Antimicrobial treatment increases the risk of infection with antimicrobial-resistant bacteria

Wednesday 09 Apr 14


Maike Koningstein

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People who have been treated with antimicrobials are at increased risk of a subsequent infection with antimicrobial-resistant Salmonella and Campylobacter. A previous treatment with antimicrobials can also make the disease progression more serious with an increased likelihood of hospitalisation, according to a PhD-project from the National Food Institute, Technical University of Denmark and Statens Serum Institut.

The problem of antimicrobial resistance due to the use of antimicrobials in food producing animals is well documented. Maike Koningstein’s PhD-project from the National Food Institute and Statens Serum Institut is one of the few studies which have estimated the association between the treatment of humans with antimicrobials and the consequences in terms of infection with antimicrobial-resistant bacteria.

Use antimicrobials prudently
"... if the use of certain types of antimicrobials increases, the number of infections with antimicrobial-resistant Salmonella and Campylobacter will likely also increase. The results emphasise yet again how crucially important it is to use antimicrobials cautiously."

The study showed an increased risk of acquiring a Salmonella or Campylobacter infection for up to a year after treatment with certain types of broad-spectrum antimicrobials. The risk was further increased if the Salmonella or Campylobacter bacteria were resistant to the type of antimicrobials used to treat the previous infection. The correlation was strongest if the prescribed drug belonged to the class of fluoroquinolones. At worst, the risk of becoming infected with fluoroquinolone-resistant Salmonella bacteria was seven times higher among people, who had previously been treated with this type of medication, compared with people who had not.

”From the results we can deduce that if the use of certain types of antimicrobials increases, the number of infections with antimicrobial-resistant Salmonella and Campylobacter will likely also increase. The results emphasise yet again how crucially important it is to use antimicrobials cautiously ,” says Maike Koningstein, who has completed her PhD at the National Food Institute.

Disturbed gut flora part of the cause

Treatment with antibiotics creates a disturbance in the gut flora by destroying the protective effect provided by the intestine’s usual composition of bacteria – regardless of whether the disease-causing bacteria are resistant or not. If the bacteria are resistant to the type of antimicrobial used to treat the patient, there is an additional effect, where bacteria that are susceptible to the drug are killed, and the drug literally creates an open space for the resistant bacteria to grow. This can have serious health consequences for the patient.

”A healthy gut flora is hugely important to the body’s ability to resist disease, and disturbances in the gut flora have been associated with various sorts of chronic diseases such as inflammatory bowel disease, which is another reason why unnecessary treatment with antimicrobials should be avoided,” adds Maike Koningstein.

Patients were more likely to experience stomach-cramps and nausea, and to end up in hospital, if their infection was due to an antimicrobial-resistant Salmonella compared to an antimicrobial-susceptible Salmonella. It may be the abovementioned disturbances in the gut flora that caused the infection to become more severe. However, it may also be because these patients generally were more susceptible to infections because of a weakened immune system – e.g. caused by another disease that lead to the treatment with antimicrobials in the first place.

Read more

Read Maike Koningstein’s PhD thesis: The interaction between human antimicrobial use and the risk of foodborne zoonotic bacteria (pdf).

Salmonella and Campylobacter bacteria cause the majority of foodborne bacterial infections in Denmark and worldwide. In Denmark, 22,609 cases of Salmonella were diagnosed from 1997 to 2005 and 31,699 cases of Campylobacter were diagnosed from 1999 to 2005. All these cases were included in the study. Data from approximately 10 times as many control cases – that is people without an infection – also formed part of the study. A further 150 people who have had a Salmonella infection were interviewed.

Please also read the latest press release from the DANMAP surveillance programme, which monitors antimicrobial use in Denmark: Mindre antibiotika til mennesker – mere til dyr (Danish only).


Resistance is bacteria’s defence against antimicrobials. Treatment with antimicrobials is meant to kill the disease-causing bacteria. Unfortunately, treatment with antimicrobials can also lead to some bacteria learning over time to protect themselves by developing resistance to the antimicrobial. For this reason it is important not to overuse antimicrobials.

Broad-spectrum antimicrobials

Antimicrobials fall into two groups: broad-spectrum and narrow-spectrum. While narrow-spectrum antimicrobials attack only a few groups of bacteria, broad-spectrum antimicrobials attack many different groups of bacteria at once.

The advantage of broad-spectrum antimicrobials is that they can be used as a treatment before it is determined which bacteria are causing the infection. However, these types of antimicrobials also kill off useful bacteria such as bacteria in the intestines. This can lead to the growth of resistant bacteria.