Beta-lactam antibiotics, which are named for the beta-lactam ring in their chemical structure,1 include the penicillins, cephalosporins and related compounds. These agents are active against many gram-positive, gram-negative and anaerobic organisms. The beta-lactam antibiotics exert their effect by interfering with the structural crosslinking of peptidoglycans in bacterial cell walls. Because many of these drugs are well absorbed after oral administration, they are clinically useful in the outpatient setting.
Resistance to Beta-Lactam Antibiotics
Bacterial resistance against beta-lactam antibiotics is increasing at a significant rate and has become a common problem in primary care medicine. There are several mechanisms of antimicrobial resistance to beta-lactam antibiotics.1–3 One important mechanism is the production of beta-lactamases, which are enzymes that cleave the beta-lactam ring.4 Beta-lactamase activity can occur in gram-positive organisms (Staphylococcus aureus and Staphylococcus epidermidis); gram-negative organisms (Haemophilus influenzae, Neisseria gonorrhoeae, Moraxella [formerly Branhamella ] catarrhalis, Escherichia coli, and Proteus, Serratia, Pseudomonas and Klebsiella species); and anaerobic organisms (Bacteroides species).
The newer beta-lactam antibiotics can be highly effective in combating infections caused by beta-lactamase–producing organisms. When used alone, beta-lactamase inhibitors (clavulanate, sulbactam and tazobactam) have weak intrinsic antibacterial activity, but their effectiveness increases when they are combined with a beta-lactam antibiotic (e.g., amoxicillin-clavulanate [Augmentin]).
|Natural penicillin||Penicillin V||Streptococcus species and oral cavity anaerobes|
|Penicillinase-resistant penicillin||Cloxacillin (Tegopen)||Methicillin-sensitive Staphylococcus aureus and Streptococcus species|
|Aminopenicillin||Amoxicillin||Same coverage as penicillin V, plus Listeria monocytogenes, Enterococcus species, Proteus mirabilis and some strains of Escherichia coli|
|Beta-lactam–beta-lactamase inhibitor combination||Amoxicillin-clavulanate (Augmentin)||Same coverage as aminopenicillins, plus betalactamase–producing strains of methicillin-sensitive S. aureus, Haemophilus influenzae and Moraxella (formerly Branhamella) catarrhalis|
|Antipseudomonal penicillin||Carbenicillin (Geocillin)||Limited activity against Pseudomonas and Klebsiella species|
|First-generation cephalosporin||Cefadroxil (Duricef)||Improved coverage of methicillin-sensitive S. aureus, E. coli, P. mirabilis and Klebsiella species|
|Second-generation cephalosporin||Cefaclor (Ceclor, Ceclor CD)||Compared with first-generation agents, better coverage of beta-lactamase–producing organisms|
|Cefprozil (Cefzil)||such as methicillin-sensitive S. aureus, H. influenzae,|
|Cefuroxime axetil (Ceftin)||M. catarrhalis, E. coli, P. mirabilis and Klebsiella species|
|Carbacephem||Loracarbef (Lorabid)||Same coverage as second-generation cephalosporins|
|Third-generation cephalosporin||Cefdinir (Omnicef)||Variable loss of Staphylococcus and Pneumococcus coverage; compared with second-generation cephalosporins, somewhat expanded coverage of gram-negative organisms; enhanced coverage of Proteus vulgaris and Providencia species|
The orally administered penicillins include natural penicillins, penicillinase-resistant penicillins, aminopenicillins, beta-lactam–beta-lactamase inhibitor combinations and antipseudomonal penicillins.6
The antibiotic properties of Penicillium mold were first noted by Fleming in 1928.1 Penicillins first became available commercially in the mid-1940s, and they remain one of the most important classes of antimicrobial agents. Despite the development of bacterial resistance, which was noted shortly after the penicillins were introduced, these drugs are still widely used, and new penicillin derivatives are being developed.
