Antibiotics and Bacterial Resistant Bacteria
antibiotics tablets work to kill bacteria, but they can also harm your good, healthy bacteria. Overusing antibiotics can promote the growth of resistant bacteria, making future infections harder to treat.
Only use antibiotics when prescribed and follow all treatment instructions. It is tempting to stop taking them as soon as your symptoms improve, but you must finish the full course of medication to prevent resistant bacteria from developing.
Penicillins
Penicillins kill bacteria by attaching to them and damaging their cell walls. Over time, some bacteria have learned to avoid being damaged by penicillins. These resistant bacteria are called gram-negative and are harder to treat. Scientists have developed new antibiotics that work in different ways to treat gram-negative infections.
Alexander Fleming discovered the first naturally occurring penicillin in 1928 from a mold called Penicillium notatum. All modern penicillins are semi-synthetic (they are made by modifying the original natural penicillins). The modification changes the antibacterial spectrum and pharmacologic properties of the drug. The core penicillin nucleus, a 4-membered b-lactam ring, is unchanged, but the side chain differs from one drug to the next. These differences are determined by the penicillium strain used to produce the drug, and a number of other factors.
The newest semi-synthetic penicillins are the fourth generation and include ampicillin, ticarcillin, carbenicillin, azlocillin, mezlocillin, piperacillin, and tazobactam. The addition of a beta-lactamase inhibitor increases the activity of these drugs against methicillin-resistant Staphylococcus aureus and other bacteria that produce b-lactamases.
The benzylpenicillins are used to prevent and treat syphilis, and the oxacillins are effective for preventing and treating infections caused by group A streptococci. Penicillins are considered safe to take during pregnancy. They enter breast milk in small amounts and have few side effects, other than allergic reactions.
Aminoglycosides
The aminoglycosides are a broad-spectrum class of antibiotics that act by binding to the 30S subunit of bacterial ribosomes, inhibiting protein synthesis and killing bacteria. They have been the cornerstone of antibacterial therapy since streptomycin (Fig. 1), a compound from the bacterium Streptomyces griseus, was first isolated and introduced into clinical use in 1944. The class includes the oxygen-dependent agents kanamycin, tobramycin, gentamicin, and amikacin. Streptomycin is distinguished from the other members of the group by its lack of the 2-deoxystreptamine moiety found in kanamycin, tobramycin, and gentamicin.
The antibacterial activity of the aminoglycosides is concentration-dependent, with higher drug concentrations providing greater bactericidal activity. Because of their poor absorption from the gastrointestinal tract, they are administered parenterally. They distribute well into extracellular fluids and are concentrated in the pleural cavity, joints, peritoneal space, and denuded skin. Concentrations of the aminoglycosides in renal tubular cells can be high enough to cause nephrotoxicity.
Aminoglycosides are used in the treatment of infections caused by multidrug resistant Gram-negative pathogens and for empiric treatment of severe sepsis, complicated intraabdominal infections, and nosocomial pneumonia, particularly when other drugs are not readily available. Family physicians should consider the potential for nephrotoxicity, ototoxicity, and neuromuscular blockade when prescribing these medications. They should also ensure that patients are receiving adequate doses of the medication to achieve therapeutic blood levels.
Quinolones
Quinolones are a broad-spectrum group of bacteriocidal antibiotics derived from the naphthyridine family. The first member of the group, nalidixic acid (Nalix), is not currently available in veterinary medicine, but other members of the family—ciprofloxacin (Cipro), marbofloxacin (Bactrim), and levofloxacin (Levaquin)—are important antibacterial agents used in both human and veterinary medicine. They are effective against a wide variety of organisms, have a good oral bioavailability, and excellent tissue penetration. The newer fluoroquinolones have expanded their activity against gram-negative organisms and also act against anaerobes. These drugs have been classified as first-generation, second-generation, and third-generation medications based on their pharmacokinetic properties and antimicrobial spectrum.
One of the most significant issues with quinolones is their ability to cause resistance. Bacteria exposed to sublethal concentrations of quinolones undergo the SOS response, a general stress response that activates DNA repair pathways that can introduce mutations. In particular, quinolones are potent inducers of the error-prone SOS DNA repair pathway mediated by RecA and LexA.
How quinolones kill bacteria is not completely understood, but it involves the binding of the drug to the bacterial topoisomerase-DNA complex and the subsequent cleavage of the complex, which results in breaks in the DNA that cannot be resealed by the topoisomerase. In addition, quinolones bind to and inhibit protein synthesis, which can result in inhibition of cell growth. These effects together can lead to slow killing of the bacteria, which is seen at quinolone concentrations that are twice the minimum inhibitory concentration (MIC).
Other Antibiotics
Bacteria are microscopic germs that live inside and on your body, as well as all around you. Most types of bacteria do not hurt you and, in fact, some (such as those in your digestive tract and on your skin) help keep you healthy. But some bacteria can cause infections that your immune system can't fight on its own. Antibiotics treat bacterial infections by killing the bacteria or making it hard for them to grow.
A large number of antibiotics work by interfering with bacterial cell wall production. These are called b-lactam antibiotics. Some interfere with the formation of the bacterial cell wall itself, while others prevent synthesis of proteins that are needed to make the bacterium's shape and cell contents stable. Antibiotics can be taken orally, in the form of pills, liquids or capsules. They can also be applied to the skin in the form of creams, ointments or sprays. They can also be given by injection or intravenously.
The overuse of antibiotics means that bacteria are becoming resistant to them. This makes it harder for doctors to prescribe them when you really need them and can lead to serious, life-threatening infections that are difficult or impossible to treat. The rise of resistant bacteria is a global health crisis and requires urgent action. It is threatening to undermine modern medicine, delaying essential cancer treatments, caesarean sections and hip replacements and jeopardising the future of agriculture, animal husbandry and horticulture.
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