In this article, I provide a simplified, practical review of the principal antibiotics that we use on a daily basis to treat bacterial infections. The antibiotics are listed in alphabetical order, either individually or by group. I focus first on the mechanism of action and spectrum of activity of the drugs used against the usual pelvic pathogens (TABLE).1 I then review their principal adverse effects, relative cost (categorized as low, intermediate, and high), and the key indications for these drugs in obstetrics and gynecology. In a forthcoming 2-part companion article, I will review how to select specific antibiotics and their dosing regimens for the most commonly encountered bacterial infections in our clinical practice.
Aminoglycoside antibiotics
The aminoglycosides include amikacin, gentamicin, plazomicin, and tobramycin.2,3 The 2 agents most commonly used in our specialty are amikacin and gentamicin. The drugs may be administered intramuscularly or intravenously, and they specifically target aerobic gram-negative bacilli. They also provide coverage against staphylococci and gonococci. Ototoxicity and nephrotoxicity are their principal adverse effects.
Aminoglycosides are used primarily as single agents to treat pyelonephritis caused by highly resistant bacteria and in combination with agents such as clindamycin and metronidazole to treat polymicrobial infections, including chorioamnionitis, puerperal endometritis, and pelvic inflammatory disease. Of all the aminoglycosides, gentamicin is clearly the least expensive.
Carbapenems
The original carbapenem widely introduced into clinical practice was imipenem-cilastatin. Imipenem, the active antibiotic, inhibits bacterial cell wall synthesis. Cilastatin inhibits renal dehydropeptidase I and, thereby, slows the metabolism of imipenem by the kidney. Other carbapenems include meropenem and ertapenem.
The carbapenems have the widest spectrum of activity against the pelvic pathogens of any antibiotic. They provide excellent coverage of aerobic and anaerobic gram-positive cocci and aerobic and anaerobic gram-negative bacilli. They do not cover methicillin-resistant Staphylococcus aureus (MRSA) and the enterococci very well.
A major adverse effect of the carbapenems is an allergic reaction, including anaphylaxis and Stevens-Johnson syndrome, and there is some minimal cross-sensitivity with the β-lactam antibiotics. Other important, but fortunately rare, adverse effects include neurotoxicity, hepatotoxicity, and Clostridium difficile colitis.4
As a group, the carbapenems are relatively more expensive than most other agents. Their principal application in our specialty is for single-agent treatment of serious polymicrobial infections, such as puerperal endometritis, pelvic cellulitis, and pelvic abscess, especially in patients who have a contraindication to the use of combination antibiotic regimens that include an aminoglycoside.1,2
Cephalosporins
The cephalosporins are β-lactam antibiotics that act by disrupting the synthesis of the bacterial cell wall. They may be administered orally, intramuscularly, and intravenously. The most common adverse effects associated with these agents are an allergic reaction, which can range from a mild rash to anaphylaxis and the Stevens-Johnson syndrome; central nervous system toxicity; and antibiotic-induced diarrhea, including C difficile colitis.1,2,4
This group of antibiotics can be confusing because it includes so many agents, and their spectrum of activity varies. I find it helpful to think about the coverage of these agents as limited spectrum versus intermediate spectrum versus extended spectrum.
The limited-spectrum cephalosporin prototypes are cephalexin (oral administration) and cefazolin (parenteral administration). This group of cephalosporins provides excellent coverage of aerobic and anaerobic gram-positive cocci. They are excellent against staphylococci, except for MRSA. Coverage is moderate for aerobic gram-negative bacilli but only limited for anaerobic gram-negative bacilli. They do not cover the enterococci. In our specialty, their principal application is for treatment of mastitis, urinary tract infections (UTIs), and wound infections and for prophylaxis against group B streptococcus (GBS) infection and post-cesarean infection.2,5 The cost of these drugs is relatively low.
The prototypes of the intermediate-spectrum cephalosporins are cefixime (oral) and ceftriaxone (parenteral). Both drugs have strong activity against aerobic and anaerobic streptococci, Neisseria gonorrhoeae, most aerobic gram-negative bacilli, and Treponema pallidum (principally, ceftriaxone). They are not consistently effective against staphylococci, particularly MRSA, and enterococci. Their key indications in obstetrics and gynecology are treatment of gonorrhea, syphilis (in penicillin-allergic patients), and acute pyelonephritis. Compared with the limited-spectrum cephalosporins, these antibiotics are moderately expensive.1,2
The 3 extended-spectrum cephalosporins used most commonly in our specialty are cefepime, cefotetan, and cefoxitin. These agents are administered intramuscularly and intravenously, and they provide very good coverage against aerobic and anaerobic gram-positive cocci, with the exception of staphylococci and enterococci. They have very good coverage against most gram-negative aerobic bacilli and excellent coverage against anerobic microorganisms. Their primary application in our specialty is for single-agent treatment of polymicrobial infections, such as puerperal endometritis and pelvic cellulitis. When used in combination with doxycycline, they are valuable in treating pelvic inflammatory disease. These drugs are more expensive than the limited-spectrum or intermediate-spectrum agents. They should not be used routinely as prophylaxis for pelvic surgery.1,2,5
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