Antibiotics and Antivirals
In 1928, Alexander Fleming at St Mary's Hospital London noticed that a mould had killed bacteria in one of his petri dishes, discovering penicillin, a drug that would go on to save an estimated 200 million lives.
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When you are sick with a cold, should you take antibiotics? Why or why not?
What do you think would happen if antibiotics stopped working against common infections?
● Know
- How antibiotics work against bacteria
- How antiviral drugs work against viruses
- The importance of proper medication use
● Understand
- Why antibiotics cannot treat viral infections
- Why completing a full course of antibiotics is important
- How different drugs target different pathogens
● Can do
- Explain why a doctor would prescribe antibiotics for one infection but not another
- Describe how antiviral drugs differ from antibiotics
- Demonstrate understanding of responsible medication use
In 1928, Alexander Fleming noticed that a mould called Penicillium notatum had dissolved a ring of bacteria on one of his petri dishes, accidentally discovering that certain chemicals could destroy bacterial cells without harming the surrounding material. That observation unlocked antibiotics: drugs that kill bacteria or stop them from reproducing by exploiting differences between bacterial cells and human cells. Because bacteria have structures that our cells lack, antibiotics can target those structures without harming us. The main mechanisms include: inhibiting cell wall synthesis (penicillin and cephalosporins), disrupting protein synthesis (tetracyclines and macrolides), interfering with DNA replication (fluoroquinolones), and disrupting bacterial metabolism (sulfonamides).
Broad-spectrum antibiotics work against many different types of bacteria, while narrow-spectrum antibiotics target specific groups. Broad-spectrum drugs are useful when the exact bacterium is unknown, but they also kill beneficial gut bacteria and create more opportunities for resistance to develop. Narrow-spectrum drugs are preferred when the pathogen has been identified.
Penicillin kills bacteria by preventing them from building strong cell walls. Human cells do not have cell walls, so penicillin leaves our cells unharmed. This is called selective toxicity, the drug harms the pathogen without harming the host.
The Australian Commission on Safety and Quality in Health Care runs antimicrobial stewardship programs in hospitals, ensuring doctors prescribe narrow-spectrum antibiotics whenever possible to slow the spread of resistant superbugs.
Antivirals are much harder to develop than antibiotics because viruses hijack the host cell own machinery. Since viruses use our cells to reproduce, any drug that attacks the virus risks attacking our own cells too. Scientists have developed several clever strategies. Entry inhibitors block viruses from entering cells. Nucleoside analogues trick the virus into incorporating fake building blocks during replication, halting reproduction. Neuraminidase inhibitors prevent newly formed viruses from escaping the host cell. Interferons boost the host own immune response.
Unlike antibiotics, which often eliminate bacterial infections completely, antivirals usually reduce symptoms and duration rather than wiping out the virus entirely. This is why vaccines are so important for viral diseases: prevention is far more effective than treatment.
Oseltamivir (Tamiflu) is a neuraminidase inhibitor used for influenza. It does not kill the flu virus directly. Instead, it prevents new virus particles from budding off infected cells, slowing the infection and giving the immune system time to catch up.
CSIRO researchers in Melbourne contributed to the development of antiviral strategies during the COVID-19 pandemic, including screening existing drugs for activity against SARS-CoV-2 and studying how the virus replicates in human cells.
- Blocking viral entry
- Inhibiting replication
- Preventing release
- Boosting immune response
- Interferons
- Entry inhibitors for HIV
- Nucleoside analogues like remdesivir
- Neuraminidase inhibitors like oseltamivir
Using antibiotics correctly is a shared responsibility between doctors and patients. The golden rules are simple but often ignored. Always complete the full antibiotic course, even if you feel better before the pills run out. Stopping early leaves surviving bacteria that may develop resistance. Never share medications or use leftover antibiotics for a new illness, the wrong antibiotic for the wrong bug does nothing except breed resistance. Never demand antibiotics for viral infections like colds and flu, antibiotics do not work on viruses, and every unnecessary prescription makes resistance more likely.
