to reach PBPs. Entry through the porin channels is determined by size, charge, and hydrophobicity.

Bacterial killing and clinical efficacy for β-lactam antibiotics is associated with the percent of time during the dosing interval that the drug concentration is above the minimum inhibitory concentration (MIC). Maximal killing occurs when the antibiotic concentration is maintained at 4 to 5 times the MIC. Carbapenems have faster killing rates than penicillins; cephalosporins have the slowest killing rates of the β-lactam class.⁴ Therefore, percentages for time above the MIC required for bacterial killing are highest for the cephalosporins and lowest for the carbapenems.⁴ Near-maximal bactericidal effect is typically observed when the free drug serum concentration exceeds the MIC for 60% to 70% of the dosing interval for cephalosporins, 50% for penicillins, and 40% for carbapenems. In vitro data in an experimental Pseudomonas aeruginosa aortic endocarditis model in rabbits suggested that bacterial resistance to β-lactams may develop if the antibiotic concentration falls below the MIC for more than half the dosing interval.⁴

All intravenous (IV) β-lactam antibiotics are recommended to be given in several daily intervals. Administering a β-lactam agent as an infusion for longer than the conventional 30- to 60-minute infusion produces a lower peak concentration of the drug while maintaining a serum drug concentration in excess of the pathogen MIC for a longer period of time. Continuous infusion of these agents is also an attractive administration method to maintain serum drug concentrations above the MIC. Several clinical trials have validated the use of extended infusion and continuous infusion β-lactams in the critically ill, and institutions may institute these administration methods to improve outcomes and reduce daily drug costs.^5–10

β-Lactams are commonly used in antibiotic combinations that may include an aminoglycoside, a fluoroquinolone, a macrolide, or another β-lactam. Combination therapy is used empirically to broaden the spectrum of activity or minimize the likelihood of resistance. In documented infection with a known organism, combination therapy may be used to provide synergistic bacterial killing in an attempt to rapidly and thoroughly eradicate the pathogen. For combinations of aminoglycosides and β-lactams, there are ample in vitro data to substantiate the potential synergistic bactericidal activity of the drugs in combination. These data are not clear for β-lactam/fluoroquinolone combinations, and there are theoretical concerns about antagonistic interactions with this combination as well as combinations of two β-lactam agents.

Mechanisms of Resistance

Bacteria resist the cytotoxic activity of the β-lactams by modifying the normal PBPs, bypassing the normal PBPs, reducing the permeability of drug through the outer membrane (gram-negative bacteria), actively removing drug from the cell through the efflux pump mechanism, and producing β-lactamases. PBP modification and bypassing of normal PBPs are the most important mechanisms of resistance in gram-positive cocci, but β-lactamases are important mechanisms of antibiotic resistance in gram-negative bacteria.¹¹

Alteration of PBPs, including decreased expression of PBPs and structural modifications to the PBPs to decrease antibiotic binding affinity, are seen in both gram-positive and gram-negative bacteria.¹¹ In gram-positive bacteria, altered PBPs occur commonly in Streptococcus pneumoniae, Enterococcus faecium, and Staphylococcus aureus. Genes encoding these PBP changes in S. pneumoniae contain segments from several different organisms, including the viridans streptococci.¹² In S. aureus and E. faecium, novel PBPs may be inducible through exposure to certain antibiotics.^13,14 These novel PBPs have a low affinity for β-lactam antibiotics. PBP alterations are best illustrated in methicillin-resistant S. aureus (MRSA). Methicillin resistance occurs through the actions of the mecA gene that encodes PBP2′ (PBP2a). MRSA produces PBP2′ as a fifth PBP in addition to the four PBPs found in all S. aureus strains.¹⁵ β-Lactam antibiotics have very low affinity for PBP2′, so the enzyme’s function continues even in the presence of β-lactams.

Gram-negative bacteria, including Neisseria meningitides, Haemophilus influenzae, and Escherichia coli, also produce altered PBPs.^11,16–19 Imipenem resistance due to altered PBPs has been reported in P. aeruginosa, Acinetobacter baumannii, and Proteus mirabilis, although this PBP alteration is not the primary mechanism responsible for most imipenem resistance.^20–22

β-Lactamase production is largely responsible for β-lactam antibiotic resistance among gram-negative bacteria in the critical care setting. β-Lactamase hydrolyzes the β-lactam ring structure within the antibiotic molecule, rendering the drug inactive. Most β-lactamases function by a serine ester hydrolysis mechanism, but a few use a zinc ion to attack the β-lactam ring.¹¹ β-Lactamase can be chromosomal (inherent within the chromosome of the organism) or can be encoded by plasmids or transposons, which are mobile genetic elements that can carry genes for resistance mechanisms. β-Lactamase production may be constitutive or inducible, and β-lactam antibiotics vary in their ability to induce β-lactamase production.^23,24 Penicillin G, ampicillin, cefoxitin, imipenem, clavulanate, and first-generation cephalosporins are strong β-lactamase inducers.²⁴ Third-generation cephalosporins, ureidopenicillins, aztreonam, and semisynthetic penicillinase-stable penicillins are weak β-lactamase inducers.²⁴

