Antibiotics

· Antibiotics = drugs that kill bacteria or inhibit bacterial growth.
· They are used to treat bacterial infections, not viral infections.
· Antibiotics are effective because they target structures or processes found in bacteria, e.g. peptidoglycan cell walls.
· Overuse and misuse of antibiotics increases antibiotic resistance, making infections harder to treat.

Penicillin: how it acts on bacteria

· Penicillin is a β-lactam antibiotic.
· It acts on bacterial cell wall synthesis.
· Bacterial cell walls contain peptidoglycan, which gives the bacterium strength and rigidity.
· Penicillin inhibits enzymes involved in forming cross-links between peptidoglycan chains.
· These enzymes are often called penicillin-binding proteins (PBPs) / transpeptidases.
· Without cross-links, the bacterial cell wall becomes weak.
· Water enters the bacterium by osmosis, causing the cell to swell and burst.
· Penicillin is most effective against actively growing/dividing bacteria, because they are building new cell walls.

This diagram shows how penicillin blocks enzymes that cross-link peptidoglycan in bacterial cell walls. Without strong cross-linking, the wall weakens and the bacterium can burst due to osmotic water entry. Source

Why antibiotics do not affect viruses

· Viruses are non-cellular and do not have the same cellular structures as bacteria.
· Viruses do not have a peptidoglycan cell wall, so penicillin has no target.
· Viruses do not carry out their own normal cellular metabolism in the same way as bacteria.
· Viruses replicate inside host cells, using host cell machinery.
· Therefore, antibiotics that target bacterial cell walls, bacterial ribosomes, or bacterial enzymes do not affect viruses.
· Antibiotics should not be used for viral infections such as colds, influenza, or most sore throats, unless there is a confirmed/suspected bacterial infection as well.

Antibiotic resistance: what it means

· Antibiotic resistance occurs when bacteria are no longer killed or inhibited by an antibiotic that previously worked.
· Resistance can arise through mutation or acquisition of resistance genes, often on plasmids.
· Resistant bacteria survive antibiotic treatment while susceptible bacteria are killed.
· Resistant bacteria then reproduce, increasing the proportion of resistant bacteria in the population.
· Resistance can spread between bacteria by horizontal gene transfer, including transfer of plasmids.
· Antibiotic resistance is an example of selection pressure: the antibiotic selects for bacteria with resistance alleles/genes.

This figure summarises how antibiotics act and how bacteria resist them. It is useful for linking resistance to mechanisms such as altered targets, reduced uptake, drug inactivation, and efflux pumps. Source

Mechanisms of antibiotic resistance

· Enzyme production: bacteria may produce enzymes such as β-lactamase, which breaks down penicillin-like antibiotics.
· Target modification: the antibiotic’s target site changes, so the antibiotic can no longer bind effectively.
· Reduced permeability: fewer antibiotics enter the bacterium.
· Efflux pumps: bacteria actively pump antibiotics out before they can act.
· Metabolic bypass: bacteria use an alternative pathway that avoids the antibiotic’s target.

Consequences of antibiotic resistance

· Infections become harder to treat.
· Previously effective antibiotics may become ineffective.
· Patients may need stronger, more expensive, or more toxic antibiotics.
· Treatment may take longer, increasing the risk of serious illness or death.
· Resistant bacteria can spread between people, hospitals, farms, and communities.
· Surgery, cancer treatment, organ transplants, and intensive care become riskier because they rely on effective antibiotics to control bacterial infections.
· Development of new antibiotics is slow, so resistance can reduce the available treatment options.

Steps to reduce the impact of antibiotic resistance

· Use antibiotics only when needed and only for bacterial infections.
· Do not use antibiotics for viral infections.
· Take antibiotics exactly as prescribed: correct dose, correct timing, full course unless told otherwise by a medical professional.
· Avoid sharing antibiotics or using leftover antibiotics.
· Doctors should prescribe narrow-spectrum antibiotics where possible, instead of broad-spectrum antibiotics.
· Use antibiotic susceptibility testing to choose the most effective antibiotic.
· Improve hygiene, sanitation, vaccination, and infection control to reduce spread of bacterial infections.
· Reduce unnecessary antibiotic use in agriculture and livestock.
· Monitor resistant strains through surveillance programmes.
· Develop new antibiotics and alternative treatments, but prevention of resistance remains essential.

This CDC infographic shows the step-by-step selection and spread of resistant bacteria. It supports exam answers on why antibiotic misuse increases resistance and why infection control matters. Source

Common exam wording to use

· “Penicillin inhibits cross-linking of peptidoglycan in bacterial cell walls.”
· “The bacterial cell wall becomes weakened, so the bacterium bursts by osmotic lysis.”
· “Antibiotics do not affect viruses because viruses lack bacterial targets, such as a peptidoglycan cell wall.”
· “Antibiotic use creates a selection pressure favouring resistant bacteria.”
· “Resistant bacteria survive, reproduce, and pass on resistance genes.”
· “Resistance can spread via plasmids and horizontal gene transfer.”