Antibiotic resistance is accelerated by misuse in both human medicine and agriculture. When farmers add antibiotics to animal feed to promote growth, resistant bacteria can spread to humans through meat and the environment. Responsible use everywhere is essential.
A patient feels better after three days of a seven-day antibiotic course and stops taking the pills. The strongest bacteria survive, multiply, and now require a stronger antibiotic next time. This is how resistance evolves in real time.
The Australian Veterinary Association and medical colleges jointly promote One Health antimicrobial stewardship, recognising that antibiotic resistance in farm animals eventually affects human hospitals.
Antibiotics and antivirals are not the only antimicrobial drugs. Antifungals treat fungal infections by disrupting fungal cell membranes or cell wall synthesis. They are essential for treating conditions like athlete foot, ringworm, and serious systemic fungal infections in immunocompromised patients. Antiparasitics treat infections caused by protists and worms. Antimalarials like artemisinin kill malaria parasites in the blood. Antihelminthics expel intestinal worms.
Every antimicrobial drug must be used responsibly to prevent resistance. Fungi, parasites, and viruses can all evolve resistance under selection pressure, just as bacteria do. The principle is universal: when a drug kills susceptible microbes but leaves resistant survivors, the resistant strain multiplies and eventually dominates.
Candida albicans is a yeast that normally lives harmlessly in the human gut. After a long course of broad-spectrum antibiotics kills competing bacteria, Candida can overgrow and cause thrush, a fungal infection that requires antifungal treatment.
The Burnet Institute in Melbourne researches new antiparasitic drugs for malaria and neglected tropical diseases, working with partners in Papua New Guinea and sub-Saharan Africa where these infections remain leading causes of death.
Wrong: "Antibiotics can cure any infection." No, antibiotics only work against bacteria. They are ineffective against viruses, fungi, and parasites. Using antibiotics for viral infections is ineffective and contributes to antimicrobial resistance.
Right: Antibiotics only work against bacterial infections. They are ineffective against viral infections like colds and flu, and unnecessary use drives antibiotic resistance.
Wrong: "It is okay to stop taking antibiotics once you feel better." No, stopping early allows the strongest bacteria to survive and multiply. These survivors may be resistant to the antibiotic, making future infections harder to treat.
Right: Always complete the full antibiotic course, even if you feel better. Stopping early allows surviving bacteria to multiply and may lead to antibiotic-resistant infections.
Wrong: "Antiviral drugs can cure viral infections completely." Usually No, most antivirals reduce symptoms and duration but do not eliminate the virus entirely. The immune system typically clears the virus. Some viruses (like HIV and herpes) persist lifelong despite antiviral treatment.
Right: Most antiviral drugs reduce symptoms and duration but do not eliminate the virus completely. The immune system typically clears the virus, and some viruses persist lifelong despite treatment.
Australian Antibiotic Use and Resistance
National Antimicrobial Resistance Strategy: Australia has a national strategy to combat antimicrobial resistance, coordinated by the Australian Commission on Safety and Quality in Health Care. The strategy promotes appropriate prescribing, surveillance of resistance patterns, and research into new treatments.
Antibiotic use in agriculture: Australia has relatively low use of antibiotics in livestock compared to many countries. The Australian Pesticides and Veterinary Medicines Authority regulates agricultural antibiotic use. However, imported meat from countries with high agricultural antibiotic use can contribute to resistant bacteria entering Australia.
Community antibiotic use: Australians are among the highest users of antibiotics in the developed world. Many prescriptions are for respiratory infections that are likely viral. The NPS MedicineWise program works to reduce unnecessary antibiotic prescribing through education for doctors and patients.
✍ Copy Into Your Books
▾Antibiotics
- Kill bacteria or stop them multiplying
- Target bacterial cell walls, ribosomes, or DNA
- Only work against bacteria, not viruses
Antivirals
- Interfere with viral replication
- Block entry, replication, or release of viruses
- Usually reduce symptoms rather than cure
Proper Use
- Complete the full course
- Do not share or use leftovers
- Do not use for viral infections
- Take antivirals early for best effect
Prescription Decisions
Antibiotic Stewardship
At the start of this lesson, you thought about how antibiotic-resistant infections could kill more people than cancer by 2050, yet millions of Australians still take antibiotics for the common cold, which is caused by a virus antibiotics can't touch.