Some measure of β-lactamase stability can be achieved through addition to the β-lactam ring of a substituent that hinders hydrolysis.²⁵ For example, the semisynthetic penicillinase-stable drugs such as oxacillin and nafcillin remain active against methicillin-susceptible S. aureus because of this ring structure manipulation. β-Lactamase stability has been difficult to achieve in compounds with activity against gram-negative bacteria and may be due to the periplasmic location of β-lactamase in the gram-negative cell structure.¹¹ Antibiotics including the β-lactams have difficulty accessing the gram-negative cell wall owing to the presence of an outer membrane. Porins within the membrane permit limited access through to the peptidoglycan layer of the cell, but the periplasmic space between the membrane and peptidoglycan layer allows β-lactamase to overwhelm the limited concentrations of drug that enter.

Third-generation cephalosporins have activity against β-lactamase-producing Enterobacteriaceae because they do not induce enzyme synthesis. However, these drugs may select spontaneous “derepressed” mutants that constitutively produce β-lactamase.¹¹ Emergence of derepressed mutants of Enterobacter spp. during third-generation cephalosporin therapy may be significant, particularly in pneumonia and bacteremia.²⁶ Through this selective pressure, organisms have developed that overproduce their chromosomal AmpC (class C) β-lactamase.²⁷ This type of β-lactamase is broad spectrum and inactivates most cephalosporins and aztreonam. AmpC resistance has been demonstrated in many clinically important gram-negative bacteria, including Acinetobacter spp., Citrobacter freundii, Enterobacter spp., E. coli, Morganella morganii, P. aeruginosa, and Serratia marcescens.^26,27 AmpC β-lactamase is not inhibited by β-lactamase inhibitors such as clavulanic acid, sulbactam, or tazobactam.²⁷ Unfortunately, these chromosomal AmpC β-lactamases have been found on plasmids worldwide, suggesting that this broad-spectrum class of enzymes may be spread much more readily in clinical settings.²⁷

Enterobacter spp. are intrinsically resistant to aminopenicillins, cefazolin, and cefoxitin due to production of constitutive chromosomal AmpC β-lactamases, which hydrolyze third-generation or expanded spectrum cephalosporins, and are resistant to inhibition by clavulanate or other β-lactamase inhibitors.²⁶ β-Lactam antibiotic exposure drives AmpC-mediated resistance, leading to development of resistance to third-generation cephalosporins and mutations that may result in permanent enzyme hyperproduction. Exposure of Enterobacter organisms to third-generation cephalosporins may select for mutant strains associated with hyperproduction of AmpC β-lactamase.²⁶

Other plasmid-mediated β-lactamases with more limited hydrolytic capacity have been found in Klebsiella pneumoniae, E. coli, Enterobacter spp., and other common Enterobacteriaceae. These so-called extended-spectrum β-lactamases (ESBL) are active against the oxyiminocephalosporins and aztreonam but not 7-α-methoxycephalosporins (cefoxitin, cefotetan) and are blocked by clavulanic acid, sulbactam, and tazobactam.²⁸ There are numerous reports of outbreaks of ESBL-producing Klebsiella and Enterobacter infections in ICUs.^28,29–35 Most organisms producing AmpC and ESBL enzymes remain susceptible to carbapenems such as imipenem. However, β-lactamase that uses zinc as an active site for β-lactam hydrolysis is able to hydrolyze carbapenems along with every other β-lactam presently available.³⁶ Carbapenemases found in Enterobacteriaceae can be either metallo-β-lactamases, expanded-spectrum oxacillinases, or clavulanic acid–inhibited β-lactamases. The most concerning carbapenemases prevalent worldwide today are the K. pneumoniae carbapenemase (KPC) enzymes, a group of mostly plasmid-encoded enzymes from K. pneumoniae. Klebsiella pneumoniae carbapenemase enzymes hydrolyze all β-lactam antibiotics including penicillins, cephalosporins, and aztreonam, although cephamycins and ceftazidime are weakly hydrolyzed.³⁷ The KPC enzymes may be mistaken for extended-spectrum β-lactamases (ESBLs), since they also hydrolyze expanded-spectrum cephalosporins, but unlike extended-spectrum β-lactamases, they also weakly hydrolyze carbapenems. The hydrolytic activity of KPC enzymes is not sufficient to produce resistance against carbapenems, but increases in MICs can occur.

