Antibiotics: Complete Guide to Types, Uses & Resistance

What Are Antibiotics and How Do They Work?

Antibiotics are medicines that fight bacterial infections by either killing bacteria (bactericidal) or stopping their growth (bacteriostatic). They work through various mechanisms such as disrupting bacterial cell wall synthesis, inhibiting protein production, or interfering with DNA replication. Antibiotics have no effect on viral infections.

In everyday language, antibiotics are often simply called "penicillin," but penicillin is just one of many different types of antibiotics, although it remains the most commonly used. The term antibiotic literally means "against life" and refers to substances that can kill or inhibit the growth of microorganisms, specifically bacteria.

Antibiotics revolutionized medicine when Alexander Fleming discovered penicillin in 1928. Before antibiotics, bacterial infections like pneumonia, tuberculosis, and wound infections were often fatal. Today, antibiotics save millions of lives every year by treating infections that would otherwise be deadly or cause serious complications.

However, antibiotics are not effective against all types of infections. They work specifically against bacteria and have no effect on viral infections such as the common cold, flu, most sore throats, bronchitis, or COVID-19. Taking antibiotics for viral infections is not only ineffective but contributes to the serious problem of antibiotic resistance.

Narrow-Spectrum vs Broad-Spectrum Antibiotics

There are many different types of bacteria, and most antibiotics only affect some of these bacterial species. Antibiotics that affect many different bacteria types are called broad-spectrum antibiotics. Antibiotics that only affect a limited number of bacteria types have a narrow spectrum.

Broad-spectrum antibiotics also affect the beneficial bacteria that normally live in your body, particularly in your gut and other areas. This is why doctors generally prefer to use narrow-spectrum antibiotics that specifically target the bacteria causing your infection while leaving the beneficial bacteria largely intact. When broad-spectrum antibiotics are necessary, you may experience more side effects related to the disruption of your normal bacterial balance.

How Antibiotics Kill or Stop Bacteria

Antibiotics work through several different mechanisms to combat bacterial infections. Understanding these mechanisms helps explain why certain antibiotics are chosen for specific infections and why they have different side effects:

• Cell wall synthesis inhibition: Antibiotics like penicillins and cephalosporins prevent bacteria from building their protective cell walls, causing them to burst and die. Human cells don't have cell walls, which is why these antibiotics don't harm our cells.
• Protein synthesis inhibition: Macrolides and tetracyclines block the bacterial machinery that produces proteins, preventing bacteria from growing and multiplying.
• DNA synthesis inhibition: Fluoroquinolones interfere with bacterial DNA replication and repair, stopping bacteria from reproducing.
• Metabolic pathway disruption: Sulfonamides block essential metabolic processes that bacteria need to survive.
• Cell membrane disruption: Some antibiotics damage the bacterial cell membrane, causing contents to leak out.

Do You Always Need Antibiotics for an Infection?

No, many infections heal without antibiotics. Your body's immune system can fight off most common infections on its own. A doctor's key task is to determine whether an infection requires antibiotic treatment or if it will resolve naturally. Taking unnecessary antibiotics harms your beneficial bacteria and contributes to resistance.

Your body is remarkably capable of fighting off many infections without medical intervention. The immune system has evolved over millions of years to detect and destroy foreign invaders, including many types of bacteria. For many common respiratory infections, the body will successfully clear the infection within several days to a couple of weeks.

When you visit a doctor for an infection, one of their most important tasks is to assess whether you actually need antibiotics. This decision is based on several factors including the type of infection, its severity, your overall health status, and whether there's a risk of complications or the infection spreading.

For many common viral respiratory infections, antibiotics will not speed up your recovery because these infections are caused by viruses, not bacteria. In these cases, rest, adequate fluid intake, and managing symptoms with appropriate over-the-counter medications is the recommended approach. Taking antibiotics when they're not needed exposes you to potential side effects and contributes to the global problem of antibiotic resistance.

When Antibiotics Are Necessary

While many infections don't require antibiotics, certain infections absolutely should be treated with appropriate antibiotics. These include serious bacterial infections where the risk of complications or the infection spreading outweighs any concerns about antibiotic use. You should never hesitate to take antibiotics when your doctor has determined that you need them.

Conditions that typically require antibiotic treatment include:

• Bacterial pneumonia: A serious lung infection that can become life-threatening without treatment
• Urinary tract infections: Especially if symptoms are severe or if the infection has spread to the kidneys
• Strep throat: Caused by group A Streptococcus bacteria, which can lead to rheumatic fever if untreated
• Bacterial meningitis: A medical emergency requiring immediate antibiotic treatment
• Bacterial skin infections: Such as cellulitis or infected wounds
• Sexually transmitted bacterial infections: Such as chlamydia, gonorrhea, and syphilis
• Lyme disease: Caused by tick-borne Borrelia bacteria

What Is Antibiotic Resistance and Why Is It Dangerous?

