Carbapenems

Question

On the internet I read articles highlighting the resistance of Pseudomonas aeruginosa against carbapenems. Still they are used in patients with cystic fibrosis to reduce the Pseudomonas aeruginosa (like they do with my daughter). Could you say something meaningful about this?

Answer

Carbapenems are antibiotics that work against different kinds of bacteria, including Pseudomonas aeruginosa. Therefore they are called “broad-spectrum antibiotics”. This is in contrast to e.g. penicillin, which is active against a much smaller family of bacteria. Carbapenems kill the bacteria and therefore they are called “bactericidal antibiotics”.
All bacteria, including Pseudomonas aeruginosa can become resistant in order to escape the 'killing' function of the antibiotics. Each bacterium develops its own way to become resistant to the antibiotics, the Pseudomonas bacterium is very good at it and can become resistant to all kinds of antibiotics (not only carbapenems) in the long term.
When a doctor decides to give particular antibiotics to a CF patient, the following is usually done:
A sputum sample is put on a culture plate. That means that sputum (coughed up phlegm) is enriched with nutrients and is put in ideal conditions to promote its growth in an incubator. After a few days, groups of bacteria can be seen. The microbiologist will distinguish different bacteria the bacterium according to their specific appearance.
To test the sensitivity of bacteria to the antibiotics, discs containing different antibiotics are put around the bacteria . After a certain time the bacteria around the “sensitive (active) ” antibiotics die. Bacteria that have become resistant to a specific antibiotic continue to grow despite the presence of the antibiotic. The larger the radius of “no bacteria” around the particular antibiotic, the better its activity. This information is communicated to the doctor, who then chooses the most appropriate antibiotics for the patient.
The test (e.g. with your daughter) could have shown that the Pseudomonas bacteria were still sensitive to carbapenems.

To prevent resistance to antibiotics, the following principles are applied:

- Combination therapy: The use of 2 antibiotics at the same time with other antibiotics decreases the risk of resistance. For the treatment of Pseudomonas, the classic combination is penicillin derived antibiotics with aminoglycosides (e.g. meropenem with tobramycin).
- A second advantage to mix antibiotics of different classes is that they each have a different way of working. That way, antibiotics can even strengthen the effect on each other, compared with each antibiotic alone.
- rotating the use of antibiotics, rather than giving the same one all the time, will also reduce the risk of resistance.

Fortunately, in patients with CF, we usually still obtain good clinical improvement even when the germs are resistant to antibiotics. That is why in several centers abroad resistance determination is no longer done routinely.

I hope this answer is of use to you
Sincerely,

Lien De Somer-Kris De Boeck-Elke Dewachter

05.11.2012