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What are antibiotics and how do they work?

Antibiotics also called antibacterials or antimicrobials are types of medications that impede the growth of bacteria. Bacteria are microscopic organisms, some of which may cause various types of illnesses.

Before bacteria can multiply and cause symptoms, the body’s immune system can destroy them. The body’s white blood cells attack harmful bacteria and kill them. Even if symptoms re occur, the body’s immune system can usually fight off the infection. However, on occasions, when the burden is too much, help is needed from “antibiotics”.

The first antibiotic was penicillin, discovered by a Scottish scientist Dr. Alexander Fleming from a mold culture. Today, over 100 different antibiotics are available to cure minor, and life-threatening infections.

Although antibiotics are useful to treat a range of infections, their effectiveness is confined to only bacterial infections. They are not effective against viral infections (for example, the common cold) and fungal infections (such as ringworm).

If afflicted by an infection, it is vital to know whether the infection is caused by bacteria or a virus. Most upper respiratory tract infections, such as the common cold, flu, sore throats, bronchitis and many sinus and ear infections are generally caused by viruses – antibiotics do not work against these viruses.

Symptom relief is the best treatment option for viral infections. Rest, fluids, and over-the-counter products may be the best treatment option against viral infections.

Types of antibiotics

Antibiotics are usually grouped together based on how they work.

A bactericidal antibiotic kills the bacteria. Penicillin is a bactericidal.A bactericidal usually either interferes with the formation of the bacterium’s cell wall or its cell contents.

A bacteriostatic stops bacteria from multiplying.

A broad-spectrum antibiotic is effective against a wide range of infections. A narrow-spectrum antibiotic on the other hand is effective only against a few types of bacteria. There are antibiotics that attack aerobic bacteria, while others work against anaerobic bacteria. Aerobic bacteria need oxygen, while anaerobic bacteria do not.

Antibiotics may be given beforehand, to prevent infection, as might be the case before surgery. This is called ‘prophylactic’ use of antibiotics. They are commonly used before bowel and orthopedic surgery.


Penicillins- for example, phenoxymethylpenicillin, flucloxacillin and amoxicillin.
Cephalosporins- for example, cefaclor, cefadroxil and cefalexin.
Tetracyclines- for example, tetracycline, doxycycline and lymecycline.
Aminoglycosides- for example, gentamicin and tobramycin.
Macrolides- for example, erythromycin, azithromycin and clarithromycin.
Sulfonamides and trimethoprim- for example, co-trimoxazole.
Metronidazole and tinidazole.
Quinolones- for example, ciprofloxacin, levofloxacin and norfloxacin


Each antibiotic is effective only against certain bacteria and parasites.You need antibiotics if you have certain serious infections caused by bacteria such as meningitis or pneumonia. In these situations, antibiotics are often life-saving. Antibiotics can also be prescribed to treat acne – a less serious condition. For acne, antibiotics can be taken by mouth or applied directly to the skin

Antibiotic Resistance

One of the main concerns of modern medicine is a condition termed as “antibiotic resistance”. If an antibiotic is overused, bacteria will emerge with new form and structure (mutation) which becomes resistant to that antibiotic.

One of the reasons for the development of antibiotic-resistant bacteria includes the common practice of prescribing antibiotics for the common cold or flu.If your sickness is because of a virus, taking antibiotics may not help the cause and can on the other hand cause harm. Also using antibiotics frequently, even when you don’t need them can add to antibiotic resistance.

There is growing concern globally regarding the overuse of antibiotics. This has led to the growing number of bacterial infections which are becoming resistant to antibacterial medications.

The ECDC (European Centre for Disease Prevention and Control) has come out with a report which says antibiotic resistance continues to be a serious public health threat globally. In a statement issued on 19th November 2012, the ECDC informed that about 25,000 people die each year in the European Union from antibiotic-resistant bacterial infections.

We are at a critical juncture where antibiotic resistance is seen as a global threat to public health.
However, efforts are on to distinguish clearly between a bacterial and viral infection through various screening tests thus helping in more accurate antibiotic prescription.


What are the side-effects of antibiotics?