Antibiotic resistance occurs when bacteria evolve to survive antibiotic treatment. This happens through genetic mutations or by acquiring resistance genes from other bacteria. Resistant infections are harder to treat, require stronger antibiotics, and can become untreatable. The WHO considers antibiotic resistance one of the top ten global public health threats.

Excessive and improper use of antibiotics has led to certain bacteria developing resistance to one or more types of antibiotics. When we say bacteria have become "resistant," we mean they have acquired the ability to survive and continue growing even in the presence of antibiotics that would normally kill them. It's important to understand that resistance is a property of the bacteria, not the person being treated. A person taking antibiotics cannot become "resistant" to them.

For many decades, new antibiotics were continuously developed that could treat infections caused by resistant bacteria. However, the pace of new antibiotic development has slowed dramatically while resistance has continued to spread. Many countries now face serious problems with bacteria that are resistant to nearly all available antibiotics, creating infections that are extremely difficult or sometimes impossible to treat.

The World Health Organization has declared antibiotic resistance one of the greatest threats to global health, food security, and development. In 2019, antibiotic-resistant bacterial infections directly caused an estimated 1.27 million deaths worldwide and contributed to 4.95 million deaths. Without effective action, this number is projected to rise dramatically in the coming decades.

How Bacteria Develop Resistance

Bacteria can develop resistance to antibiotics through several mechanisms. Understanding these helps explain why proper antibiotic use is so crucial:

• Random genetic mutations: When bacteria reproduce, small genetic changes can occasionally provide protection against antibiotics. If the bacteria then encounters the antibiotic, the resistant bacteria survive while others die, allowing the resistant strain to multiply.
• Gene transfer between bacteria: Bacteria can share genetic material, including resistance genes, with other bacteria. This horizontal gene transfer can rapidly spread resistance between different bacterial species.
• Selection pressure: When antibiotics are present, any bacteria with even slight resistance have a survival advantage. Over time, this selects for increasingly resistant bacteria.

Spread of Resistant Bacteria

The risk of spreading resistant bacteria is highest in environments where antibiotics are heavily used and where many sick people are present. This applies particularly to hospitals but also to some extent to daycare centers and care homes for the elderly.

For a healthy person, being exposed to resistant bacteria is usually not dangerous. However, for someone who is seriously ill or has undergone surgery, an infection that is difficult to treat can be very serious or even life-threatening. Travel also plays a significant role in spreading antibiotic resistance, as people can carry resistant bacteria acquired abroad back to their home countries.

What Are the Different Types of Antibiotics?

The main classes of antibiotics include penicillins (most common), cephalosporins, tetracyclines, macrolides, fluoroquinolones, and sulfonamides. Each class works differently and targets different bacteria. Your doctor selects the appropriate antibiotic based on the infection type, location, and the suspected or confirmed bacteria involved.

Choosing the right antibiotic is a critical medical decision. The most important factor is that the antibiotic must be effective against the specific bacteria causing your infection. Doctors preferentially select narrow-spectrum antibiotics when possible. However, if you are very ill or if the exact bacteria causing the infection is unclear, broad-spectrum antibiotics may be necessary. Sometimes multiple antibiotics are needed simultaneously.

The doctor must also consider where in your body the infection is located. Not all antibiotics reach all parts of the body effectively. For example, not all antibiotics can cross into the brain, so they cannot be used to treat meningitis. Other factors that influence antibiotic choice include any other medical conditions you have, other medications you're taking that might interact with certain antibiotics, your age, and the risk of specific side effects.

Penicillins - The First and Most Common Antibiotics

Penicillins are the most widely used class of antibiotics. This group includes several different active substances, with the most common being phenoxymethylpenicillin (penicillin V). Penicillin V has a narrow spectrum, meaning it is effective against a limited number of bacterial types. It is used as first-line treatment for conditions including strep throat, sinusitis, ear infections, pneumonia, and Lyme disease.

Amoxicillin is another penicillin that, unlike penicillin V, works against more types of bacteria. It is used for more complicated ear infections and lower respiratory tract infections. In some medications, amoxicillin is combined with clavulanic acid, which protects the amoxicillin from being destroyed by certain bacteria. Flucloxacillin is a penicillin often used for skin infections, and pivmecillinam is frequently used for urinary tract infections.

Do not use penicillins if you know you are allergic to these medications or to antibiotics called cephalosporins, as there can be cross-reactivity between these related drug classes.

