Beta-lactam antimicrobials, named after the active chemical component of the drug (the 4-member beta-lactam ring), include the 6-membered ring-structured penicillins, monobactams, and carbapenems; and the 7-membered ring-structured cephalosporins and cephamycins. In addition to their chemical structure, the major difference between these two subclasses of beta-lactam antimicrobials is their susceptibility to beta-lactamase destruction, with the cephalosporins, in general, being more resistant.
Mode of Action of Beta-Lactam Antimicrobials in Animals
Beta-lactam antimicrobials impair the development of bacterial cell walls by interfering with transpeptidase enzymes responsible for the formation of the cross-links between peptidoglycan strands. These enzymes are associated with a group of proteins in both gram-positive and gram-negative bacteria called the penicillin-binding proteins (PBPs). Changes in spectrum and resistance patterns among the different beta-lactam antimicrobials may be due to differences in PBP targets, given that at least nine PBPs comprise the cell wall.
During bacterial cell growth, while the peptidoglycan structure is being formed, autolysins continually cleave cell wall lattices, in anticipation of providing acceptor sites for new strands of bacterial cell synthesis. Normal bacterial growth depends on a balance between cell wall autolysis and synthesis. The beta-lactam drug mimics the PBP substrate, thus inhibiting the PBP and subsequent cell wall synthesis. In the face of continued autolysin activity, the cell wall then becomes deformed. The cell, which is generally hypertonic compared with its environment, is no longer impermeable to the flow of small molecules and is susceptible to osmotic lysis.
Beta-lactam antimicrobials, when present in sufficient concentrations, are generally bactericidal toward most bacteria (an exception is in the case of listeriosis, for which penicillins are bacteriostatic and cephalosporins are ineffective). However, at subinhibitory concentrations, beta-lactam antimicrobials do exert residual effects on bacterial structure and function that, in turn, promote host-mediated cell death.
Some bacterial isolates, when treated with inhibitors of cell-wall synthesis, undergo inhibition of growth but not lysis at usual concentrations. These tolerant organisms are defective in their production or use of autolytic enzymes and can survive exposure to beta-lactam antimicrobials. Clinically, relapses and failures in serious infections due to tolerant organisms may be prevented by the frequently synergistic effect of the aminoglycosides with beta-lactam antimicrobials.
Beta-lactam antimicrobials have little influence on formed bacterial cell walls, and like other bactericidal drugs, even susceptible organisms must be actively multiplying or growing (ie, in the log phase of growth). Beta-lactam antimicrobials are most active during the logarithmic phase of bacterial growth, and therefore static organisms are unaffected and may persist. These persisters may then develop normally after the antimicrobial is removed. Beta-lactam antimicrobials also tend to be somewhat more active in a slightly acidic environment (pH 5.5–6.5), perhaps because of enhanced membrane penetration. They also are likely to be less effective in the presence of hypertonic tissues.
Efficacy of the beta-lactam antimicrobials is related to the time that plasma or tissue drug concentrations exceed the minimum inhibitory concentration (MIC) of the infecting organism. (e.g. they are considered "time-dependent" antimicrobials).Generally, concentrations should remain above the MIC for approximately 25% (carbapenems) to 100% (amoxicillin) of the dosing interval.
Bacterial Resistance to Beta-Lactam Antimicrobials in Animals
Only microorganisms that have cell walls are susceptible to the action of beta-lactam antimicrobials. Mechanisms of antimicrobial resistance to beta-lactam antimicrobials include downregulation of porins leading to a reduction in cell permeability, increased expression of efflux pumps, production of degrading enzymes (beta-lactamases), and modification of the drug target via alterations to the PBPs. Clinical resistance in bacteria is often multifactorial and comprised of a combination of different types of resistance mechanisms or the production of a variety of beta-lactamases.
In gram-positive organisms, capsular materials may hinder access to the cytoplasmic membrane; however, this rarely limits the diffusion of the cell-wall inhibitors. Gram-negative bacteria have a restricting sieving mechanism (porins) in their outer membranes (external cell wall), which decreases the penetration of several types of antimicrobials. Different species of gram-negative bacteria exhibit varying permeability barriers to beta-lactam antimicrobials, and these impair access of the antimicrobials to the membrane-associated binding proteins.
For example, the permeability barrier of Haemophilus influenzae is readily crossed by beta-lactam antimicrobials; however, Escherichia coli presents a greater obstacle to these agents, and the outer membranes of Pseudomonas aeruginosa are not easily penetrated by most beta-lactam antimicrobials. Penicillins, aminopenicillins, first- and second-generation cephalosporins, and selected other beta-lactam antimicrobials cannot penetrate the outer membrane of P aeruginosa. In addition, porins are frequently associated with efflux proteins that effectively remove a drug even after it has successfully penetrated the lipopolysaccharide covering of gram-negative organisms.