Now that you've studied how antibiotics and antivirals work, can you explain exactly why taking antibiotics for a viral infection is useless, and actually dangerous? What do you think should change about how people use these drugs?
Q1. 1. Explain why antibiotics can cure bacterial infections but cannot treat viral infections. Use specific examples of how antibiotics target bacteria. 4 MARKS
Q2. 2. Describe the risks of misusing antibiotics (not completing courses, using leftovers, taking them for viral infections). Explain how each contributes to antimicrobial resistance. 4 MARKS
Q3. 3. Compare the mechanisms of action of antibiotics and antiviral drugs. Evaluate which is more challenging to develop and why. 4 MARKS
Revisit Your Thinking
Go back to your Think First answer. Has your understanding changed?
- Why is it important to use antibiotics and antiviral drugs responsibly?
- What could happen if antibiotics become ineffective due to resistance?
Model answers (click to reveal)
Answers
▾MCQ 1
BAntibiotics target features unique to bacteria, such as cell walls, bacterial ribosomes, and bacterial metabolic pathways, which human cells do not have.
MCQ 2
BViruses use the host cell's own machinery to replicate, so antiviral drugs must interfere with viral processes without harming human cells. This makes targeted treatment much more difficult.
MCQ 3
CAntibiotics only work against bacteria. Using them for viral infections is ineffective and contributes to antimicrobial resistance.
MCQ 4
BBactericidal antibiotics kill bacteria directly (e.g., penicillin). Bacteriostatic antibiotics stop bacteria from multiplying.
MCQ 5
BAntivirals for influenza (like oseltamivir/Tamiflu) work best when taken within 48 hours of symptom onset, as they prevent new virus particles from spreading.
Short Answer 1
Model answer: Antibiotics can cure bacterial infections because they target structures and processes that bacteria have but human cells lack. For example, penicillin inhibits bacterial cell wall synthesis, bacteria without strong cell walls burst and die, while human cells (which have no cell walls) are unaffected. Other antibiotics target bacterial ribosomes or bacterial DNA replication enzymes, which differ from human versions. Viruses, however, are not cells. They have no cell walls, ribosomes, or metabolic pathways of their own. They replicate inside host cells using the cell's machinery. Because viruses do not have the bacterial targets that antibiotics attack, antibiotics are completely ineffective against viral infections like colds, flu, and COVID-19.
Short Answer 2
Model answer: Misusing antibiotics in three ways contributes to antimicrobial resistance: (1) Not completing courses: When antibiotics are stopped early, the most susceptible bacteria are killed, but the strongest survivors remain. These survivors multiply, and their offspring may be resistant. (2) Using leftovers: Leftover antibiotics may not be the right type for the current infection. Using the wrong antibiotic applies selective pressure without killing the pathogen, allowing resistant bacteria to thrive. (3) Taking antibiotics for viral infections: Antibiotics have no effect on viruses, but they do affect the bacteria living in and on the body. This disrupts beneficial bacteria and creates opportunities for resistant strains to multiply and spread. Each misuse accelerates the evolution of antibiotic-resistant bacteria, making future infections harder to treat.
Short Answer 3
Model answer: Antibiotics work by targeting specific features of bacterial cells that human cells lack, such as cell walls (penicillin), bacterial ribosomes (tetracycline), or bacterial DNA replication enzymes (fluoroquinolones). Because bacteria are cells with their own machinery, there are multiple distinct targets to attack. Antiviral drugs must interfere with viral processes without harming host cells. This is more challenging because viruses use the host cell's own machinery to replicate. Antivirals typically work by blocking viral entry, inhibiting viral enzymes (like reverse transcriptase or neuraminidase), or preventing viral release. Developing antivirals is more difficult because: there are fewer viral-specific targets; the targets often resemble human cellular processes; and achieving effective concentrations inside cells without toxicity is challenging. Consequently, fewer antiviral drugs exist compared to antibiotics, and most reduce rather than cure viral infections.