Erythromycin
Macrolide Antibiotic: Effects, Indications and Notes
Erythromycin is an antibiotic from the macrolide group and is among the oldest and most thoroughly studied antibiotics. It was isolated from the bacterium Saccharopolyspora erythraea in the early 1950s and has been used in medicine ever since. Erythromycin is active against a broad spectrum of bacteria and is an important alternative for patients with penicillin allergy. As the first macrolide antibiotic, it paved the way for the development of newer compounds of this class such as clarithromycin and azithromycin.
Mechanism of Action
Erythromycin exerts its antibacterial effect by inhibiting bacterial protein biosynthesis. The compound binds reversibly to the 50S subunit of the bacterial ribosome. This blocks the translocation of the peptide chain during protein synthesis: new amino acids can no longer be incorporated into the growing protein chain, and the bacteria can no longer produce essential proteins.
Depending on concentration and pathogen, erythromycin can act bacteriostatically (inhibiting bacterial growth) or bactericidally (killing bacteria). At low concentrations, bacteriostatic activity generally predominates. High local concentrations, such as those achieved with topical skin application, can exert bactericidal effects.
Spectrum of Activity
Erythromycin is active against many gram-positive bacteria, including streptococci, staphylococci and pneumococci. Certain gram-negative organisms such as Haemophilus influenzae, Bordetella pertussis (whooping cough pathogen) and Legionella also fall within its spectrum. Atypical pathogens such as mycoplasmas, chlamydiae and Legionella are particularly sensitive to macrolide antibiotics, as these organisms lack a classical cell wall and therefore respond to agents that inhibit protein biosynthesis.
Erythromycin is less active against gram-negative enteric bacteria such as Escherichia coli, as these bacteria possess efflux pumps that actively expel the compound from the cell.
Indications
Respiratory Tract Infections
Erythromycin is used in infections of the upper and lower respiratory tract caused by susceptible organisms. These include scarlet fever and pharyngitis caused by streptococci, and community-acquired pneumonia, particularly when atypical pathogens such as mycoplasmas or chlamydiae are suspected. In whooping cough (pertussis), erythromycin can favourably influence the course of disease when used early, and is also used for post-exposure prophylaxis in close contacts.
Skin Infections and Acne
In dermatology, erythromycin is used both systemically and topically. Solutions, gels and creams are available for topical application directly to the skin. In acne vulgaris, erythromycin can be used topically or in combination with benzoyl peroxide to combat Cutibacterium acnes and reduce inflammation. Combination with benzoyl peroxide is important to slow the development of erythromycin resistance. Systemically, erythromycin can be used in bacterial skin infections such as erysipelas, impetigo or infected wounds.
Genital Infections
In certain chlamydial infections of the urogenital tract, erythromycin can be used, particularly when newer macrolides or tetracyclines are not appropriate. Erythromycin has also been used historically in the treatment of gonorrhoea, although the current resistance situation now strongly constrains antibiotic choice.
Penicillin Allergy
In known penicillin allergy, erythromycin represents an important therapeutic alternative, as it has an overlapping spectrum of activity with penicillins against gram-positive organisms but is chemically unrelated and does not cause cross-allergy. This is clinically particularly relevant in the treatment of streptococcal infections and pneumonia.
Prokinetic Effect in Gastroenterology
In addition to its antibacterial activity, erythromycin also has prokinetic properties: it stimulates gastrointestinal motility by binding to motilin receptors in the gastrointestinal tract. Motilin is a hormone that regulates intestinal movement. This property is occasionally utilised in gastroenterology, for example in the treatment of gastric emptying disorders (gastroparesis) or to improve gastric passage before diagnostic examinations. Substantially lower doses than those used for antibacterial therapy are generally employed in this context.
Formulations and Dosage
Erythromycin is available as tablets, capsules, granules for the preparation of a suspension, and as topical preparations (gel, solution, cream). For intravenous administration, injection solutions are available. Dosage depends on the type of infection, severity of the condition, patient age and weight, and renal and hepatic function. Exact dosage is determined by a physician and typically ranges from 1 to 4 grams daily in several divided doses.
Notes on Administration
Antibiotics should always be taken for the complete prescribed duration, even when symptoms have already improved. Early discontinuation may increase the risk of incomplete pathogen elimination and resistance development. If symptoms persist or worsen, medical advice should be sought.
Erythromycin can cause gastric irritation. Taking the dose with or after meals may improve gastric tolerability, although this depends on the specific formulation. Enteric-coated formulations should not be divided or chewed.
Antibiotic Resistance
Resistance to macrolides, including erythromycin, is an increasing problem. Resistant strains have been identified particularly in staphylococci and streptococci. Resistance genes can be transferred between bacteria, which facilitates the spread of resistance. The appropriate use of antibiotics, i.e. only for bacterial infections and on medical prescription, is an important contribution to slowing the development of resistance at both individual and societal level.
Adverse Effects
Erythromycin has a well-known side-effect profile that is considered in the treatment decision:
- Gastrointestinal complaints are frequent: nausea, abdominal cramps, diarrhoea and vomiting can occur. These are the most common reasons for treatment discontinuation and are partly related to the prokinetic effect of the compound on the gastrointestinal tract.
- Hepatic dysfunction is possible, particularly with prolonged use or certain ester forms.
- Cardiac arrhythmias (QT interval prolongation on ECG) have been described, particularly with intravenous administration and in patients with pre-existing cardiac conditions or certain drug combinations.
- Hearing disturbances can occur at high doses and are generally reversible after discontinuation.
- Allergic reactions such as rash are possible but less common than with penicillins.
Drug Interactions
Erythromycin is a potent inhibitor of the hepatic enzyme CYP3A4. It can thereby delay the metabolism of many other medications and increase their blood levels. Particularly relevant interactions occur with:
- Statins (cholesterol-lowering agents): Increased risk of myopathy or rhabdomyolysis
- Antiarrhythmics: Increased risk of cardiac arrhythmias through QT prolongation
- Anticoagulants such as warfarin: Enhanced anticoagulation effect and increased bleeding risk possible
- Certain immunosuppressants such as ciclosporin: Substantially elevated blood levels possible, which can lead to adverse effects
- Ergotamine: Increased risk of ergotism
Before starting erythromycin therapy, the treating physician should be informed of all medications being taken so that possible interactions can be detected early.
Frequently Asked Questions
Why does erythromycin often cause nausea?
Erythromycin stimulates motilin receptors in the gastrointestinal tract, which promotes intestinal motility. This prokinetic effect is deliberately used therapeutically at low doses but at antibacterial doses can lead to undesirable gastrointestinal complaints such as nausea, cramping and diarrhoea. Taking the medication with or after food may reduce this effect.
Can erythromycin be used safely in pregnancy?
Based on current data, erythromycin is one of the antibiotics that may be used in pregnancy when clearly indicated, for example for certain respiratory infections or chlamydial infections. Use should be discussed with a physician in every case. Clarithromycin and azithromycin should not be used in the first trimester of pregnancy.
What should be noted in the simultaneous use of erythromycin and statins?
Erythromycin inhibits the CYP3A4 enzyme, which metabolises many statins. This can increase statin blood levels and raise the risk of muscle damage (myopathy, rhabdomyolysis). During erythromycin therapy, statin therapy may need to be temporarily interrupted or the statin dose reduced. This should be decided by the treating physician.