Common side-effects of antibiotics include:

Feeling and being sick.
Fungal infections of the mouth, digestive tract and vagina.

Some of the rare side-effects include:

Formation of kidney stones (when taking sulphonamides).
Abnormal blood clotting (when taking some cephalosporins).
Sensitivity to sun (when taking tetracyclines).
Blood disorders (when taking trimethoprim).
Deafness (when taking erythromycin and aminoglycosides).

Allergic reactions to antibiotics

It is possible that some patients develop allergic reaction to antibiotics, particularly penicillins. Side effects may be developing a rash, swelling of the tongue and face, and difficulty breathing.

Antibiotics should be used with extreme caution and consult your doctor if:

You have reduced liver or kidney function.
You are breastfeeding
You are pregnant

Penicillins, cephalosporins, and other antibiotics may reduce the impact of oral contraceptives. If the antibiotic has caused diarrhea/vomiting the absorption of contraceptives may also be disrupted. If you are taking any of these drugs you should consider taking additional contraceptive precautions.

Blood test can help find out the patients who are in need of antibiotics

It would be difficult for you to take a course of antibiotics in the case of the prior history of antibiotic allergy. However, doctors choose a different type of antibiotic if you have had an allergic reaction to one antibiotic.

Some people are allergic to penicillin. If you have a query about a possible allergic reaction, ask your doctor or pharmacist before taking the medicine.

Allergic reactions commonly have the following symptoms:

  • Shortness of breath
  • Rash
  • Hives
  • Itching
  • Swelling of the lips, face, or tongue
  • Fainting

How to use antibiotics

One should take the complete course of antibiotics prescribed. Even though one may feel better before completing the course of medicine, one must follow through and take the entire course. If an antibiotic is stopped in midcourse, the bacteria may be partially treated and not completely killed, causing the bacteria to be resistant to the antibiotic. This can cause a serious problem if those now-resistant bacteria grow enough to cause a re-infection.

How should the medication be taken?

Antibiotics are usually taken orally( by mouth); however, they can also be administered by injection or applied directly to the affected part of the body( topical ). Some antibiotics should not be taken along with certain foods and drinks. Others should not be taken with food in your stomach – these would normally be taken about an hour before meals, or two hours after. If you are taking metronidazole do not consume alcohol. Dairy products should not be consumed if you are taking tetracyclines, as they might affect the absorption of the medication.

Some medications are taken along with a glass of milk while others only with water.Taking your antibiotics incorrectly affects their absorption, reducing or eliminating their effectiveness. Even if you are pregnant or breastfeeding there are a number of antibiotics that are thought to be safe to take. Antibiotics (other than one called rifampicin) do not interfere with the effectiveness of birth control pill. You should continue taking your pill as normal if you also need to take any antibiotics.

Some antibiotics may interact with other medicines that you might take. This may cause reactions, or reduce the effectiveness of one or other of the treatments. So, when you are prescribed an antibiotic you should tell a doctor if you take other medicines. All this goes to prove that the mode and method of administering the drug should be meticulously followed for proper results.

Most side-effects of antibiotics are not serious. Common side-effects include soft stools (faeces), diarrhea, or mild stomach upset such as feeling sick (nausea).

Notify your doctor if you have any of the following side effects:

  • Vomiting
  • Severe watery diarrhea and abdominal cramps
  • Allergic reaction (shortness of breath, hives, swelling of lips, face, or tongue, fainting)
  • Rash
  • Vaginal itching or discharge
  • White patches on the tongue

There are other factors that influence the choice of an antibiotic. These include:

  1. How severe the infection is.
  2. How well your kidneys and liver are working.
  3. Dosing schedule.
  4. Other medications you may be taking.
  5. Common side-effects.
  6. A history of having an allergy to a certain type of antibiotic.
  7. If you are pregnant or breastfeeding.

Can I buy antibiotics OTC (Over-the-counter)?
No, they can be bought from the chemist only on prescription by a qualified doctor.


It is also vital that you store your medication appropriately. Many antibiotics aimed at children need to be refrigerated (amoxicillin), while others can be stored at room temperature (Biaxin).


Written by MedPlus

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    very informative.
    Thank you and keep going, you do a great job!!