Taking antibiotics for even a short period can have a long-lasting impact on the bacteria that naturally reside in our gut, according to a new review article. And antibiotics even may turn natural gut bacteria against us in the war against drug-resistant microbes.

The drugs leave gut bacteria harboring antibiotic-resistance genes for up to two years after you take them, the researcher said. This reservoir of drug-resistant bacteria increases the chances of resistance genes being handed along to harmful, disease-causing bacteria , aiding their survival.

Antibiotics can alter the composition of bacterial populations that live in the intestine, and allow micro-organisms that are naturally resistant to the antibiotic to flourish.

The impact of antibiotics on the normal gut flora had been thought to be short-term, with any disturbances being restored several weeks after treatment. But studies are showing high levels of resistance genes in gut microbes after just seven days of antibiotic treatment, and remaining for up to two years, even if the individual has taken no further antibiotics.

The consequences of this could be potentially life-threatening, explained Dr. Cecilia Jernberg from the Swedish Institute for Infectious Disease Control, who conducted the review of those studies.

The presence of a high level of antibiotic-resistant genes in the gut increases the chance these genes could be transferred to pathogenic bacteria, she said. "This could reduce the success of future antibiotic treatments, and potentially lead to new strains of antibiotic-resistant bacteria ."

The study highlights the necessity of using antibiotics prudently.

"Antibiotic resistance is not a new problem, and there is a growing battle with multi-drug-resistant strains of pathogenic bacteria. The development of new antibiotics is slow, and so we must use the effective drugs we have left with care," Jernberg said. The new findings are "of great importance to allow rational antibiotic administration guidelines to be put in place."

The review will be published Wednesday (Nov. 3) in the journal Microbiology.