Years before antibiotic use, diarrhea-causing bacteria developed resistance

Color-enhanced scanning electron micrograph showing Salmonella Typhimurium (red) invading cultured human cells.

As soon as scientists figured out how to harness the power of antibiotic drugs, bacteria hit back. Following clinical trials of penicillin around 1941, doctors documented the spread of penicillin-resistant Staphylococcus aureus among hospital patients in 1942. By the late 1960s, more than 80 percent of S. aureus bacteria isolated in and out of hospitals turned up resistant to the revolutionary drug.

It’s a common pattern that has led to the crisis of antibiotic resistance the world is now facing. In 1945, Alexander Fleming himself—the discoverer of penicillin—even warned of such “an era… of abuses,” in which strong public demand for antibiotics would drive bacterial resistance that render the “miraculous” drugs impotent.

But the problem is not just overuse in people. And sometimes, bacteria aren’t just one step behind—they may be one step ahead, according to a new study in the Lancet Infectious Diseases.

Genetic analyses of 288 bacterial isolates collected between 1911 and 1969 from 31 countries show that Salmonella developed resistance to an antibiotic several years before that drug even hit the market. The finding suggests that the diarrhea-causing bacteria were somehow primed to withstand the semi-synthetic antibiotic ampicillin before doctors could prescribe it in the early 1960s. Thus, overuse in humans didn’t drive the emergence of that resistance.

Instead, the authors speculate that overuse of a related antibiotic—penicillin G—in animals may be to blame. During the 1950s and 1960s, farmers used low doses of penicillin G to enhance the growth of poultry and pigs, as well as prevent infections in pigs. The authors, led by bacteriologist François-Xavier Weill of the Institut Pasteur in Paris, suspect that the low doses of the drug that lingered in the waste, water, and soil around farms may have spurred the development and spread of genes that make bacteria resistant to the antibiotic. Because ampicillin is a derivative of penicillin, it may have helped make them resistant to the newer drug, too.

“Although our study cannot identify a causal link between the use of penicillin G and the emergence of transmissible ampicillin-resistance in livestock, our results suggest that the non-clinical use of penicillins like [penicillin G] may have encouraged the evolution of resistance genes in the late 1950s,” Weill said in a press statement.

History of drugs

Ampicillin was discovered in 1958 and commercialized as the first “broad spectrum” penicillin in 1961. But, like its predecessor, it quickly hit resistance. In the latter half of 1962, doctors encountered an outbreak of ampicillin-resistant Salmonella enterica serotype Typhimurium in the UK. Several such outbreaks followed in subsequent years, and researchers reported finding several strains in humans and pigs that were multi-drug resistant (including ampicillin resistant).

The timeline might suggest that use of ampicillin in humans spurred the resistance and outbreaks—they followed the drug’s 1961 release. But S. enterica serotype Typhimurium is a zoonotic germ, meaning it can cause gastrointestinal infections in a range of mammals, including livestock. The quick appearance of ampicillin-resistant strains on farms raised suspicions among scientists. In the late 1960s, researchers in the UK warned that use of penicillins on farms may indeed be behind the rapid rise in resistance.

To better understand the emergence of ampicillin resistance, Weill and colleagues gathered up 288 S. enterica serotype Typhimurium isolates collected between 1911 and 1969 in Europe from humans, animals, and feed. Each isolate was tested to see what antibiotics it could resist. The researchers also decoded the entire genome—performing whole-genome sequencing—for 225 of the isolates (skipping redundant ones).

Of the 288 isolates, 253 (88 percent) were susceptible to all antibiotics, leaving 35 that were resistant to at least one type of antibiotic. Eleven of those were resistant to ampicillin—and all of those were also resistant to penicillin G. Three of the ampicillin-resistant isolates were archived before ampicillin was released in 1961. All three were collected from humans. One was collected in France in 1959 and the other two from Tunisia in 1960.

Rebels without a cause

Genetic sequencing revealed that the 11 ampicillin-resistant isolates had a variety of ampicillin-resistance genes, which were in a variety of genome positions and on different types of shareable DNA loops, called plasmids. Yet, the plasmids in the study’s isolates were different from the ones implicated in the first ampicillin-resistant bacterial outbreaks in the UK.

As for the three isolates that were ampicillin resistant before there was ampicillin to resist, they all had a common resistance gene called blaTEM-1B. But the two from Tunisia had their bla genes located on a different plasmid than the French isolate.

“Thus, the early emergence of ampicillin resistance in S enterica serotype Typhimurium was not due to single expansion of a clonal population that had acquired a particular plasmid encoding a β lactamase [bla], but to multiple independent acquisitions of blaTEM gene-carrying plasmids by different bacterial populations,” the authors concluded.

In an accompanying commentary, researchers at the Institute of Tropical Medicine Antwerp, Belgium conclude:

Despite limitations inherent to the retrospective nature of the study… [the authors] clearly show the existence of ampicillin resistance before the drug’s commercial introduction… The findings underline the importance of One Health approaches to tackling antibiotic resistance, which state that the health of people is connected to the health of animals and the environment.

Use of antibiotics for growth promotion and prophylaxis was completely banned in Europe in 2006, but it continues elsewhere in the world. The US Food and Drug Administration has put forward guidelines to end the practice on American farms, but those guidelines are not mandatory.

Earlier this month, the World Health Organization called for an all-out ban on the use of antibiotics on healthy animals.

The Lancet Infectious Disease, 2017. DOI: 10.1016/S1473-3099(17)30705-3  (About DOIs).

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