Penicillin V, the potassium salt of phenoxymethyl penicillin, is well absorbed orally, and peak serum levels are achieved within 60 minutes. Penicillin G is not as well absorbed and is therefore less useful for oral therapy.
Penicillin V is indicated for the treatment of mild gram-positive infections of the throat, respiratory tract and soft tissues. This natural penicillin is still the drug of choice for the treatment of group A streptococcal pharyngitis in patients who are not allergic to penicillin.5 Penicillin V is also useful for anaerobic coverage in patients with oral cavity infections.
Penicillinase-resistant penicillins were developed because of the increasing resistance of staphylococci to natural penicillins. These chemically modified penicillins have a side chain that inhibits the action of penicillinase.6
The penicillinase-resistant penicillins are active against Streptococcus and Staphylococcus species, but they are not active against methicillin-resistant S. aureus, which is becoming an increasingly common organism.7 These drugs also do not have activity against gram-negative organisms.
The agents in this class with the best oral absorption are cloxacillin (Tegopen) and dicloxacillin (Dynapen). These drugs should be taken one to two hours before meals.1 Nafcillin (Unipen) and oxacillin (Prostaphlin) come in oral preparations but are poorly absorbed.
Penicillinase-resistant penicillins are primarily indicated for the treatment of skin and soft tissue infections.
The aminopenicillins were the first penicillins discovered to be active against gram-negative rods such as E. coli and H. influenzae.
Amoxicillin is more completely absorbed than ampicillin. As a result, serum amoxicillin levels are twice as high as serum ampicillin levels. Because a smaller amount of amoxicillin remains in the intestinal tract, patients treated with this agent have less diarrhea than those treated with ampicillin. However, the more complete absorption of amoxicillin makes the drug less effective than ampicillin in the treatment of Shigella enteritis. Otherwise, amoxicillin and ampicillin have almost the same spectrum of antimicrobial activity.
Bacampicillin (Spectrobid) does not have any significant advantages over the other aminopenicillins, and it is more expensive.
Orally administered amoxicillin and ampicillin are used primarily to treat mild infections such as otitis media, sinusitis, bronchitis, urinary tract infections and bacterial diarrhea. Amoxicillin is the agent of choice for the treatment of otitis media.8 Because H. influenzae and E. coli are becoming increasingly resistant to the aminopenicillins, these drugs are becoming somewhat less effective clinically.
BETA-LACTAM–BETA-LACTAMASE INHIBITOR COMBINATION
The only penicillin available in an oral combination with a beta-lactamase inhibitor is amoxicillin-clavulanate.6 This combination drug provides increased antimicrobial coverage of beta-lactamase–producing strains of S. aureus, H. influenzae, N. gonorrhoeae, E. coli, M. catarrhalis and Proteus, Klebsiella and Bacteroides species. It has little activity against Pseudomonas or methicillin-resistant S. aureus.
In clinical situations in which there is increased development of beta-lactamase–producing organisms, amoxicillin-clavulanate may be the first choice for the treatment of otitis media, sinusitis, bronchitis, urinary tract infections and skin and soft tissue infections. Because of its anaerobic coverage, amoxicillin-clavulanate is an excellent drug for treating infections caused by human and animal bites.
Common side effects include gastrointestinal distress, diarrhea (alleviated by taking the drug with food or water), rashes and Candida superinfection.
Carbenicillin (Geocillin) is the only available orally administered antipseudomonal penicillin. This drug has excellent oral absorption. However, it is metabolized so rapidly that serum levels remain low, which markedly limits its clinical usefulness.
Structurally, the cephalosporins have a beta-lactam ring (which they share with all penicillins) and a thiazolidine ring. These drugs are divided into generations based on their spectrum of antimicrobial activity.
Although the cephalosporins are often thought of as new and improved derivatives of the penicillins, they were actually discovered as naturally occurring substances separate from the penicillins. Brotzu noted the periodic clearing of microorganisms from sea water near a sewage outlet and isolated a substance with antibacterial properties that was produced by the fungus Cephalosporium acremonium.1 After further study and modification of this substance, the first commercially available cephalosporin (cephalothin) was introduced in 1962.
