Erythromycin is a macrolide antibacterial with a broad and essentially bacteriostatic action against many Gram-positive and to a lesser extent some Gram-negative bacteria, as well as other organisms including some Mycoplasma spp., Chlamydiaceae, Rickettsia spp., and spirochaetes.
Mechanism of action. Erythromycin binds reversibly to the 50S subunit of the ribosome, resulting in blockage of the transpeptidation or translocation reactions, inhibition of protein synthesis, and hence inhibition of cell growth. Its action is predominantly bacteriostatic, but high concentrations are slowly bactericidal against the more sensitive strains. Because macrolides penetrate readily into white blood cells and macrophages there has been some interest in their potential synergy with host defence mechanisms in vivo. The actions of erythromycin are increased at moderately alkaline pH (up to about 8.5), particularly in Gram-negative species, probably because of the improved cellular penetration of the nonionised form of the drug.
Spectrum of activity. Erythromycin has a broad spectrum of activity. The following pathogenic organisms are usually sensitive to erythromycin.
Gram-positive cocci, particularly streptococci such as Streptococcus pneumoniae and Str. pyogenes are sensitive. Most strains of Staphylococcus aureus remain susceptible, although resistance can emerge rapidly, and some enterococcal strains are also susceptible.
Many other Gram-positive organisms respond to erythromycin, including Bacillus anthracis, Corynebacterium diphtheriae, Erysipelothrix rhusiopathiae, and Listeria monocytogenes. Anaerobic Clostridium spp. are also usually susceptible, as is Propionibacterium acnes. Nocardia spp. vary in their susceptibility.
Gram-negative cocci including Neisseria meningitidis and N. gonorrhoeae, and Moraxella catarrhalis (Branhamella catarrhalis) are usually sensitive.
Other Gram-negative organisms vary in their susceptibility, but Bordetella spp., some Brucella strains, Flavobacterium, and Legionella spp. are usually susceptible. Haemophilus ducreyi is reportedly susceptible, but H. influenzae is somewhat less so. The Enterobacteriaceae are usually resistant, although some strains may respond at alkaline pH. Helicobacter pylori and most strains of Campylobacter jejuni are sensitive (about 10% of the latter are reported to be resistant).
Among the Gram-negative anaerobes most strains of Bacteroides fragilis and many Fusobacterium strains are resistant.
Other organisms usually sensitive to erythromycin include Actinomyces, Chlamydiaceae, rickettsias, spirochaetes such as Treponema pallidum and Borrelia burgdorferi, some mycoplasmas (notably Mycoplasma pneumoniae), and some of the opportunistic mycobacteria: Mycobacterium scrofulaceum and M. kansasii are usually susceptible, but M. intracellulare is often resistant and M. fortuitum usually so.
Fungi, yeasts, and viruses are not susceptible to erythromycin.
Activity with other antimicrobials. As with other bacteriostatic antimicrobials, the possibility of an antagonistic effect if erythromycin is given with a bactericide exists, and some antagonism has been shown in vitro between erythromycin and various penicillins and cephalosporins or gentamicin. However, in practice the results of such concurrent use are complex, and depend on the organism; in some cases synergy has been seen. Because of the adjacency of their binding sites on the ribosome, erythromycin may competitively inhibit the effects of chloramphenicol or lincosamides such as clindamycin. A synergistic effect has been seen when erythromycin was combined with a sulfonamide, notably against Haemophilus influenzae. Erythromycin has also been reported to enhance the antiplasmodial actions of chloroquine.
Resistance. Several mechanisms of acquired resistance to erythromycin have been reported of which the most common is a plasmid-mediated ability to methylate ribosomal RNA, resulting in decreased binding of the antimicrobial drug. This can result in cross-resistance between erythromycin, other macrolides, lincosamides, and streptogramin B, because they share a common binding site on the ribosome and this pattern of resistance is referred to as the MLSB phenotype. It is seen in staphylococci, and to a somewhat lesser extent in streptococci, as well as in a variety of other species including Bacteroides fragilis, Clostridium perfringens, Corynebacterium diphtheriae, Listeria, and Legionella spp.
Decreased binding of antimicrobial to the ribosome may also occur as a result of a chromosomal mutation, resulting in an alteration of the ribosomal proteins in the 50S subunit, which conveys one-step high-level erythromycin resistance. This form of resistance has been demonstrated in Escherichia coli and some strains of Str. pyogenes, and probably occurs in Staphylococcus aureus.
Other forms of erythromycin resistance may be due to the production of a plasmid-determined erythromycin esterase which can inactivate the drug, or to decreased drug penetration. The latter may be partly responsible for the intrinsic resistance of Gram-negative bacteria like the Enterobacteriaceae, but has also been shown to be acquired as a plasmid-mediated determinant in some organisms; production of a protein which increases drug efflux from the cell is thought to explain the MS form of resistance, in which organisms are resistant to 14-carbon ring macrolides and streptogramins, but retain sensitivity to 16-carbon ring macrolides and lincosamides.
The incidence of resistance varies greatly with the area and the organism concerned and, although the emergence of resistance is rarely a problem in the short-term treatment of infection, it is quite common in conditions requiring prolonged treatment such as endocarditis due to Staph. aureus. The incidence of resistance in streptococci is generally lower than in Staph. aureus but shows geographical variation and may be increasing in some countries, including the UK. In addition, localised outbreaks of resistant strains may occur and produce a much higher incidence of resistance.