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Am Fam Physician. 2000;62(3):611-620

Beta-lactam antibiotics include penicillins, cephalosporins and related compounds. As a group, these drugs are active against many gram-positive, gram-negative and anaerobic organisms. Information based on “expert opinion” and antimicrobial susceptibility testing supports certain antibiotic choices for the treatment of common infections, but less evidence-based literature is available to guide treatment decisions. Evidence in the literature supports the selection of amoxicillin as first-line antibiotic therapy for acute otitis media. Alternative drugs, such as amoxicillin-clavulanate, trimethoprim-sulfamethoxazole and cefuroxime axetil, can be used to treat resistant infections. Penicillin V remains the drug of choice for the treatment of pharyngitis caused by group A streptococci. Inexpensive narrow-spectrum drugs such as amoxicillin or trimethoprim-sulfamethoxazole are first-line therapy for sinusitis. Animal and human bites can be treated most effectively with amoxicillin-clavulanate. For most outpatient procedures, amoxicillin is the preferred agent for bacterial endocarditis prophylaxis. Beta-lactam antibiotics are usually not the first choice for empiric outpatient treatment of community-acquired pneumonia. Based on the literature, the role of beta-lactam antibiotics in the treatment of bronchitis, skin infections and urinary tract infections remains unclear.

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.13 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]).

Orally administered beta-lactam antibiotics are divided into classes based on their antimicrobial spectrum5 (Table 1).

ClassDrugAntimicrobial spectrum
Natural penicillinPenicillin VStreptococcus species and oral cavity anaerobes
Penicillinase-resistant penicillinCloxacillin (Tegopen)Methicillin-sensitive Staphylococcus aureus and Streptococcus species
Dicloxacillin (Dynapen)
Nafcillin (Unipen)*
Oxacillin (Prostaphlin)*
AminopenicillinAmoxicillinSame coverage as penicillin V, plus Listeria monocytogenes, Enterococcus species, Proteus mirabilis and some strains of Escherichia coli
Bacampicillin (Spectrobid)
Beta-lactam–beta-lactamase inhibitor combinationAmoxicillin-clavulanate (Augmentin)Same coverage as aminopenicillins, plus betalactamase–producing strains of methicillin-sensitive S. aureus, Haemophilus influenzae and Moraxella (formerly Branhamella) catarrhalis
Antipseudomonal penicillinCarbenicillin (Geocillin)Limited activity against Pseudomonas and Klebsiella species
First-generation cephalosporinCefadroxil (Duricef)Improved coverage of methicillin-sensitive S. aureus, E. coli, P. mirabilis and Klebsiella species
Cephalexin (Keflex)
Cephradine (Velosef)
Second-generation cephalosporinCefaclor (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
CarbacephemLoracarbef (Lorabid)Same coverage as second-generation cephalosporins
Third-generation cephalosporinCefdinir (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
Cefixime (Suprax)
Cefpodoxime (Vantin)
Ceftibuten (Cedax)

Oral Penicillins

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.


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.

Oral Cephalosporins

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 2.1,5,6,911 Selected differences among the oral agents in each cephalosporin generation are given in Table 3.1,5,6,911

FactorsCephalosporinsPenicillin V*
Allergic reactionsFewer immediate and delayed hypersensitivity reactions; must be avoided in patients with a history of immediate hypersensitivity to penicillinAllergic reactions common
Patient toleranceBetter taste, which increases compliance in children; fewer gastrointestinal side effectsMore gastrointestinal side effects
CostMore expensiveLess expensive
Antimicrobial spectrumBroader antibacterial spectrumNarrower antibacterial spectrum; less likely to induce antimicrobial resistance; some penicillins cover anaerobes, Listeria, Enterococcus or Pseudomonas species
First-generation agents
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
Second-generation agents
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
Third-generation agents
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.


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.

Dosages and average wholesale costs for 10 days of treatment with orally administered beta-lactam antibiotics are given in Table 4.13 Clinical indications for beta-lactam antibiotics are summarized in Table 5.

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InfectionPreferred drug(s)Alternative drug(s)
Otitis mediaAmoxicillinAmoxicillin-clavulanate (Augmentin), trimethoprim-sulfamethoxazole (Bactrim, Septra), second-generation cephalosporins, some third-generation cephalosporins, macrolide antibiotics
Streptococcal pharyngitisPenicillin VIn patients with penicillin allergy: macrolide antibiotics, first-generation cephalosporins
SinusitisAmoxicillin, trimethoprim-sulfamethoxazoleAmoxicillin-clavulanate, second-generation cephalosporins, third-generation cephalosporins
Animal and human bitesAmoxicillin-clavulanateDepends on type of bite (e.g., cefuroxime axetil [Ceftin] or doxycycline [Vibramycin] for cat bites)
Bacterial endocarditis prophylaxisAmoxicillinIn patients with penicillin allergy: clindamycin (Cleocin), cephalexin (Keflex), azithromycin (Zithromax), clarithromycin (Biaxin)
PneumoniaMacrolide antibiotics, quinolone antibioticsAmoxicillin-clavulanate, second-generation cephalosporins, third-generation cephalosporins
Bronchitis (controversial)Doxycycline, trimethoprim-sulfamethoxazole, amoxicillin-clavulanateMacrolide 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 infectionQuinolone antibiotics, trimethoprim-sulfamethoxazoleAmoxicillin, 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 guidelines1416 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.


Penicillin V remains the drug of choice for the treatment of streptococcal pharyngitis.5 Erythromycin or first-generation cephalosporins may be used in patients who are allergic to penicillin. Once-daily dosing with amoxicillin has compared favorably with thrice-daily dosing of penicillin V.20


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.


Amoxicillin-clavulanate is the drug of choice in patients who have been bitten by another human or an animal.5


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 rightsholder did not grant rights to reproduce this item in electronic media. For the missing item, see the original print version of this publication.


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.


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.

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