The effectiveness of an individual cephalosporin depends on its ability to overcome the mechanisms of resistance that bacteria have developed to combat beta-lactam antibiotics. A comparison of cephalosporins and penicillin, using penicillin V as the prototype, is presented in Table 2.1,5,6,9–11 Selected differences among the oral agents in each cephalosporin generation are given in Table 3.1,5,6,9–11
|Allergic reactions||Fewer immediate and delayed hypersensitivity reactions; must be avoided in patients with a history of immediate hypersensitivity to penicillin||Allergic reactions common|
|Patient tolerance||Better taste, which increases compliance in children; fewer gastrointestinal side effects||More gastrointestinal side effects|
|Cost||More expensive||Less expensive|
|Antimicrobial spectrum||Broader antibacterial spectrum||Narrower antibacterial spectrum; less likely to induce antimicrobial resistance; some penicillins cover anaerobes, Listeria, Enterococcus or Pseudomonas species|
|Cefadroxil (Duricef)||Kinetics allow once-daily or twice-daily dosing; convenience offset by significantly higher cost than other first-generation cephalosporins|
|Cephalexin (Keflex)||Extensive clinical experience with its use; well tolerated; good pharmacokinetics|
|Cephradine (Velosef)||Similar properties as cephalexin, but not as widely used|
|Cefaclor (Ceclor, Ceclor CD)||May cause serum sickness–like syndrome; absorption decreased by food; of second-generation cephalosporins, has highest incidence of Haemophilus influenzae resistance|
|Cefprozil (Cefzil)||Absorption not affected by food|
|Cefuroxime axetil (Ceftin)||Parenteral form available (cefuroxime sodium [Zinacef]); absorption enhanced by food; only second-generation agent labeled for the treatment of urinary tract infections|
|Cefixime (Suprax)||Oral suspension better absorbed than tablets (therefore, less likely to cause diarrhea); single oral dose indicated for the treatment of uncomplicated gonorrhea|
|Cefpodoxime (Vantin) and cefdinir (Omnicef)||Of the third-generation agents, provide best coverage of penicillin-sensitive Pneumococcus and methicillin-sensitive Staphylococcus aureus|
|Ceftibuten (Cedax)||Poor efficacy against Streptococcus pneumoniae, which limits its clinical usefulness|
The first-generation cephalosporins include cefadroxil (Duricef), cephalexin (Keflex) and cephradine (Velosef), which are similar drugs. They are all well absorbed, even in the presence of food, and they achieve high urinary concentrations. Dosages of these agents should be decreased in patients with severe renal failure.
Cefadroxil, cephalexin and cephradine are effective in the treatment of skin and soft tissue infections caused by Streptococcus species and methicillin-sensitive S. aureus. Many physicians consider these drugs to be preferable to the orally administered antistaphylococcal penicillins (cloxacillin and dicloxacillin) because they are associated with a lower incidence of gastrointestinal side effects and have a better taste.
The good urinary concentrations of first-generation cephalosporins make them second-line agents (after quinolone antibiotics and trimethoprim-sulfamethoxazole [Bactrim, Septra]) for the treatment of urinary tract infections caused by susceptible gram-negative organisms, although they are not effective against Pseudomonas or Enterococcus species. Their relative safety in pregnancy makes them a reasonable alternative for the treatment of urinary tract infections in pregnant women.
Cefadroxil, cephalexin and cephradine may be used to treat streptococcal pharyngitis in patients with delayed-reaction penicillin allergy. Indications for these agents in the treatment of other upper respiratory tract infections (bronchitis, pneumonia, otitis media and sinusitis) are unclear. First-generation cephalosporins are generally not effective against H. influenzae, M. catarrhalis and other gram-negative beta-lactamase–producing organisms.