To achieve full resistance to carbapenems, organisms must also exhibit impaired outer-membrane permeability. Clavulanic acid and tazobactam are not clinically effective against carbapenemase. Klebsiella pneumoniae carbapenemase-producing isolates are also often resistant to fluoroquinolones, aminoglycosides, and sulfamethoxazole/trimethoprim. Amikacin, gentamicin, colistin, and tigecycline typically retain activity against these enzymes. Combination therapies may be an alternative based on in vitro data, but clinical data supporting such recommendations are lacking.

Penicillins

The microbiological activity of the penicillins is shown in Tables 120-2 to 120-4. Natural penicillins are most active against non-β-lactamase-producing gram-positive aerobic and anaerobic bacteria as well as selected gram-negative cocci such as Neisseria spp. Penicillin G is effectively the only natural penicillin used in the critical care setting. Gram-positive bacteria inhibited by natural penicillins are generally more susceptible to these penicillins than to semisynthetic penicillins. Penicillin and ampicillin remain the drugs of choice for enterococcal infections, but resistance to ampicillin among enterococcal isolates in North America is nearly 20%.⁶⁰ Semisynthetic penicillins (oxacillin, nafcillin) are the agents of choice for penicillin-resistant S. aureus and Staphylococcus epidermidis, because penicillins exhibit faster bactericidal activity and improved clinical outcomes when compared with vancomycin.^61,62 Semisynthetic penicillins should be reserved for staphylococcal infections, even though they are active against streptococci. Methicillin is seldom used because of an associated higher incidence of interstitial nephritis than oxacillin or nafcillin. Nafcillin and oxacillin have similar antistaphylococcal activity and can be used interchangeably for this indication.

Ampicillin possesses the same spectrum as penicillin G and is active against gram-negative cocci and members of the family Enterobacteriaceae. Ampicillin alone is seldom used any longer in critical care settings, because β-lactamase production is common for almost all Enterobacteriaceae and staphylococci. With the addition of sulbactam to ampicillin, activity is regained against most organisms within these categories. Use of the antipseudomonal penicillins is increasingly limited to ticarcillin/clavulanate and piperacillin/tazobactam, owing to the prevalence of β-lactamase and the poor activity of these agents against this enzymatic activity. Carbenicillin and ticarcillin are less active than piperacillin against streptococci, enterococci, Haemophilus spp., and P. aeruginosa. Ticarcillin and piperacillin have good clinical activity against both gram-positive and gram-negative anaerobes, including Bacteroides fragilis, Fusobacterium, and Prevotella spp. Mezlocillin and azlocillin have similar activity to piperacillin against P. aeruginosa, but the lack of a β-lactamase inhibitor combination has dramatically reduced the use of either of these compounds in North America.

The pharmacokinetics of the penicillins and their dosing guidelines and administration are shown in Table 120-5. The pharmacokinetics of these agents has not been well investigated in critically ill patients, so extrapolation from healthy volunteers and less acutely ill patients is required. When penicillin was first available in the 1940s, the drug was administered as a continuous infusion to treat bacterial endocarditis. Nearly 70 years later, there is a resurgence of interest in using a continuous infusion or extended infusion of a β-lactam to improve bacterial killing activity and reduce development of resistance.

Piperacillin has been well studied (with and without tazobactam) as a continuous infusion. Several studies have demonstrated improved clinical cure rates and reduced overall drug exposure and drug costs compared to traditional intermittent 30- to 60-minute infusions.⁶³ Patients with ventilator-associated pneumonia (VAP) caused by gram-negative pathogens with MICs of 8 to 16 µg/mL demonstrated higher probability of clinical cure when piperacillin-tazobactam was administered by continuous compared with intermittent infusion.⁶⁴

In a study of 194 seriously ill patients with P. aeruginosa infection, the use of piperacillin/tazobactam in an extended infusion period (4-hour infusion with doses administered every 8 hours) demonstrated reduced 14-day mortality for patients with high Acute Physiology and Chronic Health Evaluation [APACHE] II scores (>17) when compared to conventional 30-minute infusions (12.2% versus 31.6%; P < 0.04).⁶⁵ Extended infusion may offer some advantages over continuous infusion. A continuous infusion requires a dedicated IV line or lumen of a catheter. This is not always practical in the critically ill, especially for patients who have limited IV access, patients who require multiple daily infusions, or in situations where drug compatibility concerns may occur. An extended infusion provides a period of time in which the IV line is available. For either administration method, intensive nursing attention is required to make sure the drug is delivered properly.