Cephalosporins - Broad-Spectrum Related to Penicillins

Cephalosporins are broad-spectrum antibiotics, meaning they are effective against many different types of bacteria. They are closely related to penicillins and share some structural similarities. The cephalosporin class includes many different active substances, and which bacteria they affect varies somewhat between different cephalosporins.

Cephalosporins may be prescribed for urinary tract infections, recurrent strep throat, and sometimes other respiratory infections. As with penicillins, you should not use cephalosporins if you are allergic to them or to penicillins.

Tetracyclines - Broad-Spectrum with Sun Sensitivity

Tetracyclines work against a large number of different bacteria. They are used for certain types of pneumonia (particularly those caused by atypical bacteria like Mycoplasma), genital infections, and sinusitis when regular penicillin has not worked or if you are allergic to regular penicillin. Certain tetracyclines can also be used to treat acne.

Tetracyclines should not be given to children under eight years old because the active substance is taken up by growing bones and can cause discoloration of tooth enamel. There is a risk of skin reactions if you are exposed to strong sunlight while taking tetracyclines, so you should usually avoid sun and tanning beds throughout the course and for two to five days after completing the course.

Macrolides - Alternative for Penicillin Allergy

Macrolides affect essentially the same bacteria as penicillins but also have effectiveness against infections caused by Mycoplasma bacteria, such as certain types of pneumonia. Macrolides are sometimes used instead of penicillin in people with penicillin allergy.

Common macrolides include azithromycin, erythromycin, clarithromycin, and roxithromycin. Macrolides can cause gastrointestinal side effects including nausea, vomiting, and stomach pain.

Fluoroquinolones - Reserved for Serious Infections

Fluoroquinolones (also called quinolones) are broad-spectrum antibiotics that work against many different types of bacteria. They are used for severe urinary tract infections, gonorrhea, prostate infections, and intestinal infections. They can also be used for certain respiratory infections, preventively against meningitis, and in cases of sepsis (blood poisoning).

Children and adolescents generally should not receive fluoroquinolones except in exceptional cases because these medications can cause joint problems. Fluoroquinolones also carry a risk of tendon damage, particularly affecting the Achilles tendon, and this risk is higher in older adults and those taking corticosteroids.

Trimethoprim and Sulfonamides

You may receive trimethoprim alone or trimethoprim in combination with sulfonamides (sometimes called sulfa drugs). Trimethoprim is used for uncomplicated urinary tract infections. The combination of trimethoprim and sulfonamides is used primarily for more severe urinary tract infections but can also sometimes be used for respiratory infections.

If you develop a rash, itching, or high fever while taking these medications, you should immediately stop the treatment and contact healthcare, as severe allergic reactions can cause breathing difficulties.

How Should You Take Antibiotics Correctly?

Take antibiotics exactly as prescribed: the correct dose, at the right times, for the full duration. Complete the entire course even if you feel better early. Some antibiotics should be taken with food, others on an empty stomach. Avoid alcohol with certain antibiotics. Never share or save leftover antibiotics.

When you start taking antibiotics, it takes some time before all the bacteria causing your infection have been eliminated. You often start feeling better after just a few days, but this doesn't mean all the bacteria are gone. It is crucial that you take the antibiotics for as long as your doctor has prescribed, even if you feel well before that.

If you stop taking antibiotics too early, some bacteria may survive and begin to multiply again. The infection can then return, potentially stronger than before, and the surviving bacteria may have developed resistance to the antibiotic.

Common Reasons Why Antibiotics Don't Work

The most common reason why you don't get better despite taking antibiotics is that the infection is not caused by bacteria but by a virus. Viral infections will not respond to antibiotic treatment. Other reasons why antibiotics might not work include:

• You received the wrong type of antibiotic for your specific infection
• The dose was too low for your infection
• You were treated for too short a time
• The bacteria are resistant to the antibiotic prescribed
• The antibiotic cannot reach the site of infection effectively

If you're not improving after several days of antibiotic treatment, contact your doctor. They may need to reconsider the diagnosis, change the antibiotic, or perform additional tests.

Food, Drink, and Drug Interactions

Different antibiotics have different requirements regarding food intake. Some antibiotics are better absorbed when taken with food, while others should be taken on an empty stomach. Check the package information or ask your pharmacist about your specific antibiotic.

Some antibiotics interact with other medications, reducing the effectiveness of either the antibiotic or the other medication. Always inform your doctor about all medications you take, including over-the-counter drugs and supplements. Certain antibiotics, particularly metronidazole and some cephalosporins, interact badly with alcohol and can cause severe nausea and vomiting.

What Are the Common Side Effects of Antibiotics?