SECOND-GENERATION CEPHALOSPORINS AND CARBACEPHEM
The second-generation cephalosporins include cefaclor (Ceclor), cefprozil (Cefzil) and cefuroxime axetil (Ceftin). Compared with first-generation cephalosporins, these drugs have improved activity against common beta-lactamase–producing respiratory pathogens such as H. influenzae and M. catarrhalis.
As a result of their widespread use, bacterial resistance to second-generation cephalosporins has greatly increased.1 In addition, second-generation cephalosporins are generally much more expensive than first-generation agents or penicillins.
Structurally, loracarbef (Lorabid) is a carbacephem rather than a cephalosporin. However, loracarbef is so similar to cefaclor in spectrum of antimicrobial activity and side effects that it is usually listed as a second-generation cephalosporin.
The second-generation cephalosporins are heavily promoted for their coverage of relatively resistant organisms (e.g., H. influenzae) that cause respiratory tract infections such as otitis media, bronchitis and sinusitis. Much less expensive agents, such as trimethoprim-sulfamethoxazole, may be preferred. Cefuroxime axetil may be considered a second-line agent for the treatment of urinary tract infections.
Third-generation cephalosporins include cefdinir (Omnicef), cefixime (Suprax), cefpodoxime (Vantin) and ceftibuten (Cedax). Secondary to better resistance to some plasmid-mediated beta lactamases, the third-generation agents demonstrate somewhat expanded coverage of gram-negative organisms compared with first- and second-generation cephalosporins. They have the advantage of convenient dosing schedules, but they are expensive.
The third-generation agents have variable loss of efficacy against gram-positive organisms, particularly Streptococcus pneumoniae and Staphylococcus species. Lack of gram-positive coverage limits the usefulness of ceftibuten in the treatment of otitis media and respiratory tract infections, except perhaps as a second-line agent when antibiotics with better gram-positive coverage have failed.11 Poor coverage of Staphylococcus species precludes the use of cefixime and ceftibuten in the treatment of skin and soft tissue infections.
Cefpodoxime and cefdinir retain good coverage of Staphylococcus and Streptococcus species.12 Thus, they are probably the more useful third-generation cephalosporins.
Practical Clinical Applications
Because of all the drugs that are available to treat common infections in the primary care setting, choosing an antibiotic can be difficult. The decision is individualized, based on the cost of treatment and the patient's financial resources, formulary restrictions from insurance companies, the availability of drug samples in the physician's office, the likelihood of a resistant organism, the severity of the infection, comorbid conditions in the patient and the risk of drug side effects.
|Infection||Preferred drug(s)||Alternative drug(s)|
|Otitis media||Amoxicillin||Amoxicillin-clavulanate (Augmentin), trimethoprim-sulfamethoxazole (Bactrim, Septra), second-generation cephalosporins, some third-generation cephalosporins, macrolide antibiotics|
|Streptococcal pharyngitis||Penicillin V||In patients with penicillin allergy: macrolide antibiotics, first-generation cephalosporins|
|Sinusitis||Amoxicillin, trimethoprim-sulfamethoxazole||Amoxicillin-clavulanate, second-generation cephalosporins, third-generation cephalosporins|
|Animal and human bites||Amoxicillin-clavulanate||Depends on type of bite (e.g., cefuroxime axetil [Ceftin] or doxycycline [Vibramycin] for cat bites)|
|Bacterial endocarditis prophylaxis||Amoxicillin||In patients with penicillin allergy: clindamycin (Cleocin), cephalexin (Keflex), azithromycin (Zithromax), clarithromycin (Biaxin)|
|Pneumonia||Macrolide antibiotics, quinolone antibiotics||Amoxicillin-clavulanate, second-generation cephalosporins, third-generation cephalosporins|
|Bronchitis (controversial)||Doxycycline, trimethoprim-sulfamethoxazole, amoxicillin-clavulanate||Macrolide antibiotics, quinolone antibiotics, second-generation cephalosporins, some third-generation cephalosporins|
|Skin and soft tissue infections (cellulitis)||First-generation cephalosporins, cloxacillin (Tegopen), dicloxacillin (Dynapen)||Macrolide antibiotics, amoxicillin-clavulanate, cefpodoxime (Vantin), cefdinir (Omnicef)|
|Urinary tract infection||Quinolone antibiotics, trimethoprim-sulfamethoxazole||Amoxicillin, amoxicillin-clavulanate, cefuroxime axetil or other cephalosporins, doxycycline, nitrofurantoin (Furadantin)|
Although information based on “expert opinion” and antimicrobial susceptibility testing is available to support certain antibiotic choices for the treatment of common infections, the literature contains much less evidence-based information on the treatment of clinical syndromes. However, clinical treatment guidelines14–16 demonstrate some consensus on antibiotic therapy for various infections.
