The era of antibiotics, launched accidentally by the Scottish bacteriologist Alexander Fleming, revolutionised the treatment of humans and animals with bacterial infections.

The era of antibiotics, launched accidentally by the Scottish bacteriologist Alexander Fleming, revolutionised the treatment of humans and animals with bacterial infections.

Antibiotics have since saved millions of lives.

An antibiotic is a substance that kills disease-causing, or pathogenic, bacteria without harming a patient.

The idea such substances might exist began in the late 19th century when medical researchers identified certain harmless bacteria in soil that could kill pathogenic bacteria.

In 1888, German microbiologist E. de Freudenreich managed to isolate an anti-bacterial substance produced by one of these but it was unstable and toxic to humans, ruling it out as a medicine.

The German scientist Paul Ehrlich (1854–1915) took a slightly different approach.

He began searching for ‘magic bullets’: chemicals that would cling to harmful bacteria, but not to human cells.

In 1909, he found one – a chemical that could cure syphilis, then a prolific killer.

It was ‘preparation 606’, which he later named ‘Salvarsan’.

But penicillin, the first antibiotic, was born one day in 1928 when Fleming (1881–1955) inadvertently let some airborne mould spores land on one of his bacterial ‘cultures’ (colonies grown for research, normally in Petri dishes).

The mould, a fungus called penicillium, thrived in the dish.

Fleming later realised his mistake – but noticed there were no bacteria around the mould.

It was clear that the fungus was producing an antibacterial chemical, which he named penicillin.

For years, Fleming’s observation had little impact because no one could isolate the active chemical produced by penicillium.

But in 1940, Oxford scientists Howard Florey (1898–1968) and Ernst Chain (1906–1979), along with colleague Norman Heatley (1911–2004), succeeded in isolating small quantities.

In 1941, Florey and Heatley went to the USA and helped develop a way of mass-producing penicillin cheaply.

The new medicine had an enormous and immediate impact, saving lives throughout World War Two.

Fleming, Florey and Chain jointly received the Nobel Prize for Medicine in 1945.

Meanwhile, another researcher, French bacteriologist René Dubos (1901–1982), isolated an antibacterial chemical produced by soil bacteria. Like Freudenreich’s discovery, Dubos’ chemical was toxic – but more effective.

It could be used on the skin, just not taken internally.

Penicillin’s effectiveness, and Dubos’ work, encouraged other researchers to search for new substances.

Concentrating on the products of organisms in soil led to the discovery of antibiotics with familiar names such as streptomycin and tetracycline.

Despite the tremendous success of antibiotics, there are problems. A long or concentrated course can kill beneficial bacteria inside your intestines.

But more worrying is that bacteria evolve quickly and fresh strains develop that are resistant to certain antibiotics.

New versions of an antibiotic can be produced, but the bacteria gradually evolve so that resistant strains become widespread.

The most infamous of these is methicillin-resistant staphylococcus aureus (MRSA), the 'super-bug’ which has been the scourge of hospitals since the 1990s and has claimed thousands of patients' lives.