Tetracyclines are a group of broad-spectrum antibiotics, which were originally derived from soil-dwelling Streptomyces bacteria.
These medications can be used to treat a wide variety of bacterial infections, including central nervous system infections like meningitis, respiratory infections like community-acquired pneumonia, gastrointestinal infections like cholera, skin conditions like acne, and genitourinary infections like chlamydia and syphilis.
In addition, tetracyclines can be used to treat rare infections like rocky mountain spotted fever, anthrax, lyme disease, and tularemia.
Tetracyclines are very effective against gram-positive bacteria like Bacillus anthracis and Clostridium spp, as well as some gram-negative bacteria like Shigella spp, Escherichia coli, Rickettsia spp, Borrelia burgdorferi, Helicobacter pylori, and Neisseria meningitidis, and finally some atypical bacteria like Mycoplasma pneumoniae, Chlamydia trachomatis, Vibrio cholerae, and Francisella tularensis.
However, some of these bacteria developed resistance against tetracyclines in time. So as a solution, tetracyclines were modified into a newer generation of antibiotics called glycylcyclines. These are commonly used in complicated skin infections and intra abdominal infections.
Glycylcyclines are very effective against some gram-positive bacteria like Streptococcus pyogenes, Clostridium perfringens, both methicillin susceptible and methicillin resistant Staphylococcus aureus; and gram-negative bacteria like Klebsiella pneumoniae.
Now, according to the duration of action and half-life, tetracyclines are divided into two groups. Short acting tetracyclines, like tetracycline itself, have a half-life of around 8 hours, while long acting ones like doxycycline and minocycline have a half-life of 16 to 22 hours.
These medications can be administered both orally or intravenously. On the other hand, glycylcyclines are only administered intravenously. The main drug in this category is tigecycline, which has a half-life of around 42 hours.
Once administered, both tetracyclines and glycylcyclines work by entering the bacteria and completely shutting down their protein synthesis.
This ultimately leads to a bacteriostatic effect, meaning they stop bacterial growth without killing the bacteria.
Now, common side effects associated with both the tetracyclines and glycylcyclines include headache and dizziness, as well as abdominal pain, nausea, vomiting, esophageal irritation and diarrhea.
In addition, they cause phototoxicity, so clients are much more likely to get sunburned. Some clients may also develop a skin rash or injection site reactions, as well as hypersensitivity reactions like exfoliative dermatitis, Stevens-Johnson syndrome, or anaphylaxis.
Another important side effect is superinfection due to fungal overgrowth in the mouth, most commonly Candida; or bacterial overgrowth in the bowel, which can rarely, but potentially, cause a Clostridioides difficile infection or CDI for short.
In addition, these medications can accumulate in teeth, causing permanent yellow or brown discoloration.
If taken during pregnancy after the fourth month of gestation, these medications can cause staining of the baby's deciduous teeth, but not their permanent teeth.
On the other hand, if these medications are used in children younger than 8 years, they can cause discoloration of permanent teeth.