In the United States, acute otitis media is routinely treated with antibiotics despite the lack of consistent supporting data and the risk of promoting drug resistance.17 When antibiotics are used, amoxicillin is considered first-line therapy in children.
Amoxicillin is given in an initial dosage of 40 to 45 mg per kg per day for 10 days.8 A higher dosage (80 to 90 mg per kg per day) may be used in children who are at increased risk for drug-resistant S. pneumoniae infection, such as children younger than two years, children who spend time in day care settings and children who have received an antibiotic within the previous three months. Amoxicillin can be given twice daily.18 If treatment failure is evident after amoxicillin has been given for three days, use of an alternative drug should be considered to provide more effective coverage of resistant organisms. Alternatives include amoxicillin-clavulanate, trimethoprim-sulfamethoxazole and cefuroxime axetil.
One meta-analysis19 supports the use of a shortened antibiotic course (five days) in patients with otitis media. This approach is associated with only a slight increase in treatment failures.
Many patients with sinusitis improve spontaneously,21 but it may be appropriate to prescribe antibiotics for patients who are sicker or at higher risk for complications.22 Inexpensive narrow-spectrum drugs such as amoxicillin or trimethoprim-sulfamethoxazole are the first choices.21 Alternative agents for the treatment of resistant infections include amoxicillin-clavulanate and second- and third-generation cephalosporins.
HUMAN AND ANIMAL BITES
Amoxicillin-clavulanate is the drug of choice in patients who have been bitten by another human or an animal.5
BACTERIAL ENDOCARDITIS PROPHYLAXIS
In patients who are able to take oral medications, amoxicillin is the preferred agent to prevent bacterial endocarditis after dental, oral, respiratory tract and esophageal procedures. If a patient is known to be allergic to penicillin, alternative antimicrobials include clindamycin (Cleocin), cephalexin, cefadroxil, azithromycin (Zithromax) and clarithromycin (Biaxin).23 Prophylactic regimens are given in Table 6.23
The beta-lactam antibiotics are not the preferred first-line agents for empiric outpatient treatment of community-acquired pneumonia.24 Quinolone antibiotics with enhanced S. pneumoniae coverage (e.g., levofloxacin [Levaquin]) or macrolide antibiotics are preferred.
Amoxicillin-clavulanate may be used for outpatient treatment of suspected aspiration pneumonia.
LESS CLEAR INDICATIONS
The benefits of beta-lactam antibiotics in the treatment of bronchitis, skin and soft tissue infections, and urinary tract infections are less clear in the evidence-based literature. The marginal benefit of antibiotics in bronchitis must be weighed against the adverse effects of the drugs and the development of antibiotic resistance.25 Penicillinase-resistant penicillins and first-generation cephalosporins are first-line choices for the treatment of skin and soft tissue infections, with macrolide antibiotics, amoxicillin-clavulanate, cefpodoxime and cefdinir as second-line agents. Quinolone antibiotics and trimethoprim-sulfamethoxazole are preferred agents for the treatment of uncomplicated urinary tract infections.