The chemical nature of beta-lactam antimicrobials (penicillins and cephalosporins) and the beta-lactamase inhibitors as well as their concentration gradients greatly influence their penetration of bacteria to their targets at the surface of the cytoplasmic membrane, giving rise to the differences among antibacterial spectra of the various classes of penicillin. Beta-lactam antimicrobials are often used in combination with other antimicrobials that disrupt the integrity of the membranes and thereby facilitate access by beta-lactam antimicrobials. The genetic loci controlling permeability generally have been considered to be chromosomally located; however, they also may be plasmid-specified genes.
The most important mechanism of bacterial resistance to beta-lactam antimicrobials is enzymatic inactivation by beta-lactamases via cleavage of the 4-member beta-lactam ring. Cleavage results in the inability of the drugs to bind to their target PBPs. More than 800 unique beta-lactamases are known, representing six major classes, with the enzyme varying with the organism and drugs targeted varying with the enzyme. The increase in the number of enzymes reflects, in part, pressure brought with the increasingly widespread use of beta-lactam antimicrobials and the continued manipulation of the drugs in an attempt to circumvent bacterial beta-lactamase production. For example, the addition of larger R groups on the beta-lactam structure rendered cephalosporins resistant to penicillinases.
However, cephalosporinases emerged with continued use of first-generation cephalosporins. Second- and third-generation cephalosporins reflect modifications, including larger R groups that hindered beta-lactamase access to the beta-lactam ring. Inhibitors of beta-lactamases (eg, clavulanic acid and sulbactam) were added to minimize penicillin destruction. Subsequently, newer beta-lactamases have emerged.
Since the approval and use of third-generation cephalosporins, resistance due to extended-spectrum beta-lactamases (ESBLs), particularly in E coli, Klebsiella, and Proteus, has emerged as a problem; ESBLs target third-generation cephalosporins (but not cephamycins such as cefoxitin). In contrast, carbapenems (eg, imipenem and meropenem) are not subject to ESBLs that target third-generation cephalosporins; however, they are subject to carbapenemases. Interestingly, clavulanic acid is not susceptible to ESBLs; susceptibility data indicating resistance to cephalosporins but susceptibility to amoxicillin-clavulanic acid indicate ESBL formation.
Beta-lactamases are produced by both gram-positive (eg, Staphylococcus aureus, S epidermidis, and S pseudintermedius but generally not enterococci) and gram-negative organisms. Some of these enzymes are active exclusively against penicillins, others are principally active against cephalosporins, and several types hydrolyze both equally. The type and concentration of beta-lactamases are also specific to bacterial species. Gram-positive beta-lactamases generally are excreted into the external environment as exoenzymes, produced in large quantity, plasmid mediated (single determinant), usually inducible (rarely constitutive), unable to initiate self-transmission (relying principally on transduction), and active primarily against penicillins. Staphylococcal strains are the main gram-positive bacteria in which beta-lactamase resistance develops, often very quickly.
However, in some gram-positive bacteria (eg, S aureus), the dominant resistance mechanism has begun to drift away from beta-lactamase production and toward alterations in PBP targets. In gram-negative bacteria, beta-lactamase production remains the dominant resistance mechanism. Gram-negative beta-lactamases generally are heterogeneous (wide range), retained within the periplasmic space, produced in small quantity, often constitutive (less often inducible), able to initiate self-transmission (conjugation mechanisms), and active against both penicillins and cephalosporins. In general, gram-negative beta-lactamases primarily act via hydrolysis of the amide bond in the 4-member beta-lactam ring. The impact of beta-lactamase protectors such as clavulanic acid may not be as positive for treatment of gram-negative versus gram-positive organisms.
Gram-negative bacteria capable of resistance as a result of beta-lactamase production include Escherichia, Haemophilus, Klebsiella, Pasteurella, Proteus, Pseudomonas, and Salmonella spp; resistance may take longer to develop in some of these strains.
Specific Bacterial-binding Proteins
Resistance to beta-lactam antimicrobial agents can be acquired via alterations in the PBP targets of these drugs. A loss or decrease in affinity of crucial PBP can lead to a sizable increase in resistance to beta-lactam antimicrobials. For example, resistance of enterococci to cephalosporins appears to reflect the lack of affinity of a PBP to this subclass of drugs. Changes in PBP-2 of Staphylococcus spp render the organism resistant to all beta-lactam antimicrobials. Methicillin resistance in Staphylococcus spp reflects acquisition of the mec gene, which results in a mutation in PBP-2. As such, no beta-lactam can bind to this protein, resulting in resistance to all beta-lactam drugs, including carbapenems and most generations of cephalosporins. Problematically, genes conferring methicillin resistance may be accompanied by genes conferring multidrug resistance.
Cell Wall–deficient Microbes
Organisms that have no cell wall, such as Mycoplasma, are intrinsically resistant to beta-lactam antimicrobials. A phenotypic form of resistance can occur when spheroplasts (incomplete cell wall) or protoplasts (absence of cell wall) are present. These so-called L-forms must be present in a hyperosmotic environment (eg, the renal medulla) to survive; otherwise, they will lyse. The clinical relevance of this form of resistance is unclear.