Pseudomonas aeruginosa is an important pathogen responsible for respiratory tract infections, but there have been reports that some of these infections do not respond to antipseudomonal agents when used alone.

For the clinical isolates of Pseudomonas aeruginosa used in this study, the MIC of levofloxacin was 2 μg/mL, while the MIC of meropenem was 4 μg/mL. No synergistic activity was shown in vitro when these two antimicrobial agents were used concurrently. However, it has become evident that a combination of levofloxacin and meropenem may exhibit bactericidal activity even against these refractory bacteria.
Let’s look at the effect of levofloxacin and meropenem when used together against these clinical isolates of Pseudomonas aeruginosa in vitro, under conditions close to the clinical setting.

This is an experimental device which programs the blood levels of two antimicrobial agents in advance and can thus simulate treatment with a single agent or two agents given concomitantly.
In this system, we use a hollow fiber cartridge, which is in common use throughout the world.
So let’s look at changes in plasma levels after these drugs are administered.
When 500 mg of levofloxacin is given once a day, plasma levels reach their peak in one hour, and then decrease gradually.
When 1,000 mg of meropenem is given three times a day, plasma levels reach their peak in 30 minutes and then fall rapidly. This is repeated three times a day.
When these two agents are given at the same time, you can obtain both of these curves.

Let’s have a look at the organisms when no drugs are added.

These cells were filmed using a time lapse where 4 seconds corresponds to one hour.

The bacteria grow to fill the entire screen in about 6 hours.

Treatment with levofloxacin was started once the organisms had grown to a certain level.

Let’s first have a look at when levofloxacin is added as a single agent.

When levofloxacin is added, the organisms stop growing and undergo bacteriolysis.

Let’s see how they look under a higher magnification. Pseudomonas aeruginosa cannot undergo cell division when levofloxacin, a fluoroquinolone that inhibits bacterial DNA synthesis, is present, and the content of the elongating bacterial cells aggregate, followed by bacteriolysis.

If you zoom out again, you can see that although most organisms appear to be killed, there are still living organisms.

Twenty-four hours later, under a different view, the bacteria have gained strength and are now growing vigorously.

When meropenem is added as a single agent,

Pseudomonas aeruginosa soon start being killed after the first treatment.

When you look at them at a higher magnification, the bacterial cells swell up like tiny balloons and then burst and die in the presence of meropenem, a carbapenem that inhibits bacterial cell wall synthesis.

When you zoom out again, you see some cells starting to grow again.

Surviving cells keep growing, but the second treatment 8 hours later soon stops the growth and induces bacteriolysis.

Thirteen hours later, when you zoom out again, you can see the cells starting to grow once more. With the third treatment given 16 hours later, cell growth can be suppressed temporarily, but the cells cannot be killed completely.

In time, the cells start growing across the entire field of view.

This time, we add levofloxacin and meropenem at the same time.

You can see Pseudomonas aeruginosa are killed very quickly, much more so than when either of the drugs is used alone.

Let’s look at them using a higher magnification from the start of treatment.

When we zoom out and continue observing the bacterial cells, they do not grow again.

Twenty-four hours later, when we look at the surrounding area, we find the cells are completely eradicated with no surviving bacteria.

This figure shows the changes in viable bacterial count over time with this simulation system. The viable bacterial count of Pseudomonas aeruginosa was 10^6 CFU/mL at the starting point. Where no drug is present, the organisms increase to 10^10 CFU/mL after 24 hours. When levofloxacin is added as a single agent, the bacterial count first falls below the detection limit, but 24 hours later it increases to a higher count than the starting point. When meropenem is added as a single agent, the bacterial count continues to fall and rise again and exceeds the count at the starting point.

However, when these two agents are added at the same time, the viable bacterial count falls below the detection limit in 2 hours and doesn’t increase again after that.

This shows the initial bactericidal activity in each case under the same magnification.

From these results, it can be seen that a combination of levofloxacin and meropenem can be expected to exhibit high antibacterial activity where a single agent is not effective.