Common side effects include digestive issues (diarrhea, nausea, stomach pain), yeast infections, and allergic reactions (rash, itching). Side effects occur because antibiotics also affect beneficial bacteria in your body. Severe allergic reactions are rare but require immediate medical attention. Contact your doctor if you experience hives, facial swelling, or breathing difficulties.

All antibiotics reduce the number of bacteria in your body - both the harmful bacteria causing your infection and the beneficial bacteria that normally live in your gut and other areas. This disruption of your normal bacterial balance is the underlying cause of many antibiotic side effects.

Reduced beneficial bacteria in the gut can cause digestive problems including loose stools and diarrhea. Reduced beneficial bacteria in the genital area can lead to yeast infections, which are particularly common in women taking antibiotics. These side effects are generally more pronounced with broad-spectrum antibiotics that affect a wider range of bacteria.

Side Effects by Antibiotic Type

Penicillins can cause nausea and vomiting. Skin rashes sometimes occur during treatment and may be due to the infection itself or be a side effect of the medication. However, if you develop hives, severe itching, fever, or facial swelling, you should stop the treatment and contact a doctor immediately. The same applies if you experience prolonged or severe diarrhea.

Cephalosporins share many of the same side effects as penicillins, including gastrointestinal upset and the potential for allergic reactions. Yeast infections are also common with these broad-spectrum antibiotics.

Tetracyclines can cause nausea and vomiting, and have the unique side effect of making your skin more sensitive to sunlight. Avoid direct sun exposure and tanning beds during treatment and for several days afterward.

Macrolides are particularly known for causing gastrointestinal side effects including nausea, vomiting, and stomach pain. These effects are due to the medications' impact on gut motility.

Fluoroquinolones have unique risks including tendon inflammation and rupture, particularly the Achilles tendon. They can also cause neurological effects and should be used with caution in older adults.

Can You Take Antibiotics During Pregnancy and Breastfeeding?

Talk to a doctor if you need to take antibiotics during pregnancy or while breastfeeding. Some antibiotics are considered safe, while others should be avoided. The risks and benefits must be carefully weighed. Untreated bacterial infections can also harm the pregnancy, so treatment decisions require professional medical guidance.

If you are pregnant or breastfeeding and develop a bacterial infection, it is essential to discuss treatment options with your doctor. Certain antibiotics are considered safe to use during pregnancy and breastfeeding, while others should be avoided because they can affect fetal development or pass into breast milk.

Penicillins are generally considered safe during pregnancy and are often the first choice for treating bacterial infections in pregnant women. Cephalosporins are also usually considered safe. However, tetracyclines should be avoided during pregnancy and in breastfeeding mothers because they can affect bone and tooth development in the baby.

The decision to treat a bacterial infection during pregnancy involves weighing the risks of the medication against the risks of leaving the infection untreated. Untreated infections can also harm both the mother and the baby, so it's important not to simply avoid all antibiotics during pregnancy. Your doctor will help you make the right decision for your specific situation.

How Do Antibiotics Affect Your Gut Microbiome?

Antibiotics disrupt the trillions of beneficial bacteria living in your gut (microbiome). This disruption can cause digestive issues, increase susceptibility to other infections, and may have longer-term health effects. The microbiome usually recovers within weeks to months, but some changes may persist longer. Probiotics may help restore balance.

Your gastrointestinal tract is home to trillions of bacteria, viruses, and fungi collectively known as the microbiome or microbiota. This complex ecosystem plays crucial roles in your health, including digestion, immune function, and even mental health. When you take antibiotics, they affect not only the bacteria causing your infection but also these beneficial gut bacteria.

The disruption of the microbiome is why digestive side effects are so common with antibiotic use. It also increases your susceptibility to certain infections while taking antibiotics - for example, Clostridioides difficile (C. diff) infection, which can cause severe diarrhea and colitis, primarily occurs after antibiotic use has disrupted the normal gut bacteria.

Research suggests that the microbiome generally recovers within a few weeks to months after completing antibiotic treatment. However, some studies indicate that certain changes may persist for longer periods. The degree of disruption depends on the type of antibiotic (broad-spectrum antibiotics cause more disruption), the duration of treatment, and individual factors.

Supporting Microbiome Recovery

While evidence is still evolving, some strategies may help support microbiome recovery during and after antibiotic treatment:

• Probiotics: Some studies suggest that probiotics (beneficial bacteria supplements) may help reduce antibiotic-associated diarrhea and support faster microbiome recovery
• Dietary fiber: Eating a diet rich in fiber provides food for beneficial gut bacteria and supports their recovery
• Fermented foods: Foods like yogurt, kefir, sauerkraut, and kimchi contain live beneficial bacteria
• Avoid unnecessary antibiotics: The best way to protect your microbiome is to only take antibiotics when truly necessary