Medication Summary
Clinical judgment, personal experience, safety, and patient compliance should direct the physician in the choice of the appropriate antimicrobial agents. [1] When choosing antimicrobials for the therapy of mixed infections, their aerobic and anaerobic antibacterial spectrum and their availability in oral or parenteral form should be considered. Some antimicrobials have a limited range of activity. For example, metronidazole is active only against anaerobes and therefore cannot be administered as a single agent for the therapy of mixed infections. Others (ie, carbapenems and the combination of penicillin and a beta-lactamase inhibitor) have wide spectra of activity against aerobes and anaerobes.
Aside from susceptibility patterns, other factors that influence the choice of antimicrobial therapy include the pharmacologic characteristics of the various drugs, their toxicity, their effect on the normal florae, and their bactericidal activity. Although identification of the infecting organisms and their antimicrobial susceptibility may be needed for selection of optimal therapy, the clinical setting and Gram-stain preparation of the specimen may indicate the types of anaerobes present in the infection and the nature of the infectious process. [1]
Although the duration of therapy for anaerobic infections is generally longer than for aerobes and facultative infections, the duration of treatment must be individualized, depending on the response. In some cases, treatment may require 6-8 weeks, but therapy may be shortened with proper surgical drainage. An anti–gram-negative enteric agent is generally added to treat Enterobacteriaceae when treating intra-abdominal infections.
The available parenteral antimicrobials for most infections include metronidazole, a penicillin (ie, ticarcillin, ampicillin, piperacillin) and a beta-lactamase inhibitor (ie, clavulanic acid, sulbactam, tazobactam), tigecycline (approved only for intra-abdominal and skin and soft tissue infections), and the carbapenems (eg, imipenem, meropenem, doripenem, ertapenem).
An agent effective against gram-negative enteric bacilli (ie, aminoglycoside) or an antipseudomonal cephalosporin (ie, cefepime) is generally added to clindamycin, metronidazole, and, occasionally, cefoxitin when treating intra-abdominal infections to provide coverage for these bacteria.
Penicillin can be added to metronidazole in the therapy of intracranial, pulmonary, or dental infections to cover microaerophilic streptococci and Actinomyces species.
A macrolide (ie, erythromycin) is added to metronidazole for upper respiratory tract infections to treat Staphylococcus aureus and aerobic streptococci.
Penicillin is added to clindamycin to supplement its coverage against Peptostreptococcus species and other gram-positive anaerobic organisms.
Penicillin is still the drug of choice for bacteremia caused by non–beta-lactamase producers. However, other agents should be used for the therapy of bacteremia caused by beta-lactamase producers.
For Chlamydia and Mycoplasma species, doxycycline is added to most regimens when treating pelvic infections.
Oral therapy is often substituted for parenteral therapy. The agents available for oral therapy include clindamycin, amoxicillin and clavulanate, and metronidazole.
Antibiotics, Other
Class Summary
Empiric antimicrobial therapy must cover all likely pathogens in the context of this clinical setting.
Penicillin G (Pfizerpen)
Interferes with synthesis of cell wall mucopeptide during active multiplication, resulting in bactericidal activity against susceptible microorganisms (beta-lactam).
Cefoxitin (Mefoxin)
Second-generation cephalosporin indicated for gram-positive cocci and gram-negative rod infections. Infections caused by cephalosporin- or penicillin-resistant gram-negative bacteria may respond.
Cefotetan
Second-generation cephalosporin indicated for infections caused by susceptible gram-positive cocci and gram-negative rods. Dosage and route of administration depend on condition of patient, severity of infection, and susceptibility of causative organism.
Clindamycin (Cleocin)
Lincosamide for treatment of serious skin and soft tissue staphylococcal infections. Also effective against aerobic and anaerobic streptococci (except enterococci). Inhibits bacterial growth, possibly by blocking dissociation of peptidyl tRNA from ribosomes causing RNA-dependent protein synthesis to arrest.
Amoxicillin and clavulanate (Augmentin)
Amoxicillin inhibits bacterial cell wall synthesis by binding to penicillin-binding proteins. Addition of clavulanate inhibits beta-lactamase–producing bacteria.
Good alternative antibiotic for patients allergic to or intolerant to macrolides. Usually is well tolerated and provides good coverage for most infectious agents. Not effective against Mycoplasma and Legionella species. Half-life of oral dosage form is 1-1.3 h. Has good tissue penetration but does not enter cerebrospinal fluid.
For children >3 mo, base dosing protocol on amoxicillin content. Because of different ratios of amoxicillin to clavulanic acid in 250-mg tab (250/125) vs 250-mg chewable tab (250/62.5), do not use 250-mg tab until child weighs >40 kg.
Ticarcillin and clavulanate potassium (Timentin)
Inhibits biosynthesis of cell wall mucopeptide and is effective during stage of active growth. Antipseudomonal penicillin plus beta-lactamase inhibitor that provides coverage against most gram-positive organisms, most gram-negative organisms, and most anaerobes. Contains 4.7-5.0 mEq of Na+/g.
Chloramphenicol
Binds to 50S bacterial ribosomal subunits and inhibits bacterial growth by inhibiting protein synthesis. Effective against gram-negative and gram-positive bacteria.
Imipenem and cilastatin (Primaxin)
Carbapenem used for treatment of multiple organism infections in which other agents do not have wide-spectrum therapeutic activity or are contraindicated because of potential toxicity.
Meropenem (Merrem)
Broad-spectrum carbapenem antibiotic that inhibits cell wall synthesis and has bactericidal activity. Effective against most gram-positive and gram-negative bacteria. Has slightly increased activity against gram-negative bacteria and slightly decreased activity against staphylococci and streptococci compared with imipenem. Also less likely to cause seizures compared with imipenem.
Metronidazole (Flagyl)
Imidazole ring-based antibiotic active against various anaerobic bacteria and protozoa. Used in combination with other antimicrobial agents (except for C difficile enterocolitis).
Ertapenem (Invanz)
Bactericidal activity results from inhibition of cell wall synthesis and is mediated through ertapenem binding to penicillin-binding proteins. Stable against hydrolysis by a variety of beta-lactamases, including penicillinases, cephalosporinases, and extended-spectrum beta-lactamases. Hydrolyzed by metallo-beta-lactamases.
Moxifloxacin (Avelox)
Inhibits the A subunits of DNA gyrase, resulting in inhibition of bacterial DNA replication and transcription.
Tigecycline (Tygacil)
A glycylcycline antibiotic that is structurally similar to tetracycline antibiotics. Inhibits bacterial protein translation by binding to 30S ribosomal subunit and blocks entry of amino-acyl tRNA molecules in ribosome A site. Indicated for complicated skin and skin structure infections caused by E coli, E faecalis (vancomycin-susceptible isolates only), S aureus (methicillin-susceptible and -resistant isolates), S agalactiae, S anginosus group (includes S anginosus, S intermedius, and S constellatus), S pyogenes, and B fragilis.
