thumb|A colored electron microscopy image of methicillin-resistant staphylococcus aureus ([[Methicillin-resistant Staphylococcus aureus|MRSA), a bacterium commonly targeted by broad-spectrum antibiotics]]
A broad-spectrum antibiotic is an antibiotic that acts on the two major bacterial groups, Gram-positive and Gram-negative, or any antibiotic that acts against a wide range of disease-causing bacteria. These medications are used when a bacterial infection is suspected but the group of bacteria is unknown (also called empiric therapy) or when infection with multiple groups of bacteria is suspected. This is in contrast to a narrow-spectrum antibiotic, which is effective against only a specific group of bacteria. Although powerful, broad-spectrum antibiotics pose specific risks, particularly the disruption of native, normal bacteria and the development of antimicrobial resistance. An example of a commonly used broad-spectrum antibiotic is ampicillin.
Empiric antibiotic therapy
thumb|395x395px|Simplified diagram showing common disease-causing bacteria and the antibiotics which act against them.
Empiric antibiotic therapy refers to the use of antibiotics to treat a suspected bacterial infection despite lack of a specific bacterial diagnosis. Definitive diagnosis of the species of bacteria often occurs through culture of blood, sputum, or urine, and can be delayed by 24 to 72 hours. Antibiotics are generally given after the culture specimen has been taken from the patient in order to preserve the bacteria in the specimen and ensure accurate diagnosis. As a side-effect of therapy, antibiotics can change the body's normal microbial content by attacking indiscriminately both the pathological and naturally occurring, beneficial or harmless bacteria found in the intestines, lungs and bladder. The destruction of the body's normal bacterial flora is thought to disrupt immunity, nutrition, and lead to a relative overgrowth in some bacteria or fungi. An overgrowth of drug-resistant microorganisms can lead to a secondary infection such as Clostridioides difficile ("C. diff") or candidiasis ("thrush"). although a later study indicated a link between acne vulgaris and IBS irrespective of the use of antibiotics. Likewise, the use of minocycline in acne vulgaris has been associated with skin and gut dysbiosis.
Examples of broad-spectrum antibiotics
In humans:
- Doxycycline
- Minocycline
- Aminoglycosides (except for streptomycin)
- Ampicillin
- Amoxicillin/clavulanic acid (Augmentin)
- Azithromycin
- Carbapenems (e.g. imipenem)
- Piperacillin/tazobactam
- Quinolones (e.g. ciprofloxacin)
- Tetracycline-class drugs (except sarecycline)
- Chloramphenicol
- Ticarcillin
- Trimethoprim/sulfamethoxazole (Bactrim)
- Ofloxacin
In veterinary medicine, co-amoxiclav, (in small animals); penicillin & streptomycin and oxytetracycline (in farm animals); penicillin and potentiated sulfonamides (in horses).